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C# Implicit Conversion from Enum to different Enum [duplicate]

Is it possible to define an implicit conversion of enums in c#?
something that could achieve this?
public enum MyEnum
{
one = 1, two = 2
}
MyEnum number = MyEnum.one;
long i = number;
If not, why not?

There is a solution. Consider the following:
public sealed class AccountStatus
{
public static readonly AccountStatus Open = new AccountStatus(1);
public static readonly AccountStatus Closed = new AccountStatus(2);
public static readonly SortedList<byte, AccountStatus> Values = new SortedList<byte, AccountStatus>();
private readonly byte Value;
private AccountStatus(byte value)
{
this.Value = value;
Values.Add(value, this);
}
public static implicit operator AccountStatus(byte value)
{
return Values[value];
}
public static implicit operator byte(AccountStatus value)
{
return value.Value;
}
}
The above offers implicit conversion:
AccountStatus openedAccount = 1; // Works
byte openedValue = AccountStatus.Open; // Works
This is a fair bit more work than declaring a normal enum (though you can refactor some of the above into a common generic base class). You can go even further by having the base class implement IComparable & IEquatable, as well as adding methods to return the value of DescriptionAttributes, declared names, etc, etc.
I wrote a base class (RichEnum<>) to handle most fo the grunt work, which eases the above declaration of enums down to:
public sealed class AccountStatus : RichEnum<byte, AccountStatus>
{
public static readonly AccountStatus Open = new AccountStatus(1);
public static readonly AccountStatus Closed = new AccountStatus(2);
private AccountStatus(byte value) : base (value)
{
}
public static implicit operator AccountStatus(byte value)
{
return Convert(value);
}
}
The base class (RichEnum) is listed below.
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Resources;
namespace Ethica
{
using Reflection;
using Text;
[DebuggerDisplay("{Value} ({Name})")]
public abstract class RichEnum<TValue, TDerived>
: IEquatable<TDerived>,
IComparable<TDerived>,
IComparable, IComparer<TDerived>
where TValue : struct , IComparable<TValue>, IEquatable<TValue>
where TDerived : RichEnum<TValue, TDerived>
{
#region Backing Fields
/// <summary>
/// The value of the enum item
/// </summary>
public readonly TValue Value;
/// <summary>
/// The public field name, determined from reflection
/// </summary>
private string _name;
/// <summary>
/// The DescriptionAttribute, if any, linked to the declaring field
/// </summary>
private DescriptionAttribute _descriptionAttribute;
/// <summary>
/// Reverse lookup to convert values back to local instances
/// </summary>
private static SortedList<TValue, TDerived> _values;
private static bool _isInitialized;
#endregion
#region Constructors
protected RichEnum(TValue value)
{
if (_values == null)
_values = new SortedList<TValue, TDerived>();
this.Value = value;
_values.Add(value, (TDerived)this);
}
#endregion
#region Properties
public string Name
{
get
{
CheckInitialized();
return _name;
}
}
public string Description
{
get
{
CheckInitialized();
if (_descriptionAttribute != null)
return _descriptionAttribute.Description;
return _name;
}
}
#endregion
#region Initialization
private static void CheckInitialized()
{
if (!_isInitialized)
{
ResourceManager _resources = new ResourceManager(typeof(TDerived).Name, typeof(TDerived).Assembly);
var fields = typeof(TDerived)
.GetFields(BindingFlags.Static | BindingFlags.GetField | BindingFlags.Public)
.Where(t => t.FieldType == typeof(TDerived));
foreach (var field in fields)
{
TDerived instance = (TDerived)field.GetValue(null);
instance._name = field.Name;
instance._descriptionAttribute = field.GetAttribute<DescriptionAttribute>();
var displayName = field.Name.ToPhrase();
}
_isInitialized = true;
}
}
#endregion
#region Conversion and Equality
public static TDerived Convert(TValue value)
{
return _values[value];
}
public static bool TryConvert(TValue value, out TDerived result)
{
return _values.TryGetValue(value, out result);
}
public static implicit operator TValue(RichEnum<TValue, TDerived> value)
{
return value.Value;
}
public static implicit operator RichEnum<TValue, TDerived>(TValue value)
{
return _values[value];
}
public static implicit operator TDerived(RichEnum<TValue, TDerived> value)
{
return value;
}
public override string ToString()
{
return _name;
}
#endregion
#region IEquatable<TDerived> Members
public override bool Equals(object obj)
{
if (obj != null)
{
if (obj is TValue)
return Value.Equals((TValue)obj);
if (obj is TDerived)
return Value.Equals(((TDerived)obj).Value);
}
return false;
}
bool IEquatable<TDerived>.Equals(TDerived other)
{
return Value.Equals(other.Value);
}
public override int GetHashCode()
{
return Value.GetHashCode();
}
#endregion
#region IComparable Members
int IComparable<TDerived>.CompareTo(TDerived other)
{
return Value.CompareTo(other.Value);
}
int IComparable.CompareTo(object obj)
{
if (obj != null)
{
if (obj is TValue)
return Value.CompareTo((TValue)obj);
if (obj is TDerived)
return Value.CompareTo(((TDerived)obj).Value);
}
return -1;
}
int IComparer<TDerived>.Compare(TDerived x, TDerived y)
{
return (x == null) ? -1 :
(y == null) ? 1 :
x.Value.CompareTo(y.Value);
}
#endregion
public static IEnumerable<TDerived> Values
{
get
{
return _values.Values;
}
}
public static TDerived Parse(string name)
{
foreach (TDerived value in _values.Values)
if (0 == string.Compare(value.Name, name, true) || 0 == string.Compare(value.DisplayName, name, true))
return value;
return null;
}
}
}

You can't do implict conversions (except for zero), and you can't write your own instance methods - however, you can probably write your own extension methods:
public enum MyEnum { A, B, C }
public static class MyEnumExt
{
public static int Value(this MyEnum foo) { return (int)foo; }
static void Main()
{
MyEnum val = MyEnum.A;
int i = val.Value();
}
}
This doesn't give you a lot, though (compared to just doing an explicit cast).
One of the main times I've seen people want this is for doing [Flags] manipulation via generics - i.e. a bool IsFlagSet<T>(T value, T flag); method. Unfortunately, C# 3.0 doesn't support operators on generics, but you can get around this using things like this, which make operators fully available with generics.

struct PseudoEnum
{
public const int
INPT = 0,
CTXT = 1,
OUTP = 2;
};
// ...
var arr = new String[3];
arr[PseudoEnum.CTXT] = "can";
arr[PseudoEnum.INPT] = "use";
arr[PseudoEnum.CTXT] = "as";
arr[PseudoEnum.CTXT] = "array";
arr[PseudoEnum.OUTP] = "index";

I adapted Mark's excellent RichEnum generic baseclass.
Fixing
a number of compilation problems due to missing bits from his libraries (notably: the resource dependent display names weren't completely removed; they are now)
initialization wasn't perfect: if the first thing you did was access the static .Values property from the base class, you'd get a NPE. Fixed this by forcing the base class to curiously-recursively (CRTP) force the static construction of TDerived just in time during CheckInitialized
finally moved CheckInitialized logic into a static constructor (to avoid the penalty of checking each time, the race condition on multithreaded initialization; perhaps this was an impossibility solved by my bullet 1.?)
Kudos to Mark for the splendid idea + implementation, here's to you all:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Resources;
namespace NMatrix
{
[DebuggerDisplay("{Value} ({Name})")]
public abstract class RichEnum<TValue, TDerived>
: IEquatable<TDerived>,
IComparable<TDerived>,
IComparable, IComparer<TDerived>
where TValue : struct, IComparable<TValue>, IEquatable<TValue>
where TDerived : RichEnum<TValue, TDerived>
{
#region Backing Fields
/// <summary>
/// The value of the enum item
/// </summary>
public readonly TValue Value;
/// <summary>
/// The public field name, determined from reflection
/// </summary>
private string _name;
/// <summary>
/// The DescriptionAttribute, if any, linked to the declaring field
/// </summary>
private DescriptionAttribute _descriptionAttribute;
/// <summary>
/// Reverse lookup to convert values back to local instances
/// </summary>
private static readonly SortedList<TValue, TDerived> _values = new SortedList<TValue, TDerived>();
#endregion
#region Constructors
protected RichEnum(TValue value)
{
this.Value = value;
_values.Add(value, (TDerived)this);
}
#endregion
#region Properties
public string Name
{
get
{
return _name;
}
}
public string Description
{
get
{
if (_descriptionAttribute != null)
return _descriptionAttribute.Description;
return _name;
}
}
#endregion
#region Initialization
static RichEnum()
{
var fields = typeof(TDerived)
.GetFields(BindingFlags.Static | BindingFlags.GetField | BindingFlags.Public)
.Where(t => t.FieldType == typeof(TDerived));
foreach (var field in fields)
{
/*var dummy =*/ field.GetValue(null); // forces static initializer to run for TDerived
TDerived instance = (TDerived)field.GetValue(null);
instance._name = field.Name;
instance._descriptionAttribute = field.GetCustomAttributes(true).OfType<DescriptionAttribute>().FirstOrDefault();
}
}
#endregion
#region Conversion and Equality
public static TDerived Convert(TValue value)
{
return _values[value];
}
public static bool TryConvert(TValue value, out TDerived result)
{
return _values.TryGetValue(value, out result);
}
public static implicit operator TValue(RichEnum<TValue, TDerived> value)
{
return value.Value;
}
public static implicit operator RichEnum<TValue, TDerived>(TValue value)
{
return _values[value];
}
public static implicit operator TDerived(RichEnum<TValue, TDerived> value)
{
return value;
}
public override string ToString()
{
return _name;
}
#endregion
#region IEquatable<TDerived> Members
public override bool Equals(object obj)
{
if (obj != null)
{
if (obj is TValue)
return Value.Equals((TValue)obj);
if (obj is TDerived)
return Value.Equals(((TDerived)obj).Value);
}
return false;
}
bool IEquatable<TDerived>.Equals(TDerived other)
{
return Value.Equals(other.Value);
}
public override int GetHashCode()
{
return Value.GetHashCode();
}
#endregion
#region IComparable Members
int IComparable<TDerived>.CompareTo(TDerived other)
{
return Value.CompareTo(other.Value);
}
int IComparable.CompareTo(object obj)
{
if (obj != null)
{
if (obj is TValue)
return Value.CompareTo((TValue)obj);
if (obj is TDerived)
return Value.CompareTo(((TDerived)obj).Value);
}
return -1;
}
int IComparer<TDerived>.Compare(TDerived x, TDerived y)
{
return (x == null) ? -1 :
(y == null) ? 1 :
x.Value.CompareTo(y.Value);
}
#endregion
public static IEnumerable<TDerived> Values
{
get
{
return _values.Values;
}
}
public static TDerived Parse(string name)
{
foreach (TDerived value in Values)
if (0 == string.Compare(value.Name, name, true))
return value;
return null;
}
}
}
A sample of usage that I ran on mono:
using System.ComponentModel;
using System;
namespace NMatrix
{
public sealed class MyEnum : RichEnum<int, MyEnum>
{
[Description("aap")] public static readonly MyEnum my_aap = new MyEnum(63000);
[Description("noot")] public static readonly MyEnum my_noot = new MyEnum(63001);
[Description("mies")] public static readonly MyEnum my_mies = new MyEnum(63002);
private MyEnum(int value) : base (value) { }
public static implicit operator MyEnum(int value) { return Convert(value); }
}
public static class Program
{
public static void Main(string[] args)
{
foreach (var enumvalue in MyEnum.Values)
Console.WriteLine("MyEnum {0}: {1} ({2})", (int) enumvalue, enumvalue, enumvalue.Description);
}
}
}
Producing the output
[mono] ~/custom/demo # gmcs test.cs richenum.cs && ./test.exe
MyEnum 63000: my_aap (aap)
MyEnum 63001: my_noot (noot)
MyEnum 63002: my_mies (mies)
Note: mono 2.6.7 requires an extra explicit cast that is not required when using mono 2.8.2...

You cannot declare implicit conversions on enum types, because they can't define methods. The C# implicit keyword compiles into a method starting with 'op_', and it wouldn't work in this case.

You probably could, but not for the enum (you can't add a method to it). You could add an implicit conversion to you own class to allow an enum to be converted to it,
public class MyClass {
public static implicit operator MyClass ( MyEnum input ) {
//...
}
}
MyClass m = MyEnum.One;
The question would be why?
In general .Net avoids (and you should too) any implicit conversion where data can be lost.

enums are largely useless for me because of this, OP.
I end up doing pic-related all the time:
the simple solution
classic example problem is the VirtualKey set for detecting keypresses.
enum VKeys : ushort
{
a = 1,
b = 2,
c = 3
}
// the goal is to index the array using predefined constants
int[] array = new int[500];
var x = array[VKeys.VK_LSHIFT];
problem here is you can't index the array with the enum because it can't implicitly convert enum to ushort (even though we even based the enum on ushort)
in this specific context, enums are obsoleted by the following datastructure
. . . .
public static class VKeys
{
public const ushort
a = 1,
b = 2,
c = 3;
}

I found even easier solution taken from here https://codereview.stackexchange.com/questions/7566/enum-vs-int-wrapper-struct I pasted the code below from that link just in case it does not work in the future.
struct Day
{
readonly int day;
public static readonly Day Monday = 0;
public static readonly Day Tuesday = 1;
public static readonly Day Wednesday = 2;
public static readonly Day Thursday = 3;
public static readonly Day Friday = 4;
public static readonly Day Saturday = 5;
public static readonly Day Sunday = 6;
private Day(int day)
{
this.day = day;
}
public static implicit operator int(Day value)
{
return value.day;
}
public static implicit operator Day(int value)
{
return new Day(value);
}
}

I created this utility to help me convert an Enum to PrimitiveEnum and PrimitiveEnum to byte, sbyte, short, ushort, int, uint, long, or ulong.
So, this technically converts any enum to any its primitive value.
public enum MyEnum
{
one = 1, two = 2
}
PrimitiveEnum number = MyEnum.one;
long i = number;
See commit at https://github.com/McKabue/McKabue.Extentions.Utility/blob/master/src/McKabue.Extentions.Utility/Enums/PrimitiveEnum.cs
using System;
namespace McKabue.Extentions.Utility.Enums
{
/// <summary>
/// <see href="https://stackoverflow.com/q/261663/3563013">
/// Can we define implicit conversions of enums in c#?
/// </see>
/// </summary>
public struct PrimitiveEnum
{
private Enum _enum;
public PrimitiveEnum(Enum _enum)
{
this._enum = _enum;
}
public Enum Enum => _enum;
public static implicit operator PrimitiveEnum(Enum _enum)
{
return new PrimitiveEnum(_enum);
}
public static implicit operator Enum(PrimitiveEnum primitiveEnum)
{
return primitiveEnum.Enum;
}
public static implicit operator byte(PrimitiveEnum primitiveEnum)
{
return Convert.ToByte(primitiveEnum.Enum);
}
public static implicit operator sbyte(PrimitiveEnum primitiveEnum)
{
return Convert.ToSByte(primitiveEnum.Enum);
}
public static implicit operator short(PrimitiveEnum primitiveEnum)
{
return Convert.ToInt16(primitiveEnum.Enum);
}
public static implicit operator ushort(PrimitiveEnum primitiveEnum)
{
return Convert.ToUInt16(primitiveEnum.Enum);
}
public static implicit operator int(PrimitiveEnum primitiveEnum)
{
return Convert.ToInt32(primitiveEnum.Enum);
}
public static implicit operator uint(PrimitiveEnum primitiveEnum)
{
return Convert.ToUInt32(primitiveEnum.Enum);
}
public static implicit operator long(PrimitiveEnum primitiveEnum)
{
return Convert.ToInt64(primitiveEnum.Enum);
}
public static implicit operator ulong(PrimitiveEnum primitiveEnum)
{
return Convert.ToUInt64(primitiveEnum.Enum);
}
}
}

If you define the base of the enum as a long then you can perform explicit conversion. I don't know if you can use implicit conversions as enums cannot have methods defined on them.
public enum MyEnum : long
{
one = 1,
two = 2,
}
MyEnum number = MyEnum.one;
long i = (long)number;
Also, be aware with this that an uninitalised enumeration will default to the 0 value, or the first item - so in the situation above it would probably be best to define zero = 0 as well.

I've worked around an issue with sehe's answer when running the code on MS .net (non-Mono). For me specifically the issue occurred on .net 4.5.1 but other versions seem affected, too.
The issue
accessing a public static TDervied MyEnumValue by reflection (via FieldInfo.GetValue(null) does not initialize said field.
The workaround
Instead of assigning names to TDerived instances upon the static initializer of RichEnum<TValue, TDerived> this is done lazily on first access of TDerived.Name. The code:
public abstract class RichEnum<TValue, TDerived> : EquatableBase<TDerived>
where TValue : struct, IComparable<TValue>, IEquatable<TValue>
where TDerived : RichEnum<TValue, TDerived>
{
// Enforcing that the field Name (´SomeEnum.SomeEnumValue´) is the same as its
// instances ´SomeEnum.Name´ is done by the static initializer of this class.
// Explanation of initialization sequence:
// 1. the static initializer of ´RichEnum<TValue, TDerived>´ reflects TDervied and
// creates a list of all ´public static TDervied´ fields:
// ´EnumInstanceToNameMapping´
// 2. the static initializer of ´TDerive´d assigns values to these fields
// 3. The user is now able to access the values of a field.
// Upon first access of ´TDervied.Name´ we search the list
// ´EnumInstanceToNameMapping´ (created at step 1) for the field that holds
// ´this´ instance of ´TDerived´.
// We then get the Name for ´this´ from the FieldInfo
private static readonly IReadOnlyCollection<EnumInstanceReflectionInfo>
EnumInstanceToNameMapping =
typeof(TDerived)
.GetFields(BindingFlags.Static | BindingFlags.GetField | BindingFlags.Public)
.Where(t => t.FieldType == typeof(TDerived))
.Select(fieldInfo => new EnumInstanceReflectionInfo(fieldInfo))
.ToList();
private static readonly SortedList<TValue, TDerived> Values =
new SortedList<TValue, TDerived>();
public readonly TValue Value;
private readonly Lazy<string> _name;
protected RichEnum(TValue value)
{
Value = value;
// SortedList doesn't allow duplicates so we don't need to do
// duplicate checking ourselves
Values.Add(value, (TDerived)this);
_name = new Lazy<string>(
() => EnumInstanceToNameMapping
.First(x => ReferenceEquals(this, x.Instance))
.Name);
}
public string Name
{
get { return _name.Value; }
}
public static implicit operator TValue(RichEnum<TValue, TDerived> richEnum)
{
return richEnum.Value;
}
public static TDerived Convert(TValue value)
{
return Values[value];
}
protected override bool Equals(TDerived other)
{
return Value.Equals(other.Value);
}
protected override int ComputeHashCode()
{
return Value.GetHashCode();
}
private class EnumInstanceReflectionInfo
{
private readonly FieldInfo _field;
private readonly Lazy<TDerived> _instance;
public EnumInstanceReflectionInfo(FieldInfo field)
{
_field = field;
_instance = new Lazy<TDerived>(() => (TDerived)field.GetValue(null));
}
public TDerived Instance
{
get { return _instance.Value; }
}
public string Name { get { return _field.Name; } }
}
}
which - in my case - is based upon EquatableBase<T>:
public abstract class EquatableBase<T>
where T : class
{
public override bool Equals(object obj)
{
if (this == obj)
{
return true;
}
T other = obj as T;
if (other == null)
{
return false;
}
return Equals(other);
}
protected abstract bool Equals(T other);
public override int GetHashCode()
{
unchecked
{
return ComputeHashCode();
}
}
protected abstract int ComputeHashCode();
}
Note
The above code does not incorporate all features of Mark's original answer!
Thanks
Thanks to Mark for providing his RichEnum implementation and thanks to sehe for providing some improvements!

Here's a different flavour based on adminSoftDK's answer.
/// <summary>
/// Based on https://datatracker.ietf.org/doc/html/rfc4346#appendix-A.1
/// </summary>
[DebuggerDisplay("{_value}")]
public struct HandshakeContentType
{
#region Types
public const byte ChangeCipher = 0x14;
public const byte Alert = 0x15;
public const byte Handshake = 0x16;
public const byte ApplicationData = 0x17;
#endregion
byte _value;
private HandshakeContentType(byte value)
{
_value = value;
switch (_value)
{
case ChangeCipher:
case Alert:
case Handshake:
case ApplicationData:
break;
default:
throw new InvalidOperationException($"An invalid handshake content type (${value}) was provided.");
}
}
#region Methods
public static implicit operator byte(HandshakeContentType type) => type._value;
public static implicit operator HandshakeContentType(byte b) => new HandshakeContentType(b);
#endregion
}
This allows you to use this struct with switch statements which I think is pretty awesome.

#BatteryBackupUnit
Hey this sounds like a cool solution but could you explain this part here?
Since im getting with .NET 4.7.2 an "InvalidCastException" out of this sadly :/
_name = new Lazy<string>(
() => EnumInstanceToNameMapping
.First(x => ReferenceEquals(this, x.Instance))
.Name);
I dont know why, i have created a derived type of the RichEnum and initialized as everything u did in the example but i getthis annyoingg exception..
Would be glad of some help to this since i like this approach alot tbh.

I don't have enough rep to add a comment, but I was inspired by the 'struct' comment here:
https://stackoverflow.com/a/39141171/12135042
Here is how I did it:
public enum DaysOfWeek
{
Sunday = 0,
Monday = 1,
Tuesday = 2,
Wednesday = 3,
Thursday = 4,
Friday = 5,
Saturday = 7,
}
public struct Weekends
{
private Weekends(DaysOfWeek day){ Day = day; }
public readonly DaysOfWeek Day;
public static Weekends Sunday = new(DaysOfWeek.Sunday);
public static Weekends Saturday = new(DaysOfWeek.Saturday);
public static implicit operator DaysOfWeek(Weekends value) => value.Mode;
}
I feel this gets the best of both worlds here, since you get your super enum, and easily accessible structs that are statically accessibly acting as subsets of the superenum.

Introducing implicit conversions for enum types would break type safety, so I'd not recommend to do that. Why would you want to do that? The only use case for this I've seen is when you want to put the enum values into a structure with a pre-defined layout. But even then, you can use the enum type in the structure and just tell the Marshaller what he should do with this.

How to define an enum with string value?

I am trying to define an Enum and add valid common separators which used in CSV or similar files. Then I am going to bind it to a ComboBox as a data source so whenever I add or remove from the Enum definition, I would not need to change anything in the combo box.
The problem is how can I define enum with string representation, something like:
public enum SeparatorChars{Comma = ",", Tab = "\t", Space = " "}

You can't - enum values have to be integral values. You can either use attributes to associate a string value with each enum value, or in this case if every separator is a single character you could just use the char value:
enum Separator
{
Comma = ',',
Tab = '\t',
Space = ' '
}
(EDIT: Just to clarify, you can't make char the underlying type of the enum, but you can use char constants to assign the integral value corresponding to each enum value. The underlying type of the above enum is int.)
Then an extension method if you need one:
public string ToSeparatorString(this Separator separator)
{
// TODO: validation
return ((char) separator).ToString();
}

You can achieve it but it will require a bit of work.
Define an attribute class which will contain the string value for enum.
Define an extension method which will return back the value from the attribute. Eg..GetStringValue(this Enum value) will return attribute value.
Then you can define the enum like this..
public enum Test : int {
[StringValue("a")]
Foo = 1,
[StringValue("b")]
Something = 2
}
To get back the value from Attribute Test.Foo.GetStringValue();
Refer : Enum With String Values In C#

As far as I know, you will not be allowed to assign string values to enum. What you can do is create a class with string constants in it.
public static class SeparatorChars
{
public static String Comma { get { return ",";} }
public static String Tab { get { return "\t,";} }
public static String Space { get { return " ";} }
}

For a simple enum of string values (or any other type):
public static class MyEnumClass
{
public const string
MyValue1 = "My value 1",
MyValue2 = "My value 2";
}
Usage: string MyValue = MyEnumClass.MyValue1;

Maybe it's too late, but here it goes.
We can use the attribute EnumMember to manage Enum values.
public enum UnitOfMeasure
{
[EnumMember(Value = "KM")]
Kilometer,
[EnumMember(Value = "MI")]
Miles
}
This way the result value for UnitOfMeasure will be KM or MI. This also can be seen in Andrew Whitaker answer.

You can't do this with enums, but you can do it like that:
public static class SeparatorChars
{
public static string Comma = ",";
public static string Tab = "\t";
public static string Space = " ";
}

A class that emulates enum behaviour but using string instead of int can be created as follows...
public class GrainType
{
private string _typeKeyWord;
private GrainType(string typeKeyWord)
{
_typeKeyWord = typeKeyWord;
}
public override string ToString()
{
return _typeKeyWord;
}
public static GrainType Wheat = new GrainType("GT_WHEAT");
public static GrainType Corn = new GrainType("GT_CORN");
public static GrainType Rice = new GrainType("GT_RICE");
public static GrainType Barley = new GrainType("GT_BARLEY");
}
Usage...
GrainType myGrain = GrainType.Wheat;
PrintGrainKeyword(myGrain);
then...
public void PrintGrainKeyword(GrainType grain)
{
Console.Writeline("My Grain code is " + grain.ToString()); // Displays "My Grain code is GT_WHEAT"
}

It is kind of late for answer, but maybe it helps someone in future. I found it easier to use struct for this kind of problem.
Following sample is copy pasted part from MS code:
namespace System.IdentityModel.Tokens.Jwt
{
//
// Summary:
// List of registered claims from different sources http://tools.ietf.org/html/rfc7519#section-4
// http://openid.net/specs/openid-connect-core-1_0.html#IDToken
public struct JwtRegisteredClaimNames
{
//
// Summary:
// http://tools.ietf.org/html/rfc7519#section-4
public const string Actort = "actort";
//
// Summary:
// http://tools.ietf.org/html/rfc7519#section-4
public const string Typ = "typ";
//
// Summary:
// http://tools.ietf.org/html/rfc7519#section-4
public const string Sub = "sub";
//
// Summary:
// http://openid.net/specs/openid-connect-frontchannel-1_0.html#OPLogout
public const string Sid = "sid";
//
// Summary:
// http://tools.ietf.org/html/rfc7519#section-4
public const string Prn = "prn";
//
// Summary:
// http://tools.ietf.org/html/rfc7519#section-4
public const string Nbf = "nbf";
//
// Summary:
// http://tools.ietf.org/html/rfc7519#section-4
public const string Nonce = "nonce";
//
// Summary:
// http://tools.ietf.org/html/rfc7519#section-4
public const string NameId = "nameid";
}
}

You can't, because enum can only be based on a primitive numeric type.
You could try using a Dictionary instead:
Dictionary<String, char> separators = new Dictionary<string, char>
{
{"Comma", ','},
{"Tab", '\t'},
{"Space", ' '},
};
Alternatively, you could use a Dictionary<Separator, char> or Dictionary<Separator, string> where Separator is a normal enum:
enum Separator
{
Comma,
Tab,
Space
}
which would be a bit more pleasant than handling the strings directly.

For people arriving here looking for an answer to a more generic question, you can extend the static class concept if you want your code to look like an enum.
The following approach works when you haven't finalised the enum names you want and the enum values are the string representation of the enam name; use nameof() to make your refactoring simpler.
public static class Colours
{
public static string Red => nameof(Red);
public static string Green => nameof(Green);
public static string Blue => nameof(Blue);
}
This achieves the intention of an enum that has string values (such as the following pseudocode):
public enum Colours
{
"Red",
"Green",
"Blue"
}

I created a base class for creating string-valued enums in .NET. It is just one C# file that you can copy & paste into your projects, or install via NuGet package named StringEnum.
Usage:
///<completionlist cref="HexColor"/>
class HexColor : StringEnum<HexColor>
{
public static readonly HexColor Blue = New("#FF0000");
public static readonly HexColor Green = New("#00FF00");
public static readonly HexColor Red = New("#000FF");
}
Features
Your StringEnum looks somewhat similar to a regular enum:
// Static Parse Method
HexColor.Parse("#FF0000") // => HexColor.Red
HexColor.Parse("#ff0000", caseSensitive: false) // => HexColor.Red
HexColor.Parse("invalid") // => throws InvalidOperationException
// Static TryParse method.
HexColor.TryParse("#FF0000") // => HexColor.Red
HexColor.TryParse("#ff0000", caseSensitive: false) // => HexColor.Red
HexColor.TryParse("invalid") // => null
// Parse and TryParse returns the preexistent instances
object.ReferenceEquals(HexColor.Parse("#FF0000"), HexColor.Red) // => true
// Conversion from your `StringEnum` to `string`
string myString1 = HexColor.Red.ToString(); // => "#FF0000"
string myString2 = HexColor.Red; // => "#FF0000" (implicit cast)
Intellisense will suggest the enum name if the class is annotated with the xml comment <completitionlist>. (Works in both C# and VB): i.e.
Instalation
Either:
Install latest StringEnum NuGet package, which is based on .Net Standard 1.0 so it runs on .Net Core >= 1.0, .Net Framework >= 4.5, Mono >= 4.6, etc.
Or paste the following StringEnum base class to your project. (latest version)
public abstract class StringEnum<T> : IEquatable<T> where T : StringEnum<T>, new()
{
protected string Value;
private static IList<T> valueList = new List<T>();
protected static T New(string value)
{
if (value == null)
return null; // the null-valued instance is null.
var result = new T() { Value = value };
valueList.Add(result);
return result;
}
public static implicit operator string(StringEnum<T> enumValue) => enumValue.Value;
public override string ToString() => Value;
public static bool operator !=(StringEnum<T> o1, StringEnum<T> o2) => o1?.Value != o2?.Value;
public static bool operator ==(StringEnum<T> o1, StringEnum<T> o2) => o1?.Value == o2?.Value;
public override bool Equals(object other) => this.Value.Equals((other as T)?.Value ?? (other as string));
bool IEquatable<T>.Equals(T other) => this.Value.Equals(other.Value);
public override int GetHashCode() => Value.GetHashCode();
/// <summary>
/// Parse the <paramref name="value"/> specified and returns a valid <typeparamref name="T"/> or else throws InvalidOperationException.
/// </summary>
/// <param name="value">The string value representad by an instance of <typeparamref name="T"/>. Matches by string value, not by the member name.</param>
/// <param name="caseSensitive">If true, the strings must match case sensitivity.</param>
public static T Parse(string value, bool caseSensitive = false)
{
var result = TryParse(value, caseSensitive);
if (result == null)
throw new InvalidOperationException((value == null ? "null" : $"'{value}'") + $" is not a valid {typeof(T).Name}");
return result;
}
/// <summary>
/// Parse the <paramref name="value"/> specified and returns a valid <typeparamref name="T"/> or else returns null.
/// </summary>
/// <param name="value">The string value representad by an instance of <typeparamref name="T"/>. Matches by string value, not by the member name.</param>
/// <param name="caseSensitive">If true, the strings must match case sensitivity.</param>
public static T TryParse(string value, bool caseSensitive = false)
{
if (value == null) return null;
if (valueList.Count == 0) System.Runtime.CompilerServices.RuntimeHelpers.RunClassConstructor(typeof(T).TypeHandle); // force static fields initialization
var field = valueList.FirstOrDefault(f => f.Value.Equals(value,
caseSensitive ? StringComparison.Ordinal
: StringComparison.OrdinalIgnoreCase));
// Not using InvariantCulture because it's only supported in NETStandard >= 2.0
if (field == null)
return null;
return field;
}
}
For Newtonsoft.Json serialization support, copy this extended version instead. StringEnum.cs
I realized after the fact that this code is similar to Ben's answer. I sincerely wrote it from scratch. However I think it has a few extras, like the <completitionlist> hack, the resulting class looks more like an Enum, no use of reflection on Parse(), the NuGet package and repo where I will hopefully address incoming issues and feedback.

Building on some of the answers here I have implemented a reusable base class that mimics the behaviour of an enum but with string as the underlying type. It supports various operations including:
getting a list of possible values
converting to string
comparison with other instances via .Equals, ==, and !=
conversion to/from JSON using a JSON.NET JsonConverter
This is the base class in it's entirety:
public abstract class StringEnumBase<T> : IEquatable<T>
where T : StringEnumBase<T>
{
public string Value { get; }
protected StringEnumBase(string value) => this.Value = value;
public override string ToString() => this.Value;
public static List<T> AsList()
{
return typeof(T)
.GetProperties(BindingFlags.Public | BindingFlags.Static)
.Where(p => p.PropertyType == typeof(T))
.Select(p => (T)p.GetValue(null))
.ToList();
}
public static T Parse(string value)
{
List<T> all = AsList();
if (!all.Any(a => a.Value == value))
throw new InvalidOperationException($"\"{value}\" is not a valid value for the type {typeof(T).Name}");
return all.Single(a => a.Value == value);
}
public bool Equals(T other)
{
if (other == null) return false;
return this.Value == other?.Value;
}
public override bool Equals(object obj)
{
if (obj == null) return false;
if (obj is T other) return this.Equals(other);
return false;
}
public override int GetHashCode() => this.Value.GetHashCode();
public static bool operator ==(StringEnumBase<T> a, StringEnumBase<T> b) => a?.Equals(b) ?? false;
public static bool operator !=(StringEnumBase<T> a, StringEnumBase<T> b) => !(a?.Equals(b) ?? false);
public class JsonConverter<T> : Newtonsoft.Json.JsonConverter
where T : StringEnumBase<T>
{
public override bool CanRead => true;
public override bool CanWrite => true;
public override bool CanConvert(Type objectType) => ImplementsGeneric(objectType, typeof(StringEnumBase<>));
private static bool ImplementsGeneric(Type type, Type generic)
{
while (type != null)
{
if (type.IsGenericType && type.GetGenericTypeDefinition() == generic)
return true;
type = type.BaseType;
}
return false;
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
JToken item = JToken.Load(reader);
string value = item.Value<string>();
return StringEnumBase<T>.Parse(value);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
if (value is StringEnumBase<T> v)
JToken.FromObject(v.Value).WriteTo(writer);
}
}
}
And this is how you would implement your "string enum":
[JsonConverter(typeof(JsonConverter<Colour>))]
public class Colour : StringEnumBase<Colour>
{
private Colour(string value) : base(value) { }
public static Colour Red => new Colour("red");
public static Colour Green => new Colour("green");
public static Colour Blue => new Colour("blue");
}
Which could be used like this:
public class Foo
{
public Colour colour { get; }
public Foo(Colour colour) => this.colour = colour;
public bool Bar()
{
if (this.colour == Colour.Red || this.colour == Colour.Blue)
return true;
else
return false;
}
}
I hope someone finds this useful!

What I have recently begun doing is using Tuples
public static (string Fox, string Rabbit, string Horse) Animals = ("Fox", "Rabbit", "Horse");
...
public static (string Comma, string Tab, string Space) SeparatorChars = (",", "\t", " ");

I wish there were a more elegant solution, like just allowing string type enum in the language level, but it seems that it is not supported yet. The code below is basically the same idea as other answers, but I think it is shorter and it can be reused. All you have to do is adding a [Description("")] above each enum entry and add a class that has 10 lines.
The class:
public static class Extensions
{
public static string ToStringValue(this Enum en)
{
var type = en.GetType();
var memInfo = type.GetMember(en.ToString());
var attributes = memInfo[0].GetCustomAttributes(typeof(DescriptionAttribute), false);
var stringValue = ((DescriptionAttribute)attributes[0]).Description;
return stringValue;
}
}
Usage:
enum Country
{
[Description("Deutschland")]
Germany,
[Description("Nippon")]
Japan,
[Description("Italia")]
Italy,
}
static void Main(string[] args)
{
Show(new[] {Country.Germany, Country.Japan, Country.Italy});
void Show(Country[] countries)
{
foreach (var country in countries)
{
Debug.WriteLine(country.ToStringValue());
}
}
}

Well first you try to assign strings not chars, even if they are just one character. use ',' instead of ",". Next thing is, enums only take integral types without char you could use the unicode value, but i would strongly advice you not to do so.
If you are certain that these values stay the same, in differnt cultures and languages, i would use a static class with const strings.

While it is really not possible to use a char or a string as the base for an enum, i think this is not what you really like to do.
Like you mentioned you'd like to have an enum of possibilities and show a string representation of this within a combo box. If the user selects one of these string representations you'd like to get out the corresponding enum. And this is possible:
First we have to link some string to an enum value. This can be done by using the DescriptionAttribute like it is described here or here.
Now you need to create a list of enum values and corresponding descriptions. This can be done by using the following method:
/// <summary>
/// Creates an List with all keys and values of a given Enum class
/// </summary>
/// <typeparam name="T">Must be derived from class Enum!</typeparam>
/// <returns>A list of KeyValuePair<Enum, string> with all available
/// names and values of the given Enum.</returns>
public static IList<KeyValuePair<T, string>> ToList<T>() where T : struct
{
var type = typeof(T);
if (!type.IsEnum)
{
throw new ArgumentException("T must be an enum");
}
return (IList<KeyValuePair<T, string>>)
Enum.GetValues(type)
.OfType<T>()
.Select(e =>
{
var asEnum = (Enum)Convert.ChangeType(e, typeof(Enum));
return new KeyValuePair<T, string>(e, asEnum.Description());
})
.ToArray();
}
Now you'll have a list of key value pairs of all enums and their description. So let's simply assign this as a data source for a combo box.
var comboBox = new ComboBox();
comboBox.ValueMember = "Key"
comboBox.DisplayMember = "Value";
comboBox.DataSource = EnumUtilities.ToList<Separator>();
comboBox.SelectedIndexChanged += (sender, e) =>
{
var selectedEnum = (Separator)comboBox.SelectedValue;
MessageBox.Show(selectedEnum.ToString());
}
The user sees all the string representations of the enum and within your code you'll get the desired enum value.

Adding still something more as some of the answers here are missing the point of using enums. Naturally one option is is to have well defined strings as static variables etc, but then you're opening your interface for illegal values also, i.e. you need to validate the input. With enums it's guaranteed that only allowed values are passed to your interface.
enum Separator
{
Comma,
Tab,
Space,
CRLF,
SoFunny
}
In addition to this you can use e.g. an internal Dictionary for the mapping purposes.
private readonly Dictionary<Separator, string> separatorMap = new Dictionary<Separator, string>()
{
{ Separator.Comma, "," },
{ Separator.Tab, "\t" },
{ Separator.Space, " " },
{ Separator.CRLF, "\r\n" },
{ Separator.SoFunny, "Your Mom" }
};
Even more sophisticated method for doing this is to create a static class to give the enum new capabilities and handle the mapping there.
Example of using code above would be like.
public string TransformToSingleString(List<string> values, Separator separator)
{
var separateWith = separatorMap[separator];
...
}

Enumaration Class
public sealed class GenericDateTimeFormatType
{
public static readonly GenericDateTimeFormatType Format1 = new GenericDateTimeFormatType("dd-MM-YYYY");
public static readonly GenericDateTimeFormatType Format2 = new GenericDateTimeFormatType("dd-MMM-YYYY");
private GenericDateTimeFormatType(string Format)
{
_Value = Format;
}
public string _Value { get; private set; }
}
Enumaration Consuption
public static void Main()
{
Country A = new Country();
A.DefaultDateFormat = GenericDateTimeFormatType.Format1;
Console.ReadLine();
}

We can't define enumeration as string type. The approved types for an enum are byte, sbyte, short, ushort, int, uint, long, or ulong.
If you need more details on enumeration please follow below link,that link will help you to understand enumeration.
Enumeration
#narendras1414

It works for me..
public class ShapeTypes
{
private ShapeTypes() { }
public static string OVAL
{
get
{
return "ov";
}
private set { }
}
public static string SQUARE
{
get
{
return "sq";
}
private set { }
}
public static string RECTANGLE
{
get
{
return "rec";
}
private set { }
}
}

Edit the display name of enumeration members in a PropertyGrid

I have a property grid that I am using for users to be able to configure objects for any plugin that is written to be used in my application. I would like to be able to tell developers writing plugins to use the ComponentModel Attributes for their members like so:
[CategoryAttribute("On Screen Display Settings"),
DescriptionAttribute("Whether or not to show the session timer."),
DisplayName("Show Session Timer")]
public bool ShowTimer
{
get;
set;
}
This works great. Now I would like for the members of an enumeration to be able to be edited as well. i.e.
public enum Resolution_ : byte
{
DCIF,
CIF,
QCIF,
[DisplayName("4CIF")]
CIF4,
[DisplayName("2CIF")]
CIF2
}
So that they are displayed in the PropertyGrid's list like so:
DCIF
CIF
QCIF
CIF4
CIF2
Along with any Descriptions and Display names they may have with them.
It seems that I can only do this with properties, events, and methods. Does anyone know how I can do this for an enumeration?

You will have to make an EnumConverter class and decorate your property with a TypeConverter attribute in order to do this.
See this Using PropertyGrid in .NET, it's a fun example:
Imagine that you want more than two items in list. The boolean type is not enough; you need to set Description attributes with a name for every element in enum.
enum DrinkDoses {
[Description("Half of litre")]
litre,
[Description("One litre")]
oneLitre,
[Description("Two litres")]
twoLitre,
[Description("Three litres")]
threeLitres,
[Description("Four litres")]
fourLitres,
[Description("Death dose, five litres")]
fiveLitres
}
In another class you need to utilize the type EnumConverter.
class DrinkDosesConverter : EnumConverter {
private Type enumType;
public DrinkDosesConverter(Type type) : base(type) {
enumType = type;
}
public override bool CanConvertTo(ITypeDescriptorContext context, Type destType) {
return destType == typeof(string);
}
public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture,
object value, Type destType) {
FieldInfo fi = enumType.GetField(Enum.GetName(enumType, value));
DescriptionAttribute dna = (DescriptionAttribute)Attribute.GetCustomAttribute(fi,
typeof(DescriptionAttribute));
if (dna != null)
return dna.Description;
else
return value.ToString();
}
public override bool CanConvertFrom(ITypeDescriptorContext context, Type srcType) {
return srcType == typeof(string);
}
public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture,
object value) {
foreach (FieldInfo fi in enumType.GetFields()) {
DescriptionAttribute dna = (DescriptionAttribute)Attribute.GetCustomAttribute(fi,
typeof(DescriptionAttribute));
if ((dna != null) && ((string)value == dna.Description))
return Enum.Parse(enumType, fi.Name);
}
return Enum.Parse(enumType, (string)value);
}
}
Third, you need set the attribute TypeConverter for displaying the property.
class DrinkerDoses {
DrinkDoses doses;
[DisplayName("Doses")]
[Description("Drinker doses")]
[Category("Alcoholics drinking")]
[TypeConverter(typeof(DrinkDosesConverter))]
public DrinkDoses Doses {
get { return doses; }
set { doses = value; }
}
int dataInt;
public int DataInt {
get { return dataInt; }
set { dataInt = value; }
}
}

You can attach a custom TypeConverter implementation to the property whose type is your enumeration and override the GetStandardValuesSupported and GetStandardValues to return a custom list of items to show in the drop-down list in the PropertyGrid. You can then override ConvertFrom/ConvertTo methods to handle converting values to/from your enumeration type.
You may also want to override GetStandardValuesExclusive and have it return "true" so the user can't type anything into the property value.
So, something like this:
public class MyTypeConverter : TypeConverter
{
//Override GetStandardValuesExclusive,
//GetStandardValues and GetStandardValuesSupported
}
public class SomeClass
{
[TypeConverter(typeof(MyTypeConverter))]
public Resolution SomePropertry
{
...
}
}
In your implementation of GetStandardValues/ConvertFrom/ConvertTo you could then use Reflection to pull out the DisplayNameAttribute (or DescriptionAttribute, which may be more suited to this task) attributes of the various enum members to show that text instead of hard-coding the list of items to show.

The answer I gave here Has a working example of this. Here is the specific code from that example that you want:
/// <summary>
/// This attribute is used to represent a string value
/// for a value in an enum.
/// </summary>
public class StringValueAttribute : Attribute {
#region Properties
/// <summary>
/// Holds the stringvalue for a value in an enum.
/// </summary>
public string StringValue { get; protected set; }
#endregion
#region Constructor
/// <summary>
/// Constructor used to init a StringValue Attribute
/// </summary>
/// <param name="value"></param>
public StringValueAttribute(string value) {
this.StringValue = value;
}
#endregion
}
public static class MyExtension
{
public static string GetStringValue(this Enum value)
{
// Get the type
Type type = value.GetType();
// Get fieldinfo for this type
FieldInfo fieldInfo = type.GetField(value.ToString());
// Get the stringvalue attributes
StringValueAttribute[] attribs = fieldInfo.GetCustomAttributes(
typeof(StringValueAttribute), false) as StringValueAttribute[];
// Return the first if there was a match.
return attribs.Length > 0 ? attribs[0].StringValue : null;
}
public static String[] GetEnumNames(Type t)
{
Array enumValueArray= Enum.GetValues(t);
string[] enumStrings = new String[enumValueArray.Length];
for(int i = 0; i< enumValueArray.Length; ++i)
{
enumStrings[i] = GetStringValue((test)enumValueArray.GetValue(i));
}
return enumStrings;
}
}
enum test
{
[StringValue("test ONE")]
test1,
[StringValue("test TWO")]
test2
}

This also seems to work:
[AttributeUsage(AttributeTargets.Field)]
public class EnumDisplayNameAttribute : System.ComponentModel.DisplayNameAttribute
{
public EnumDisplayNameAttribute(string data) : base(data) { }
}
public enum Resolution_ : byte
{
DCIF,
CIF,
QCIF,
[EnumDisplayName("4CIF")]
CIF4,
[EnumDisplayName("2CIF")]
CIF2
}
Components looking for a DisplayName attribute via Reflection will find one, and as far as I can tell this works. Is there a reason why this might be a bad idea?

Custom attribute on property - Getting type and value of attributed property

I have the following custom attribute, which can be applied on properties:
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public class IdentifierAttribute : Attribute
{
}
For example:
public class MyClass
{
[Identifier()]
public string Name { get; set; }
public int SomeNumber { get; set; }
public string SomeOtherProperty { get; set; }
}
There will also be other classes, to which the Identifier attribute could be added to properties of different type:
public class MyOtherClass
{
public string Name { get; set; }
[Identifier()]
public int SomeNumber { get; set; }
public string SomeOtherProperty { get; set; }
}
I then need to be able to get this information in my consuming class.
For example:
public class TestClass<T>
{
public void GetIDForPassedInObject(T obj)
{
var type = obj.GetType();
//type.GetCustomAttributes(true)???
}
}
What's the best way of going about this?
I need to get the type of the [Identifier()] field (int, string, etc...) and the actual value, obviously based on the type.

Something like the following,, this will use only the first property it comes accross that has the attribute, of course you could place it on more than one..
public object GetIDForPassedInObject(T obj)
{
var prop = typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance)
.FirstOrDefault(p => p.GetCustomAttributes(typeof(IdentifierAttribute), false).Count() ==1);
object ret = prop !=null ? prop.GetValue(obj, null) : null;
return ret;
}

public class TestClass<T>
{
public void GetIDForPassedInObject(T obj)
{
PropertyInfo[] properties =
obj.GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance);
PropertyInfo IdProperty = (from PropertyInfo property in properties
where property.GetCustomAttributes(typeof(Identifier), true).Length > 0
select property).First();
if(null == IdProperty)
throw new ArgumentException("obj does not have Identifier.");
Object propValue = IdProperty.GetValue(entity, null)
}
}

A bit late but here is something I did for enums (could be any object also) and getting the description attribute value using an extension (this could be a generic for any attribute):
public enum TransactionTypeEnum
{
[Description("Text here!")]
DROP = 1,
[Description("More text here!")]
PICKUP = 2,
...
}
Getting the value:
var code = TransactionTypeEnum.DROP.ToCode();
Extension supporting all my enums:
public static string ToCode(this TransactionTypeEnum val)
{
return GetCode(val);
}
public static string ToCode(this DockStatusEnum val)
{
return GetCode(val);
}
public static string ToCode(this TrailerStatusEnum val)
{
return GetCode(val);
}
public static string ToCode(this DockTrailerStatusEnum val)
{
return GetCode(val);
}
public static string ToCode(this EncodingType val)
{
return GetCode(val);
}
private static string GetCode(object val)
{
var attributes = (DescriptionAttribute[])val.GetType().GetField(val.ToString()).GetCustomAttributes(typeof(DescriptionAttribute), false);
return attributes.Length > 0 ? attributes[0].Description : string.Empty;
}

Here is a more real-word example. We use an extension method and check if a property contains a FieldMetaDataAttribute (a custom attribute in my source code base)
with valid Major and MinorVersion. What is of general interest is the part where we use the parent class type and GetProperties and retrieve the ProperyInfo and then use GetCustomAttribute to retrieve a attribute FieldMetaDataAttribute in this special case. Use this code for inspiration how to do more generic way of retrieving a custom attribute. This can of course be polished to make a general method to retrieve a given attribute of any property of a class instance.
/// <summary>
/// Executes the action if not the field is deprecated
/// </summary>
/// <typeparam name="TProperty"></typeparam>
/// <typeparam name="TForm"></typeparam>
/// <param name="form"></param>
/// <param name="memberExpression"></param>
/// <param name="actionToPerform"></param>
/// <returns>True if the action was performed</returns>
public static bool ExecuteActionIfNotDeprecated<TForm, TProperty>(this TForm form, Expression<Func<TForm, TProperty>> memberExpression, Action actionToPerform)
{
var memberExpressionConverted = memberExpression.Body as MemberExpression;
if (memberExpressionConverted == null)
return false;
string memberName = memberExpressionConverted.Member.Name;
PropertyInfo matchingProperty = typeof(TForm).GetProperties(BindingFlags.Public | BindingFlags.Instance)
.FirstOrDefault(p => p.Name == memberName);
if (matchingProperty == null)
return false; //should not occur
var fieldMeta = matchingProperty.GetCustomAttribute(typeof(FieldMetadataAttribute), true) as FieldMetadataAttribute;
if (fieldMeta == null)
{
actionToPerform();
return true;
}
var formConverted = form as FormDataContract;
if (formConverted == null)
return false;
if (fieldMeta.DeprecatedFromMajorVersion > 0 && formConverted.MajorVersion > fieldMeta.DeprecatedFromMajorVersion)
{
//major version of formConverted is deprecated for this field - do not execute action
return false;
}
if (fieldMeta.DeprecatedFromMinorVersion > 0 && fieldMeta.DeprecatedFromMajorVersion > 0
&& formConverted.MinorVersion >= fieldMeta.DeprecatedFromMinorVersion
&& formConverted.MajorVersion >= fieldMeta.DeprecatedFromMajorVersion)
return false; //the field is expired - do not invoke action
actionToPerform();
return true;
}

What are your favorite extension methods for C#? (codeplex.com/extensionoverflow)

Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
Let's make a list of answers where you post your excellent and favorite extension methods.
The requirement is that the full code must be posted and a example and an explanation on how to use it.
Based on the high interest in this topic I have setup an Open Source Project called extensionoverflow on Codeplex.
Please mark your answers with an acceptance to put the code in the Codeplex project.
Please post the full sourcecode and not a link.
Codeplex News:
24.08.2010 The Codeplex page is now here: http://extensionoverflow.codeplex.com/
11.11.2008 XmlSerialize / XmlDeserialize is now Implemented and Unit Tested.
11.11.2008 There is still room for more developers. ;-) Join NOW!
11.11.2008 Third contributer joined ExtensionOverflow, welcome to BKristensen
11.11.2008 FormatWith is now Implemented and Unit Tested.
09.11.2008 Second contributer joined ExtensionOverflow. welcome to chakrit.
09.11.2008 We need more developers. ;-)
09.11.2008 ThrowIfArgumentIsNull in now Implemented and Unit Tested on Codeplex.

public static bool In<T>(this T source, params T[] list)
{
if(null==source) throw new ArgumentNullException("source");
return list.Contains(source);
}
Allows me to replace:
if(reallyLongIntegerVariableName == 1 ||
reallyLongIntegerVariableName == 6 ||
reallyLongIntegerVariableName == 9 ||
reallyLongIntegerVariableName == 11)
{
// do something....
}
and
if(reallyLongStringVariableName == "string1" ||
reallyLongStringVariableName == "string2" ||
reallyLongStringVariableName == "string3")
{
// do something....
}
and
if(reallyLongMethodParameterName == SomeEnum.Value1 ||
reallyLongMethodParameterName == SomeEnum.Value2 ||
reallyLongMethodParameterName == SomeEnum.Value3 ||
reallyLongMethodParameterName == SomeEnum.Value4)
{
// do something....
}
With:
if(reallyLongIntegerVariableName.In(1,6,9,11))
{
// do something....
}
and
if(reallyLongStringVariableName.In("string1","string2","string3"))
{
// do something....
}
and
if(reallyLongMethodParameterName.In(SomeEnum.Value1, SomeEnum.Value2, SomeEnum.Value3, SomeEnum.Value4)
{
// do something....
}

I have various extension methods in my MiscUtil project (full source is available there - I'm not going to repeat it here). My favourites, some of which involve other classes (such as ranges):
Date and time stuff - mostly for unit tests. Not sure I'd use them in production :)
var birthday = 19.June(1976);
var workingDay = 7.Hours() + 30.Minutes();
Ranges and stepping - massive thanks to Marc Gravell for his operator stuff to make this possible:
var evenNaturals = 2.To(int.MaxValue).Step(2);
var daysSinceBirth = birthday.To(DateTime.Today).Step(1.Days());
Comparisons:
var myComparer = ProjectionComparer.Create(Person p => p.Name);
var next = myComparer.ThenBy(p => p.Age);
var reversed = myComparer.Reverse();
Argument checking:
x.ThrowIfNull("x");
LINQ to XML applied to anonymous types (or other types with appropriate properties):
// <Name>Jon</Name><Age>32</Age>
new { Name="Jon", Age=32}.ToXElements();
// Name="Jon" Age="32" (as XAttributes, obviously)
new { Name="Jon", Age=32}.ToXAttributes()
Push LINQ - would take too long to explain here, but search for it.

string.Format shortcut:
public static class StringExtensions
{
// Enable quick and more natural string.Format calls
public static string F(this string s, params object[] args)
{
return string.Format(s, args);
}
}
Example:
var s = "The co-ordinate is ({0}, {1})".F(point.X, point.Y);
For quick copy-and-paste go here.
Don't you find it more natural to type "some string".F("param") instead of string.Format("some string", "param") ?
For a more readable name, try one of these suggestion:
s = "Hello {0} world {1}!".Fmt("Stack", "Overflow");
s = "Hello {0} world {1}!".FormatBy("Stack", "Overflow");
s = "Hello {0} world {1}!".FormatWith("Stack", "Overflow");
s = "Hello {0} world {1}!".Display("Stack", "Overflow");
s = "Hello {0} world {1}!".With("Stack", "Overflow");
..

Are these any use?
public static bool CoinToss(this Random rng)
{
return rng.Next(2) == 0;
}
public static T OneOf<T>(this Random rng, params T[] things)
{
return things[rng.Next(things.Length)];
}
Random rand;
bool luckyDay = rand.CoinToss();
string babyName = rand.OneOf("John", "George", "Radio XBR74 ROCKS!");

public static class ComparableExtensions
{
public static bool Between<T>(this T actual, T lower, T upper) where T : IComparable<T>
{
return actual.CompareTo(lower) >= 0 && actual.CompareTo(upper) < 0;
}
}
Example:
if (myNumber.Between(3,7))
{
// ....
}

The extension method:
public static void AddRange<T, S>(this ICollection<T> list, params S[] values)
where S : T
{
foreach (S value in values)
list.Add(value);
}
The method applies for all types and lets you add a range of items to a list as parameters.
Example:
var list = new List<Int32>();
list.AddRange(5, 4, 8, 4, 2);

By all means put this in the codeplex project.
Serializing / Deserializing objects to XML:
/// <summary>Serializes an object of type T in to an xml string</summary>
/// <typeparam name="T">Any class type</typeparam>
/// <param name="obj">Object to serialize</param>
/// <returns>A string that represents Xml, empty otherwise</returns>
public static string XmlSerialize<T>(this T obj) where T : class, new()
{
if (obj == null) throw new ArgumentNullException("obj");
var serializer = new XmlSerializer(typeof(T));
using (var writer = new StringWriter())
{
serializer.Serialize(writer, obj);
return writer.ToString();
}
}
/// <summary>Deserializes an xml string in to an object of Type T</summary>
/// <typeparam name="T">Any class type</typeparam>
/// <param name="xml">Xml as string to deserialize from</param>
/// <returns>A new object of type T is successful, null if failed</returns>
public static T XmlDeserialize<T>(this string xml) where T : class, new()
{
if (xml == null) throw new ArgumentNullException("xml");
var serializer = new XmlSerializer(typeof(T));
using (var reader = new StringReader(xml))
{
try { return (T)serializer.Deserialize(reader); }
catch { return null; } // Could not be deserialized to this type.
}
}

ForEach for IEnumerables
public static class FrameworkExtensions
{
// a map function
public static void ForEach<T>(this IEnumerable<T> #enum, Action<T> mapFunction)
{
foreach (var item in #enum) mapFunction(item);
}
}
Naive example:
var buttons = GetListOfButtons() as IEnumerable<Button>;
// click all buttons
buttons.ForEach(b => b.Click());
Cool example:
// no need to type the same assignment 3 times, just
// new[] up an array and use foreach + lambda
// everything is properly inferred by csc :-)
new { itemA, itemB, itemC }
.ForEach(item => {
item.Number = 1;
item.Str = "Hello World!";
});
Note:
This is not like Select because Select expects your function to return something as for transforming into another list.
ForEach simply allows you to execute something for each of the items without any transformations/data manipulation.
I made this so I can program in a more functional style and I was surprised that List has a ForEach while IEnumerable does not.
Put this in the codeplex project

My conversion extensions which allow you to do:
int i = myString.To<int>();
Here it is, as posted on TheSoftwareJedi.com
public static T To<T>(this IConvertible obj)
{
return (T)Convert.ChangeType(obj, typeof(T));
}
public static T ToOrDefault<T>
(this IConvertible obj)
{
try
{
return To<T>(obj);
}
catch
{
return default(T);
}
}
public static bool ToOrDefault<T>
(this IConvertible obj,
out T newObj)
{
try
{
newObj = To<T>(obj);
return true;
}
catch
{
newObj = default(T);
return false;
}
}
public static T ToOrOther<T>
(this IConvertible obj,
T other)
{
try
{
return To<T>obj);
}
catch
{
return other;
}
}
public static bool ToOrOther<T>
(this IConvertible obj,
out T newObj,
T other)
{
try
{
newObj = To<T>(obj);
return true;
}
catch
{
newObj = other;
return false;
}
}
public static T ToOrNull<T>
(this IConvertible obj)
where T : class
{
try
{
return To<T>(obj);
}
catch
{
return null;
}
}
public static bool ToOrNull<T>
(this IConvertible obj,
out T newObj)
where T : class
{
try
{
newObj = To<T>(obj);
return true;
}
catch
{
newObj = null;
return false;
}
}
You can ask for default (calls blank constructor or "0" for numerics) on failure, specify a "default" value (I call it "other"), or ask for null (where T : class). I've also provided both silent exception models, and a typical TryParse model that returns a bool indicating the action taken, and an out param holds the new value.
So our code can do things like this
int i = myString.To<int>();
string a = myInt.ToOrDefault<string>();
//note type inference
DateTime d = myString.ToOrOther(DateTime.MAX_VALUE);
double d;
//note type inference
bool didItGiveDefault = myString.ToOrDefault(out d);
string s = myDateTime.ToOrNull<string>();
I couldn't get Nullable types to roll into the whole thing very cleanly. I tried for about 20 minutes before I threw in the towel.

I have an extension method for logging exceptions:
public static void Log(this Exception obj)
{
//your logging logic here
}
And it is used like this:
try
{
//Your stuff here
}
catch(Exception ex)
{
ex.Log();
}
[sorry for posting twice; the 2nd one is better designed :-)]

public static class StringExtensions {
/// <summary>
/// Parses a string into an Enum
/// </summary>
/// <typeparam name="T">The type of the Enum</typeparam>
/// <param name="value">String value to parse</param>
/// <returns>The Enum corresponding to the stringExtensions</returns>
public static T EnumParse<T>(this string value) {
return StringExtensions.EnumParse<T>(value, false);
}
public static T EnumParse<T>(this string value, bool ignorecase) {
if (value == null) {
throw new ArgumentNullException("value");
}
value = value.Trim();
if (value.Length == 0) {
throw new ArgumentException("Must specify valid information for parsing in the string.", "value");
}
Type t = typeof(T);
if (!t.IsEnum) {
throw new ArgumentException("Type provided must be an Enum.", "T");
}
return (T)Enum.Parse(t, value, ignorecase);
}
}
Useful to parse a string into an Enum.
public enum TestEnum
{
Bar,
Test
}
public class Test
{
public void Test()
{
TestEnum foo = "Test".EnumParse<TestEnum>();
}
}
Credit goes to Scott Dorman
--- Edit for Codeplex project ---
I have asked Scott Dorman if he would mind us publishing his code in the Codeplex project. This is the reply I got from him:
Thanks for the heads-up on both the SO post and the CodePlex project. I have upvoted your answer on the question. Yes, the code is effectively in the public domain currently under the CodeProject Open License (http://www.codeproject.com/info/cpol10.aspx).
I have no problems with this being included in the CodePlex project, and if you want to add me to the project (username is sdorman) I will add that method plus some additional enum helper methods.

I find this one pretty useful:
public static class PaulaBean
{
private static String paula = "Brillant";
public static String GetPaula<T>(this T obj) {
return paula;
}
}
You may use it on CodePlex.

DateTimeExtensions
Examples:
DateTime firstDayOfMonth = DateTime.Now.First();
DateTime lastdayOfMonth = DateTime.Now.Last();
DateTime lastFridayInMonth = DateTime.Now.Last(DayOfWeek.Friday);
DateTime nextFriday = DateTime.Now.Next(DayOfWeek.Friday);
DateTime lunchTime = DateTime.Now.SetTime(11, 30);
DateTime noonOnFriday = DateTime.Now.Next(DayOfWeek.Friday).Noon();
DateTime secondMondayOfMonth = DateTime.Now.First(DayOfWeek.Monday).Next(DayOfWeek.Monday).Midnight();

gitorious.org/cadenza is a full library of some of the most useful extension methods I've seen.

Here is one I use frequently for presentation formatting.
public static string ToTitleCase(this string mText)
{
if (mText == null) return mText;
System.Globalization.CultureInfo cultureInfo = System.Threading.Thread.CurrentThread.CurrentCulture;
System.Globalization.TextInfo textInfo = cultureInfo.TextInfo;
// TextInfo.ToTitleCase only operates on the string if is all lower case, otherwise it returns the string unchanged.
return textInfo.ToTitleCase(mText.ToLower());
}

Here's a to-and-from for Roman numerals. Not often used, but could be handy. Usage:
if ("IV".IsValidRomanNumeral())
{
// Do useful stuff with the number 4.
}
Console.WriteLine("MMMDCCCLXXXVIII".ParseRomanNumeral());
Console.WriteLine(3888.ToRomanNumeralString());
The source:
public static class RomanNumeralExtensions
{
private const int NumberOfRomanNumeralMaps = 13;
private static readonly Dictionary<string, int> romanNumerals =
new Dictionary<string, int>(NumberOfRomanNumeralMaps)
{
{ "M", 1000 },
{ "CM", 900 },
{ "D", 500 },
{ "CD", 400 },
{ "C", 100 },
{ "XC", 90 },
{ "L", 50 },
{ "XL", 40 },
{ "X", 10 },
{ "IX", 9 },
{ "V", 5 },
{ "IV", 4 },
{ "I", 1 }
};
private static readonly Regex validRomanNumeral = new Regex(
"^(?i:(?=[MDCLXVI])((M{0,3})((C[DM])|(D?C{0,3}))"
+ "?((X[LC])|(L?XX{0,2})|L)?((I[VX])|(V?(II{0,2}))|V)?))$",
RegexOptions.Compiled);
public static bool IsValidRomanNumeral(this string value)
{
return validRomanNumeral.IsMatch(value);
}
public static int ParseRomanNumeral(this string value)
{
if (value == null)
{
throw new ArgumentNullException("value");
}
value = value.ToUpperInvariant().Trim();
var length = value.Length;
if ((length == 0) || !value.IsValidRomanNumeral())
{
throw new ArgumentException("Empty or invalid Roman numeral string.", "value");
}
var total = 0;
var i = length;
while (i > 0)
{
var digit = romanNumerals[value[--i].ToString()];
if (i > 0)
{
var previousDigit = romanNumerals[value[i - 1].ToString()];
if (previousDigit < digit)
{
digit -= previousDigit;
i--;
}
}
total += digit;
}
return total;
}
public static string ToRomanNumeralString(this int value)
{
const int MinValue = 1;
const int MaxValue = 3999;
if ((value < MinValue) || (value > MaxValue))
{
throw new ArgumentOutOfRangeException("value", value, "Argument out of Roman numeral range.");
}
const int MaxRomanNumeralLength = 15;
var sb = new StringBuilder(MaxRomanNumeralLength);
foreach (var pair in romanNumerals)
{
while (value / pair.Value > 0)
{
sb.Append(pair.Key);
value -= pair.Value;
}
}
return sb.ToString();
}
}

A convenient way to deal with sizes:
public static class Extensions {
public static int K(this int value) {
return value * 1024;
}
public static int M(this int value) {
return value * 1024 * 1024;
}
}
public class Program {
public void Main() {
WSHttpContextBinding serviceMultipleTokenBinding = new WSHttpContextBinding() {
MaxBufferPoolSize = 2.M(), // instead of 2097152
MaxReceivedMessageSize = 64.K(), // instead of 65536
};
}
}

For Winform Controls:
/// <summary>
/// Returns whether the function is being executed during design time in Visual Studio.
/// </summary>
public static bool IsDesignTime(this Control control)
{
if (LicenseManager.UsageMode == LicenseUsageMode.Designtime)
{
return true;
}
if (control.Site != null && control.Site.DesignMode)
{
return true;
}
var parent = control.Parent;
while (parent != null)
{
if (parent.Site != null && parent.Site.DesignMode)
{
return true;
}
parent = parent.Parent;
}
return false;
}
/// <summary>
/// Sets the DropDownWidth to ensure that no item's text is cut off.
/// </summary>
public static void SetDropDownWidth(this ComboBox comboBox)
{
var g = comboBox.CreateGraphics();
var font = comboBox.Font;
float maxWidth = 0;
foreach (var item in comboBox.Items)
{
maxWidth = Math.Max(maxWidth, g.MeasureString(item.ToString(), font).Width);
}
if (comboBox.Items.Count > comboBox.MaxDropDownItems)
{
maxWidth += SystemInformation.VerticalScrollBarWidth;
}
comboBox.DropDownWidth = Math.Max(comboBox.Width, Convert.ToInt32(maxWidth));
}
IsDesignTime Usage:
public class SomeForm : Form
{
public SomeForm()
{
InitializeComponent();
if (this.IsDesignTime())
{
return;
}
// Do something that makes the visual studio crash or hang if we're in design time,
// but any other time executes just fine
}
}
SetDropdownWidth Usage:
ComboBox cbo = new ComboBox { Width = 50 };
cbo.Items.Add("Short");
cbo.Items.Add("A little longer");
cbo.Items.Add("Holy cow, this is a really, really long item. How in the world will it fit?");
cbo.SetDropDownWidth();
I forgot to mention, feel free to use these on Codeplex...

The ThrowIfArgumentIsNull is a nice way to do that null check we all should do.
public static class Extensions
{
public static void ThrowIfArgumentIsNull<T>(this T obj, string parameterName) where T : class
{
if (obj == null) throw new ArgumentNullException(parameterName + " not allowed to be null");
}
}
Below is the way to use it and it works on all classes in your namespace or wherever you use the namespace its within.
internal class Test
{
public Test(string input1)
{
input1.ThrowIfArgumentIsNull("input1");
}
}
It's ok to use this code on the CodePlex project.

I miss the Visual Basic's With statement when moving to C#, so here it goes:
public static void With<T>(this T obj, Action<T> act) { act(obj); }
And here's how to use it in C#:
someVeryVeryLonggggVariableName.With(x => {
x.Int = 123;
x.Str = "Hello";
x.Str2 = " World!";
});
Saves a lot of typing!
Compare this to:
someVeryVeryLonggggVariableName.Int = 123;
someVeryVeryLonggggVariableName.Str = "Hello";
someVeryVeryLonggggVariableName.Str2 = " World!";
put in codeplex project

Takes a camelCaseWord or PascalCaseWord and "wordifies" it, ie camelCaseWord => camel Case Word
public static string Wordify( this string camelCaseWord )
{
// if the word is all upper, just return it
if( !Regex.IsMatch( camelCaseWord, "[a-z]" ) )
return camelCaseWord;
return string.Join( " ", Regex.Split( camelCaseWord, #"(?<!^)(?=[A-Z])" ) );
}
I often use it in conjuction with Capitalize
public static string Capitalize( this string word )
{
return word[0].ToString( ).ToUpper( ) + word.Substring( 1 );
}
Example usage
SomeEntityObject entity = DataAccessObject.GetSomeEntityObject( id );
List<PropertyInfo> properties = entity.GetType().GetPublicNonCollectionProperties( );
// wordify the property names to act as column headers for an html table or something
List<string> columns = properties.Select( p => p.Name.Capitalize( ).Wordify( ) ).ToList( );
Free to use in codeplex project

I found this one helpful
public static IEnumerable<T> EmptyIfNull<T>(this IEnumerable<T> pSeq)
{
return pSeq ?? Enumerable.Empty<T>();
}
It removes the null check in the calling code. You could now do
MyList.EmptyIfNull().Where(....)

Convert a double to string formatted using the specified culture:
public static class ExtensionMethods
{
public static string ToCurrency(this double value, string cultureName)
{
CultureInfo currentCulture = new CultureInfo(cultureName);
return (string.Format(currentCulture, "{0:C}", value));
}
}
Example:
double test = 154.20;
string testString = test.ToCurrency("en-US"); // $154.20

Below is an extension method that adapts Rick Strahl's code (and the comments too) to stop you having to guess or read the byte order mark of a byte array or text file each time you convert it to a string.
The snippet allows you to simply do:
byte[] buffer = File.ReadAllBytes(#"C:\file.txt");
string content = buffer.GetString();
If you find any bugs please add to the comments. Feel free to include it in the Codeplex project.
public static class Extensions
{
/// <summary>
/// Converts a byte array to a string, using its byte order mark to convert it to the right encoding.
/// Original article: http://www.west-wind.com/WebLog/posts/197245.aspx
/// </summary>
/// <param name="buffer">An array of bytes to convert</param>
/// <returns>The byte as a string.</returns>
public static string GetString(this byte[] buffer)
{
if (buffer == null || buffer.Length == 0)
return "";
// Ansi as default
Encoding encoding = Encoding.Default;
/*
EF BB BF UTF-8
FF FE UTF-16 little endian
FE FF UTF-16 big endian
FF FE 00 00 UTF-32, little endian
00 00 FE FF UTF-32, big-endian
*/
if (buffer[0] == 0xef && buffer[1] == 0xbb && buffer[2] == 0xbf)
encoding = Encoding.UTF8;
else if (buffer[0] == 0xfe && buffer[1] == 0xff)
encoding = Encoding.Unicode;
else if (buffer[0] == 0xfe && buffer[1] == 0xff)
encoding = Encoding.BigEndianUnicode; // utf-16be
else if (buffer[0] == 0 && buffer[1] == 0 && buffer[2] == 0xfe && buffer[3] == 0xff)
encoding = Encoding.UTF32;
else if (buffer[0] == 0x2b && buffer[1] == 0x2f && buffer[2] == 0x76)
encoding = Encoding.UTF7;
using (MemoryStream stream = new MemoryStream())
{
stream.Write(buffer, 0, buffer.Length);
stream.Seek(0, SeekOrigin.Begin);
using (StreamReader reader = new StreamReader(stream, encoding))
{
return reader.ReadToEnd();
}
}
}
}

Here's one I just created today.
// requires .NET 4
public static TReturn NullOr<TIn, TReturn>(this TIn obj, Func<TIn, TReturn> func,
TReturn elseValue = default(TReturn)) where TIn : class
{ return obj != null ? func(obj) : elseValue; }
// versions for CLR 2, which doesn't support optional params
public static TReturn NullOr<TIn, TReturn>(this TIn obj, Func<TIn, TReturn> func,
TReturn elseValue) where TIn : class
{ return obj != null ? func(obj) : elseValue; }
public static TReturn NullOr<TIn, TReturn>(this TIn obj, Func<TIn, TReturn> func)
where TIn : class
{ return obj != null ? func(obj) : default(TReturn); }
It lets you do this:
var lname = thingy.NullOr(t => t.Name).NullOr(n => n.ToLower());
which is more fluent and (IMO) easier to read than this:
var lname = (thingy != null ? thingy.Name : null) != null
? thingy.Name.ToLower() : null;

"Please mark your answers with an acceptance to put the code in the Codeplex project."
Why? All the Stuff on this site under CC-by-sa-2.5, so just put your Extension overflow Project under the same license and you can freely use it.
Anyway, here is a String.Reverse function, based on this question.
/// <summary>
/// Reverse a String
/// </summary>
/// <param name="input">The string to Reverse</param>
/// <returns>The reversed String</returns>
public static string Reverse(this string input)
{
char[] array = input.ToCharArray();
Array.Reverse(array);
return new string(array);
}

I got tired of tedious null-checking while pulling values from MySqlDataReader, so:
public static DateTime? GetNullableDateTime(this MySqlDataReader dr, string fieldName)
{
DateTime? nullDate = null;
return dr.IsDBNull(dr.GetOrdinal(fieldName)) ? nullDate : dr.GetDateTime(fieldName);
}
public static string GetNullableString(this MySqlDataReader dr, string fieldName)
{
return dr.IsDBNull(dr.GetOrdinal(fieldName)) ? String.Empty : dr.GetString(fieldName);
}
public static char? GetNullableChar(this MySqlDataReader dr, string fieldName)
{
char? nullChar = null;
return dr.IsDBNull(dr.GetOrdinal(fieldName)) ? nullChar : dr.GetChar(fieldName);
}
Of course this could be used with any SqlDataReader.
Both hangy and Joe had some good comments on how to do this, and I have since had an opportunity to implement something similar in a different context, so here is another version:
public static int? GetNullableInt32(this IDataRecord dr, int ordinal)
{
int? nullInt = null;
return dr.IsDBNull(ordinal) ? nullInt : dr.GetInt32(ordinal);
}
public static int? GetNullableInt32(this IDataRecord dr, string fieldname)
{
int ordinal = dr.GetOrdinal(fieldname);
return dr.GetNullableInt32(ordinal);
}
public static bool? GetNullableBoolean(this IDataRecord dr, int ordinal)
{
bool? nullBool = null;
return dr.IsDBNull(ordinal) ? nullBool : dr.GetBoolean(ordinal);
}
public static bool? GetNullableBoolean(this IDataRecord dr, string fieldname)
{
int ordinal = dr.GetOrdinal(fieldname);
return dr.GetNullableBoolean(ordinal);
}

It irritated me that LINQ gives me an OrderBy that takes a class implementing IComparer as an argument, but does not support passing in a simple anonymous comparer function. I rectified that.
This class creates an IComparer from your comparer function...
/// <summary>
/// Creates an <see cref="IComparer{T}"/> instance for the given
/// delegate function.
/// </summary>
internal class ComparerFactory<T> : IComparer<T>
{
public static IComparer<T> Create(Func<T, T, int> comparison)
{
return new ComparerFactory<T>(comparison);
}
private readonly Func<T, T, int> _comparison;
private ComparerFactory(Func<T, T, int> comparison)
{
_comparison = comparison;
}
#region IComparer<T> Members
public int Compare(T x, T y)
{
return _comparison(x, y);
}
#endregion
}
...and these extension methods expose my new OrderBy overloads on enumerables. I doubt this works for LINQ to SQL, but it's great for LINQ to Objects.
public static class EnumerableExtensions
{
/// <summary>
/// Sorts the elements of a sequence in ascending order by using a specified comparison delegate.
/// </summary>
public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector,
Func<TKey, TKey, int> comparison)
{
var comparer = ComparerFactory<TKey>.Create(comparison);
return source.OrderBy(keySelector, comparer);
}
/// <summary>
/// Sorts the elements of a sequence in descending order by using a specified comparison delegate.
/// </summary>
public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector,
Func<TKey, TKey, int> comparison)
{
var comparer = ComparerFactory<TKey>.Create(comparison);
return source.OrderByDescending(keySelector, comparer);
}
}
You're welcome to put this on codeplex if you like.

This one is for MVC it adds the ability to generate a <label /> tag to the Html variable that is available in every ViewPage. Hopefully it will be of use to others trying to develop similar extensions.
Use:
<%= Html.Label("LabelId", "ForId", "Text")%>
Output:
<label id="LabelId" for="ForId">Text</label>
Code:
public static class HtmlHelperExtensions
{
public static string Label(this HtmlHelper Html, string #for, string text)
{
return Html.Label(null, #for, text);
}
public static string Label(this HtmlHelper Html, string #for, string text, object htmlAttributes)
{
return Html.Label(null, #for, text, htmlAttributes);
}
public static string Label(this HtmlHelper Html, string #for, string text, IDictionary<string, object> htmlAttributes)
{
return Html.Label(null, #for, text, htmlAttributes);
}
public static string Label(this HtmlHelper Html, string id, string #for, string text)
{
return Html.Label(id, #for, text, null);
}
public static string Label(this HtmlHelper Html, string id, string #for, string text, object htmlAttributes)
{
return Html.Label(id, #for, text, new RouteValueDictionary(htmlAttributes));
}
public static string Label(this HtmlHelper Html, string id, string #for, string text, IDictionary<string, object> htmlAttributes)
{
TagBuilder tag = new TagBuilder("label");
tag.MergeAttributes(htmlAttributes);
if (!string.IsNullOrEmpty(id))
tag.MergeAttribute("id", Html.AttributeEncode(id));
tag.MergeAttribute("for", Html.AttributeEncode(#for));
tag.SetInnerText(Html.Encode(text));
return tag.ToString(TagRenderMode.Normal);
}
}

Turn this:
DbCommand command = connection.CreateCommand();
command.CommandText = "SELECT #param";
DbParameter param = command.CreateParameter();
param.ParameterName = "#param";
param.Value = "Hello World";
command.Parameters.Add(param);
... into this:
DbCommand command = connection.CreateCommand("SELECT {0}", "Hello World");
... using this extension method:
using System;
using System.Data.Common;
using System.Globalization;
using System.Reflection;
namespace DbExtensions {
public static class Db {
static readonly Func<DbConnection, DbProviderFactory> getDbProviderFactory;
static readonly Func<DbCommandBuilder, int, string> getParameterName;
static readonly Func<DbCommandBuilder, int, string> getParameterPlaceholder;
static Db() {
getDbProviderFactory = (Func<DbConnection, DbProviderFactory>)Delegate.CreateDelegate(typeof(Func<DbConnection, DbProviderFactory>), typeof(DbConnection).GetProperty("DbProviderFactory", BindingFlags.Instance | BindingFlags.NonPublic).GetGetMethod(true));
getParameterName = (Func<DbCommandBuilder, int, string>)Delegate.CreateDelegate(typeof(Func<DbCommandBuilder, int, string>), typeof(DbCommandBuilder).GetMethod("GetParameterName", BindingFlags.Instance | BindingFlags.NonPublic, Type.DefaultBinder, new Type[] { typeof(Int32) }, null));
getParameterPlaceholder = (Func<DbCommandBuilder, int, string>)Delegate.CreateDelegate(typeof(Func<DbCommandBuilder, int, string>), typeof(DbCommandBuilder).GetMethod("GetParameterPlaceholder", BindingFlags.Instance | BindingFlags.NonPublic, Type.DefaultBinder, new Type[] { typeof(Int32) }, null));
}
public static DbProviderFactory GetProviderFactory(this DbConnection connection) {
return getDbProviderFactory(connection);
}
public static DbCommand CreateCommand(this DbConnection connection, string commandText, params object[] parameters) {
if (connection == null) throw new ArgumentNullException("connection");
return CreateCommandImpl(GetProviderFactory(connection).CreateCommandBuilder(), connection.CreateCommand(), commandText, parameters);
}
private static DbCommand CreateCommandImpl(DbCommandBuilder commandBuilder, DbCommand command, string commandText, params object[] parameters) {
if (commandBuilder == null) throw new ArgumentNullException("commandBuilder");
if (command == null) throw new ArgumentNullException("command");
if (commandText == null) throw new ArgumentNullException("commandText");
if (parameters == null || parameters.Length == 0) {
command.CommandText = commandText;
return command;
}
object[] paramPlaceholders = new object[parameters.Length];
for (int i = 0; i < paramPlaceholders.Length; i++) {
DbParameter dbParam = command.CreateParameter();
dbParam.ParameterName = getParameterName(commandBuilder, i);
dbParam.Value = parameters[i] ?? DBNull.Value;
command.Parameters.Add(dbParam);
paramPlaceholders[i] = getParameterPlaceholder(commandBuilder, i);
}
command.CommandText = String.Format(CultureInfo.InvariantCulture, commandText, paramPlaceholders);
return command;
}
}
}
More ADO.NET extension methods: DbExtensions