Develop Reference

Method with Multiple Return Types

I've looked through many questions that are similar to this, but none of them really touched on what I precisely want to do. What I am trying to do is read from an external source a list of variables that also include their data type into a string array:
Example:
ID/Key Type Value/Data;
varName1 bool true;
varName2 string str;
varName3 int 5;
I then store these are objects into a dictionary as objects containing several strings, with the ID also serving as the key.
What I want to do is now create a method that uses a switch statement that casts the string into the correct datatype, and returns it without having to specify anything in the method call. The function should look something like this:
public ??? Method(string key)
{
if(dictionary.ContainsKey(ID))
{
Var temp = dictionary[ID];
switch (temp.Type)
{
case "bool":
return Convert.ToBoolean(temp.Value);
case "int"
return Convert.ToInt(temp.Value);
case "string"
return temp.Value;
}
}
return "NULL";
}
The method call should look something like this:
int x = Method(string key);
string word = Method(string key);
bool isTrue = Method(string key);
Maybe I've missed something, but I have yet to find something that really does something quite like this. Any and all thoughts about this are welcome as well.

In C# 7 you have the option to return multiple values from a method like this:
public (string SomeString, int SomeInt) DoSomething() { ... }
You can get the values like this:
var result = DoSomething();
Console.WriteLine(result.SomeString);
Console.WriteLine(result.SomeInt.ToString());
Or
(var someString, var someInt) = DoSomething();
Console.WriteLine(someString);
Console.WriteLine(someInt.ToString());
This works below the surface with a Tuple and you are not restricted to only 2 values. I don't know how many you can return but I suggest when you need to return that many values, create a class.
More info: https://blogs.msdn.microsoft.com/dotnet/2016/08/24/whats-new-in-csharp-7-0/

Update
I believe a lot of people are arriving at this question because they're looking for ways to return multiple values generally, not necessarily for the purposes given in the original question. If this is what you want, there are a few options to choose from.
If the combination of your returned types represents a concept that may be useful outside of your method call, consider creating a type to represent that concept. C#'s records provide a nice, concise way to do that:
public record ExtractedValue(bool? BooleanValue, string? StringValue, int? IntValue);
public ExtractedValue Method(string key)
{
...
}
If this is the only place these values will appear together, and it's not really worth coming up with a named type to represent the values, you can also use a Value Tuple. Just be aware that there are some behavioral implications that might bite you if you plan to use the type for things like serialization.
public (bool? BooleanValue, string? StringValue, int? IntValue) Method(string key)
{
...
}
Original Answer
The compiler has no way to distinguish between the three method calls you've provided, because they all look like Method(key);
One option is to return an object and then expect the consuming code to cast it to what they want:
public object Method(string key)
{
if(dictionary.ContainsKey(key))
{
var temp = dictionary[key];
switch (temp.Type)
{
case "bool":
return Convert.ToBoolean(temp.Value);
case "int"
return Convert.ToInt(temp.Value);
case "string"
return temp.Value;
}
}
return "NULL";
}
...
int x = (int) Method(key);
string word = (string) Method(key);
bool isTrue = (bool) Method(key);
You could also use the dynamic keyword to make the cast implicit:
public dynamic Method(string key)
{
if(dictionary.ContainsKey(key))
{
var temp = dictionary[key];
switch (temp.Type)
{
case "bool":
return Convert.ToBoolean(temp.Value);
case "int"
return Convert.ToInt(temp.Value);
case "string"
return temp.Value;
}
}
return "NULL";
}
...
int x = Method(key);
string word = Method(key);
bool isTrue = Method(key);
However, dynamic is a very powerful concept, and it's easy for it to get out of hand, so you have to be really careful with that.
It seems to me that you're expecting your calling code to know which type of object it's expecting to get for each key. It seems like maybe the best approach is to just let the user supply that information:
public T Method<T>(string key)
{
if(dictionary.ContainsKey(key))
return (T) Convert.ChangeType(dictionary[key].Value, typeof(T));
return default(T);
}
...
int x = Method<int>(key);
string word = Method<string>(key);
bool isTrue = Method<bool>(key);
That way, there's no need to track the Type value in your dictionary objects in the first place.

The return type of a function must be typed. As with any other variable or operation, any type that inherits from the specified type is a valid return value (which is why object allows anything as a value).
Personally i dont think it is useful to make one method with multiple return types but if you really want to have one method with multiple return types, you could use the dynamic type in .NET 4.0:
private static void Main(string[] args)
{
int x = Method("varName3");
string word = Method("varName2");
bool isTrue = Method("varName1");
}
private static dynamic Method(string key)
{
var dictionary = new Dictionary<string, KeyValuePair<Type, object>>()
{
{ "varName1", new KeyValuePair<Type, object>(typeof(bool), false) },
{ "varName2", new KeyValuePair<Type, object>(typeof(string), "str") },
{ "varName3", new KeyValuePair<Type, object>(typeof(int), 5) },
};
if (dictionary.ContainsKey(key))
{
return dictionary[key].Value;
}
return null;
}
Hope it helps

How to restrict a variable to a fixed set of strings?

If I want to restrict the values of the spicelevel column in the database to 1, 2 and 3, I could do something like
private enum SpiceLevel
{
Low=1,
Medium=2,
Hot=3
}
Then in the code I could do (int)SpiceLevel.Low to pick 1 as the spice level.
Now what if I have a need where I can only accept "Red Rose","White Rose" and "Black Rose" for the values of a column in the database? What is a graceful way to handle this?
I am thinking of storing them in a config file or constants, but neither is as graceful as enums. Any ideas?
Update:
The answer here worked for me

You can use a property for this
public string[] AllowedRoses = new string[] { "Red Rose", "White Rose" ,"Black Rose" };
string _Rose = "Red Rose";
public string Rose
{
get
{
return _Rose;
}
set
{
if (!AllowedRoses.Any(x => x == value))
throw new ArgumentException("Not valid rose");
_Rose = value;
}
}

I can see the following options:
verify the value in the setter (see for example l4V's answer)
conceptually, you're thinking about an enum. So you could do the following:
enum RoseType { RedRose, WhiteRose, BlackRose };
and then provide appropriate conversion from this enum to string. Two convenient options how to do it are described here: Enum ToString with user friendly strings. One is to use a custom Description attribute, and the second (I'd prefer this one) to provide an extension method:
public static class RoseTypeExtensions
{
public static string GetString(this RoseType #this)
{
switch (#this)
{
case RoseType.RedRose:
return "Red Rose";
case RoseType.WhiteRose:
return "White Rose";
case RoseType.BlackRose:
return "Black Rose";
default:
throw new InvalidOperationException();
}
}
}
create a set of constants:
public class RoseType
{
public readonly RoseType RedRose = new RoseType("Red Rose");
public readonly RoseType WhiteRose = new RoseType("White Rose");
public readonly RoseType BlackRose = new RoseType("Black Rose");
public string Content { get; private set; }
private RoseType(string content)
{
this.Content = content;
}
public override string ToString()
{
return this.Content;
}
}
As Oskar Berggren correctly pointed out in the comment, RoseType should also provide other standard overrides beside ToString: Equals, GetHashCode, operator== and operator!=.

There is no really good solution. All of them require the database to be "synchronized" with the enum in C#.
Simplest solution:
What you said store the enum values as integers in the database.
Almost as simple but less efficient:
Store the values as strings in the database and convert between string and enum with anEnumVar.ToString() and Enum.Parse (or any of the other parse methods in Enum).
Complex but flexible:
Have a sort of enum in the database: a table with string values and ids and then use foreign keys to that table where you want to save the enums. This allows you to either select/update/insert using the numeric value or the string value (via a join).
It also maintains integrity as it is not possible to store an integer which has no corresponding enum value.
The downside is the complexity.

Create a mapping of string to enum with Dictionary<string, SpiceLevel> to associate the string to the Enum. Wrap them in a class.
You could also use a Decorator attribute [Name("Red Rose"] Low=1, and get that from the enum itself, but that involves reflection, which has some performance issues, especially when iterating through enum values to find the one with the matching attribute.
public static class Spice
{
public enum Level
{
Low = 1,
Medium = 2,
Hot = 3
}
private static readonly Dictionary<string, Level> spices = new Dictionary<string, Level>{
{ "Red Rose", Level.Low },
{ "White Rose", Level.Medium },
{ "Black Rose", Level.Hot },
};
public static bool TryGet(string spiceName, out Level spiceLevel) => spices.TryGetValue(spiceName, out spiceLevel);
public static string SpiceName(Level target) => Enum.GetName(typeof(Spice.Level), target);
}
/// <summary>
/// Some tests to validate it works. This could be a unit test or just in a console app
/// </summary>
public class SpiceTest
{
public void VerifyBlackRoseIsHot()
{
string subject = "Black Rose";
Spice.Level expectedSpice;
// Here's the ease of use. Pass a string, get an enum and whether it's a valid string
var result = Spice.TryGet(subject, out expectedSpice);
//Some Assertion from a unit test library
Assert.True(result, $"Unable to find spice '{subject}', when it should exist");
Assert.True(Spice.Level.Hot.Equals(expectedSpice), $"The returned spice '{ Spice.SpiceName(expectedSpice) }' was not the value 'Hot' as expected");
}
}

How to represent enum values with space?

I have enums like the following in my DB:
"Random Type", "Random Type1", "NewRandom"
Normally, I would represent the values in an enum like:
enum myTypes
{
Random Type = 0,...
}
but this is not possible, so I tried using a class
static class myTypes
{
public const string RandomType = "Random Type";
public const string NewRandom = "NewRandom";
}
This way, I can use the class like an Enum, but I'm wondering if this is the best implementation possible? Or is there away around creating Enums to allow space?
Thanks.
EDIT:
Please, I would also love to know whether there is anything wrong with my current implementation. I have a feeling my current implementation is better than most suggested solutions here.
Thanks

No, you can't do that. Enums are just typesafe ints.
There is a solution available, and I quite like it. Use the DescriptionAttribute.
You'd use it like this:
static enum myTypes
{
[Description("Random Type")]
RandomType,
[Descripton("New Random")]
NewRandom
}
and then you'd also need this extension method:
public static string GetDescription<T>(this T en) where T : struct, IConvertible
{
Type type = typeof(T);
if (!type.IsEnum)
{
throw new ArgumentException("The type is not an enum");
}
MemberInfo[] memInfo = type.GetMember(en.ToString());
if (memInfo != null && memInfo.Length > 0)
{
object[] attrs = memInfo[0].GetCustomAttributes(typeof(DescriptionAttribute), false);
if (attrs != null && attrs.Length > 0)
{
return ((DescriptionAttribute)attrs[0]).Description;
}
}
return en.ToString();
}
And then with that, you could just do this:
myTypes.RandomType.GetDescription();

Enums are much similar to numbers (Integers specifically), rather than strings or so. Adhering to numbered Enums yields you to easy casting, flags-composition (e.g. AND, OR, etc).
I wouldn't use string constant in place of Enums, unless that will bring you more benefits than penalties.
If your goal is to describe to the user the Enum options, I'd suggest to consider to enrich each item with a Description attribute. It's a metadata, rather a real data, but it's also pretty easy to read using reflection.
Cheers

What I do is I define custom attribute [DisplayName(string)] that can be attached to enum values. You define your enum with display name on the values you wish were named with spaces / special characters:
public enum Test
{
None = 0,
[DisplayName("My Value")]
MyValue = 1,
[DisplayName("Spęćiał")]
Special = 2
}
Your implementation in addition to getting enum value name should also check if DisplayName attribute is set, and if so, it should take display name instead.

I would go with display name attributes:
[AttributeUsage(AttributeTargets.Field)]
public class EnumDisplayNameAttribute : DisplayNameAttribute
{
public EnumDisplayNameAttribute()
: base(string.Empty)
{
}
public EnumDisplayNameAttribute(string displayName)
: base(displayName)
{
}
}
public static class EnumExtensions
{
public static string ToDisplayName(this Enum enumValue)
{
var builder = new StringBuilder();
var fields = GetEnumFields(enumValue);
if (fields[0] != null)
for (int i = 0; i < fields.Length; i++)
{
var value = fields[i]
.GetCustomAttributes(typeof(EnumDisplayNameAttribute), false)
.OfType<EnumDisplayNameAttribute>()
.FirstOrDefault();
builder.Append(value != null
? value.DisplayName
: enumValue.ToString());
if (i != fields.Length - 1)
builder.Append(", ");
}
return builder.ToString();
}
private static FieldInfo[] GetEnumFields(Enum enumValue)
{
var type = enumValue.GetType();
return enumValue
.ToString()
.Split(new[] { ',', ' ' }, StringSplitOptions.RemoveEmptyEntries)
.Select(type.GetField)
.ToArray();
}
}
usage for type:
public enum MyType
{
[DisplayName("Random Type")]
RandomType,
[DisplayName("New Random")]
NewRandom
}
would be:
var enumVariable = MyType.RandomType;
var stringRepresentation = enumVariable.ToDisplayName();
note that with that approach you would get ToString values if you omit attribute for some enum members.

You should probably not use strings as type indicators in your database. Use integers instead. If you like, you can have a "type table" in your database, where you can store the type names, instead of repeating them through the tables that uses them.
If you do this, then you can convert the integers from the database to enums as suggested above.

You can use Typesafe Enum pattern to achieve your goal.
Idea is to wrap your enum around a class. I guess this is what you want -
public class MyTypes
{
#region Enum Values
public static MyTypes RandomType = new MyTypes(0, "Random Type");
public static MyTypes NewRandom = new MyTypes(1, "New Random");
#endregion
#region Private members
private int id;
private string value;
private MyTypes(int id, string value)
{
this.id = id;
this.value = value;
}
#endregion
#region Overriden members
public override string ToString()
{
return value;
}
#endregion
public static List<MyTypes> GetValues()
{
return new List<MyTypes>() { MyTypes.RandomType, MyTypes.NewRandom };
}
}

C# Enum return string with breaks

I need my enum to return a specific string, but I can't work out how to make it return a string with breaks in it without having a method to do the conversion. Is it possible to make LicenseTypes.DISCOUNT_EARLY_ADOPTER return DISCOUNT EARLY-ADOPTER without the helper method?
// All license types
public enum LicenseTypes
{
DISCOUNT,
DISCOUNT_EARLY_ADOPTER,
COMMERCIAL,
COMMERCIAL_EARLY_ADOPTER
}
// Convert enum to correct string
public static string LicenseTypeToString(LicenseTypes Enum)
{
if (Enum == LicenseTypes.COMMERCIAL)
return "COMMERCIAL";
else if (Enum == LicenseTypes.COMMERCIAL_EARLY_ADOPTER)
return "COMMERCIAL EARLY-ADOPTER";
else if (Enum == LicenseTypes.DISCOUNT)
return "DISCOUNT";
else if (Enum == LicenseTypes.DISCOUNT_EARLY_ADOPTER)
return "DISCOUNT EARLY-ADOPTER";
else
return "ERROR";
}

Firstly, a separate option from a helper method is simply to have a Dictionary<LicenseTypes, string> which you populate once. That would probably be the simplest approach, to be honest:
private static readonly Dictionary<LicenseTypes, string> LicenseDesciptions =
new Dictionary<LicenseTypes, string>
{
{ LicenseTypes.COMMERCIAL, "COMMERCIAL" },
{ LicenseTypes.COMMERCIAL_EARLY_ADOPTER, "COMMERCIAL EARLY-ADOPTER" },
{ LicenseTypes.DOMESTIC, "DOMESTIC" },
{ LicenseTypes.DOMESTIC_EARLY_ADOPTER, "DOMESTIC EARLY-ADOPTER" },
};
(As noted in comments, another alternative is a switch/case... but I personally prefer this way, as effectively you've got a data mapping, so it makes sense to use a data structure rather than an execution flow structure. It also means you can swap out dictionaries for different languages etc if you want.)
Secondly, one option would be to decorate each enum value with a [Description] attribute (or your own attribute if you want), and find that out with reflection - Unconstrained Melody has an extension method which can do that very easily:
// Throws ArgumentOutOfRangeException if the licenseType value isn't defined
// or doesn't have a description.
string description = licenseType.GetDescription();
Also, following .NET naming conventions it should be:
public enum LicenseType // Singular as it's not a Flags enum
{
Discount,
DiscountEarlyAdopter,
Commercial,
CommercialEarlyAdopter
}

A little reflection, and Attribute magic, and this should just answer it :)
Getting attributes of Enum's value
Might also make it an extension method.
here's how it should look (also added generics):
public static class MyExtensionsClass
{
public static string ToDescriptionString<T>(this T val)
where T : struct, IConvertible
{
if (typeof(T).IsEnum)
{
var type = val.GetType();
var memInfo = type.GetMember(val.ToString());
var attributes = memInfo[0].GetCustomAttributes(typeof(DescriptionAttribute), false);
return ((DescriptionAttribute)attributes[0]).Description;
}
return ""; //all paths must return a value
}
}
public enum LicenseTypes
{
[Description("DISCOUNT")]
DISCOUNT,
[Description("DISCOUNT EARLY-ADOPTER")]
DISCOUNT_EARLY_ADOPTER,
[Description("COMMERCIAL")]
COMMERCIAL,
[Description("COMMERCIAL EARLY-ADOPTER")]
COMMERCIAL_EARLY_ADOPTER
}
Thank you, guys that wrote these:
Enhance enums using extension methods
Create Generic method constraining T to an Enum
Good luck!

While it doesn't eliminate the helper method, note that in your case you could just special-case the values that you can't get using ToString:
switch (Enum) {
case LicenseTypes.COMMERCIAL_EARLY_ADOPTER:
return "COMMERCIAL EARLY-ADOPTER";
case LicenseTypes.DISCOUNT_EARLY_ADOPTER:
return "DISCOUNT EARLY-ADOPTER";
default
return Enum.ToString();
}

I use this from Google's dotnet client - StringValueAttribute.cs
and Utilities.cs
public enum LicenseType
{
[StringValue("DISCOUNT")] Discount,
[StringValue("DISCOUNT EARLY-ADOPTER")] DiscountEarlyAdopter,
[StringValue("COMMERCIAL")] Commercial,
[StringValue("COMMERCIAL EARLY-ADOPTER")] CommercialEarlyAdopter
}
Then you can simply do this:
licenseType.GetStringValue();

Convert to string using "G" format, then replace "_" (underscore) with spaces:
LicensceTypes license = LicenseTypes.COMMERCIAL_EARLY_ADOPTERS;
string licenseDescription = license.ToString("G").Replace('_', ' ');
// licenseDescription = "COMMERCIAL EARLY ADOPTERS"

I think I would use a class to avoid this scenario. :/
public class LicenceType
{
private string name;
public LicenceType(string Name)
{
this.name = Name;
}
public override string ToString()
{
return name;
}
}
public static class LicenceTypes
{
public static LicenceType DISCOUNT = new LicenceType("DISCOUNT");
public static LicenceType DISCOUNT_EARLY_ADOPTER= new LicenceType("DISCOUNT EARLY-ADOPTER");
public static LicenceType COMMERCIAL= new LicenceType("COMMERCIAL");
public static LicenceType COMMERCIAL_EARLY_ADOPTER= new LicenceType("COMMERCIAL EARLY-ADOPTER");
}

convert an enum to another type of enum

I have an enum of for example 'Gender' (Male =0 , Female =1) and I have another enum from a service which has its own Gender enum (Male =0 , Female =1, Unknown =2)
My question is how can I write something quick and nice to convert from their enum to mine?

Given Enum1 value = ..., then if you mean by name:
Enum2 value2 = (Enum2) Enum.Parse(typeof(Enum2), value.ToString());
If you mean by numeric value, you can usually just cast:
Enum2 value2 = (Enum2)value;
(with the cast, you might want to use Enum.IsDefined to check for valid values, though)

Using an extension method works quite neatly, when using the two conversion methods suggested by Nate:
public static class TheirGenderExtensions
{
public static MyGender ToMyGender(this TheirGender value)
{
// insert switch statement here
}
}
public static class MyGenderExtensions
{
public static TheirGender ToTheirGender(this MyGender value)
{
// insert switch statement here
}
}
Obviously there's no need to use separate classes if you don't want to. My preference is to keep extension methods grouped by the classes/structures/enumerations they apply to.

Just cast one to int and then cast it to the other enum (considering that you want the mapping done based on value):
Gender2 gender2 = (Gender2)((int)gender1);

If we have:
enum Gender
{
M = 0,
F = 1,
U = 2
}
and
enum Gender2
{
Male = 0,
Female = 1,
Unknown = 2
}
We can safely do
var gender = Gender.M;
var gender2 = (Gender2)(int)gender;
Or even
var enumOfGender2Type = (Gender2)0;
If you want to cover the case where an enum on the right side of the '=' sign has more values than the enum on the left side - you will have to write your own method/dictionary to cover that as others suggested.

To be thorough I normally create a pair of functions, one that takes Enum 1 and returns Enum 2 and another that takes Enum 2 and returns Enum 1. Each consists of a case statement mapping inputs to outputs and the default case throws an exception with a message complaining about an unexpected value.
In this particular case you could take advantage of the fact that the integer values of Male and Female are the same, but I'd avoid that as it's hackish and subject to breakage if either enum changes in the future.

You could write a simple generic extension method like this
public static T ConvertTo<T>(this object value)
where T : struct,IConvertible
{
var sourceType = value.GetType();
if (!sourceType.IsEnum)
throw new ArgumentException("Source type is not enum");
if (!typeof(T).IsEnum)
throw new ArgumentException("Destination type is not enum");
return (T)Enum.Parse(typeof(T), value.ToString());
}

I wrote this answer because I believe there are fundamental issues with the majority of answers already provided, and the ones that are acceptable are incomplete.
Mapping by enum integer value
This approach is bad simply because it assumes that the integer values of both MyGender and TheirGender will always remain comparable. In practice, it is very rare that you can guarantee this even within a single project, let alone a separate service.
The approach we take should be something that can be used for other enum-mapping cases. We should never assume that one enum identically relates to another - especially when we may not have control over one or another.
Mapping by enum string value
This is a little better, as MyGender.Male will still convert to TheirGender.Male even if the integer representation is changed, but still not ideal.
I would discourage this approach as it assumes the name values will not change, and will always remain identical. Considering future enhancements, you cannot guarantee that this will be the case; consider if MyGender.NotKnown was added. It is likely that you would want this to map to TheirGender.Unknown, but this would not be supported.
Also, it is generally bad to assume that one enum equates to another by name, as this might not be the case in some contexts. As mentioned earlier, an ideal approach would work for other enum-mapping requirements.
Explicitly mapping enums
This approach explictly maps MyGender to TheirGender using a switch statement.
This is better as:
Covers the case where the underlying integer value changes.
Covers the case where the enum names changes (i.e. no assumptions - the developer will need to update the code to handle the scenario - good).
Handles cases where enum values cannot be mapped.
Handles cases where new enum values are added and cannot be mapped by default (again, no assumptions made - good).
Can easily be updated to support new enum values for either MyGender or TheirGender.
The same approach can be taken for all enum mapping requirements.
Assuming we have the following enums:
public enum MyGender
{
Male = 0,
Female = 1,
}
public enum TheirGender
{
Male = 0,
Female = 1,
Unknown = 2,
}
We can create the following function to "convert from their enum to mine":
public MyGender GetMyGender(TheirGender theirGender)
{
switch (theirGender)
{
case TheirGender.Male:
return MyGender.Male;
case TheirGender.Female:
return MyGender.Female;
default:
throw new InvalidEnumArgumentException(nameof(theirGender), (int)theirGender, typeof(TheirGender));
}
}
A previous answer suggested returning a nullable enum (TheirGender?) and returning null for any unmatched input. This is bad; null is not the same as an unknown mapping. If the input cannot be mapped, an exception should be thrown, else the method should be named more explictly to the behaviour:
public TheirGender? GetTheirGenderOrDefault(MyGender myGender)
{
switch (myGender)
{
case MyGender.Male:
return TheirGender.Male;
case MyGender.Female:
return TheirGender.Female;
default:
return default(TheirGender?);
}
}
Additional considerations
If it is likely that this method will be required more than once in various parts of the solution, you could consider creating an extension method for this:
public static class TheirGenderExtensions
{
public static MyGender GetMyGender(this TheirGender theirGender)
{
switch (theirGender)
{
case TheirGender.Male:
return MyGender.Male;
case TheirGender.Female:
return MyGender.Female;
default:
throw new InvalidEnumArgumentException(nameof(theirGender), (int)theirGender, typeof(TheirGender));
}
}
}
If you are using C#8, you can use the syntax for switch expressions and expression bodies to neaten up the code:
public static class TheirGenderExtensions
{
public static MyGender GetMyGender(this TheirGender theirGender)
=> theirGender switch
{
TheirGender.Male => MyGender.Male,
TheirGender.Female => MyGender.Female,
_ => throw new InvalidEnumArgumentException(nameof(theirGender), (int)theirGender, typeof(TheirGender))
};
}
If you will only ever be mapping the enums within a single class, then an extension method may be overkill. In this case, the method can be declared within the class itself.
Furthermore, if the mapping will only ever take place within a single method, then you can declare this as a local function:
public static void Main()
{
Console.WriteLine(GetMyGender(TheirGender.Male));
Console.WriteLine(GetMyGender(TheirGender.Female));
Console.WriteLine(GetMyGender(TheirGender.Unknown));
static MyGender GetMyGender(TheirGender theirGender)
=> theirGender switch
{
TheirGender.Male => MyGender.Male,
TheirGender.Female => MyGender.Female,
_ => throw new InvalidEnumArgumentException(nameof(theirGender), (int)theirGender, typeof(TheirGender))
};
}
Here's a dotnet fiddle link with the above example.
tl;dr:
Do not:
Map enums by integer value
Map enums by name
Do:
Map enums explicitly using a switch statement
Throw an exception when a value cannot be mapped rather than returning null
Consider using extension methods

you could write a simple function like the following:
public static MyGender ConvertTo(TheirGender theirGender)
{
switch(theirGender)
{
case TheirGender.Male:
break;//return male
case TheirGender.Female:
break;//return female
case TheirGender.Unknown:
break;//return whatever
}
}

Here's an extension method version if anyone is interested
public static TEnum ConvertEnum<TEnum >(this Enum source)
{
return (TEnum)Enum.Parse(typeof(TEnum), source.ToString(), true);
}
// Usage
NewEnumType newEnum = oldEnumVar.ConvertEnum<NewEnumType>();

In case when the enum members have different values, you can apply something like this:
public static MyGender? MapToMyGender(this Gender gender)
{
return gender switch
{
Gender.Male => MyGender.Male,
Gender.Female => MyGender.Female,
Gender.Unknown => null,
_ => throw new InvalidEnumArgumentException($"Invalid gender: {gender}")
};
}
Then you can call: var myGender = gender.MapToMyGender();
Update:
This previous code works only with C# 8.
For older versions of C#, you can use the switch statement instead of the switch expression:
public static MyGender? MapToMyGender(this Gender gender)
{
switch (gender)
{
case Gender.Male:
return MyGender.Male;
case Gender.Female:
return MyGender.Female;
case Gender.Unknown:
return null;
default:
throw new InvalidEnumArgumentException($"Invalid gender: {gender}")
};
}

Based on Justin's answer above I came up with this:
/// <summary>
/// Converts Enum Value to different Enum Value (by Value Name) See https://stackoverflow.com/a/31993512/6500501.
/// </summary>
/// <typeparam name="TEnum">The type of the enum to convert to.</typeparam>
/// <param name="source">The source enum to convert from.</param>
/// <returns></returns>
/// <exception cref="InvalidOperationException"></exception>
public static TEnum ConvertTo<TEnum>(this Enum source)
{
try
{
return (TEnum) Enum.Parse(typeof(TEnum), source.ToString(), ignoreCase: true);
}
catch (ArgumentException aex)
{
throw new InvalidOperationException
(
$"Could not convert {source.GetType().ToString()} [{source.ToString()}] to {typeof(TEnum).ToString()}", aex
);
}
}

public static TEnum ConvertByName<TEnum>(this Enum source, bool ignoreCase = false) where TEnum : struct
{
// if limited by lack of generic enum constraint
if (!typeof(TEnum).IsEnum)
{
throw new InvalidOperationException("enumeration type required.");
}
TEnum result;
if (!Enum.TryParse(source.ToString(), ignoreCase, out result))
{
throw new Exception("conversion failure.");
}
return result;
}

I wrote a set extension methods a while back that work for several different kinds of Enums. One in particular works for what you are trying to accomplish and handles Enums with the FlagsAttribute as well as Enums with different underlying types.
public static tEnum SetFlags<tEnum>(this Enum e, tEnum flags, bool set, bool typeCheck = true) where tEnum : IComparable
{
if (typeCheck)
{
if (e.GetType() != flags.GetType())
throw new ArgumentException("Argument is not the same type as this instance.", "flags");
}
var flagsUnderlyingType = Enum.GetUnderlyingType(typeof(tEnum));
var firstNum = Convert.ToUInt32(e);
var secondNum = Convert.ToUInt32(flags);
if (set)
firstNum |= secondNum;
else
firstNum &= ~secondNum;
var newValue = (tEnum)Convert.ChangeType(firstNum, flagsUnderlyingType);
if (!typeCheck)
{
var values = Enum.GetValues(typeof(tEnum));
var lastValue = (tEnum)values.GetValue(values.Length - 1);
if (newValue.CompareTo(lastValue) > 0)
return lastValue;
}
return newValue;
}
From there you can add other more specific extension methods.
public static tEnum AddFlags<tEnum>(this Enum e, tEnum flags) where tEnum : IComparable
{
SetFlags(e, flags, true);
}
public static tEnum RemoveFlags<tEnum>(this Enum e, tEnum flags) where tEnum : IComparable
{
SetFlags(e, flags, false);
}
This one will change types of Enums like you are trying to do.
public static tEnum ChangeType<tEnum>(this Enum e) where tEnum : IComparable
{
return SetFlags(e, default(tEnum), true, false);
}
Be warned, though, that you CAN convert between any Enum and any other Enum using this method, even those that do not have flags. For example:
public enum Turtle
{
None = 0,
Pink,
Green,
Blue,
Black,
Yellow
}
[Flags]
public enum WriteAccess : short
{
None = 0,
Read = 1,
Write = 2,
ReadWrite = 3
}
static void Main(string[] args)
{
WriteAccess access = WriteAccess.ReadWrite;
Turtle turtle = access.ChangeType<Turtle>();
}
The variable turtle will have a value of Turtle.Blue.
However, there is safety from undefined Enum values using this method. For instance:
static void Main(string[] args)
{
Turtle turtle = Turtle.Yellow;
WriteAccess access = turtle.ChangeType<WriteAccess>();
}
In this case, access will be set to WriteAccess.ReadWrite, since the WriteAccess Enum has a maximum value of 3.
Another side effect of mixing Enums with the FlagsAttribute and those without it is that the conversion process will not result in a 1 to 1 match between their values.
public enum Letters
{
None = 0,
A,
B,
C,
D,
E,
F,
G,
H
}
[Flags]
public enum Flavors
{
None = 0,
Cherry = 1,
Grape = 2,
Orange = 4,
Peach = 8
}
static void Main(string[] args)
{
Flavors flavors = Flavors.Peach;
Letters letters = flavors.ChangeType<Letters>();
}
In this case, letters will have a value of Letters.H instead of Letters.D, since the backing value of Flavors.Peach is 8. Also, a conversion from Flavors.Cherry | Flavors.Grape to Letters would yield Letters.C, which can seem unintuitive.

I know that's an old question and have a lot of answers, However I find that using a switch statement as in the accepted answer is somewhat cumbersome, so here are my 2 cents:
My personal favorite method is to use a dictionary, where the key is the source enum and the value is the target enum - so in the case presented on the question my code would look like this:
var genderTranslator = new Dictionary<TheirGender, MyGender>();
genderTranslator.Add(TheirGender.Male, MyGender.Male);
genderTranslator.Add(TheirGender.Female, MyGender.Female);
genderTranslator.Add(TheirGender.Unknown, MyGender.Unknown);
// translate their to mine
var myValue = genderTranslator[TheirValue];
// translate mine to their
var TheirValue = genderTranslator .FirstOrDefault(x => x.Value == myValue).Key;;
Of course, this can be wrapped in a static class and be used as an extension methods:
public static class EnumTranslator
{
private static Dictionary<TheirGender, MyGender> GenderTranslator = InitializeGenderTranslator();
private static Dictionary<TheirGender, MyGender> InitializeGenderTranslator()
{
var translator = new Dictionary<TheirGender, MyGender>();
translator.Add(TheirGender.Male, MyGender.Male);
translator.Add(TheirGender.Female, MyGender.Female);
translator.Add(TheirGender.Unknown, MyGender.Unknown);
return translator;
}
public static MyGender Translate(this TheirGender theirValue)
{
return GenderTranslator[theirValue];
}
public static TheirGender Translate(this MyGender myValue)
{
return GenderTranslator.FirstOrDefault(x => x.Value == myValue).Key;
}
}

You can use ToString() to convert the first enum to its name, and then Enum.Parse() to convert the string back to the other Enum. This will throw an exception if the value is not supported by the destination enum (i.e. for an "Unknown" value)

For grins you can do this
public enum TestNum {
{
[EnumConvert( NewType.AType )]
Test1,
[EnumConvert( NewType.BType )]
Test2
}
public enum NewType {
AType,
BType
}
public static class EnumExtensions {
public static Enum GetEnum(this Enum value ) {
var attribute = (EnumConvertAttribute)value.GetType()
.GetField( value.ToString() )
.GetCustomAttribute( false )
.Where( a => a is EnumConvertAttribute
.FirstOrDefault();
if( attribute == null ) {
throw new ArgumentNullException();
}
try {
return attribute.TargetEnum;
} catch ( Exception ex ) {
throw new InvalidArgumentException();
}
}
}
public class EnumConvertAttribute : Attribute {
public Enum TargetEnum;
public EnumConvertAttribute( object e ) {
TargetEnum = (Enum)e;
}
}
call:
var t1 = TestNum.Test2;
var t2 = t1.GetEnum();
t2 = NewType.BType;
It works, assuming you don't mind boxing. Converting enums a lot is not good practice anyways. You do get strong typing, you don't have 2 methods for conversions to create a bug eventually when maintaining, and your not praying that the string or int comparisons don't fail.

Enum1 var1;
Enum2 var2;
var2 = Enum2.fromValue(var1.getValue())
public enum Enum2 {
FEMALE("FEMALE"),
MALE("MALE")
private String value;
public static Enum2 fromValue(String value) {
for (Enum2 b : Enum2.values()) {
if (b.value.equals(value)) {
return b;
}
}
throw new IllegalArgumentException("Unexpected value '" + value + "'");
}
}