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Create variable of a generic class from a generic class

I want to create a class that can keep a number between to values. Here for I have created my first two generic classes.
The first called LimitRang and keep two limit rang variables; one for the lower rang and for the top rang. The second called NumberLimitRang and keep the number and the first class LimitRang.
When I try to create the variable mvarRang, how is a LimitRang var, in the constructor of NumberLimitRang I receive the error : 'Cannot implicitly convert type 'VariableTypes.Supplement.LimitRang' to 'T'.
My code for LimitRang:
namespace VariableTypes.Supplement
{
/// <summary>
/// Manage the boundaries for a number.
/// </summary>
/// <typeparam name="T">The numberic type for the limit parameters</typeparam>
public class LimitRang<T> where T : IComparable
{
private T mvarLowestLimitNumber;
private T mvarHighestLimitNumber;
/// <summary>
/// The default constructor (between 0 and 100)
/// </summary>
public LimitRang()
{
if (Functions.IsNumericType(typeof(T)))
{
try
{
mvarLowestLimitNumber = (T)Convert.ChangeType(0, typeof(T));
mvarHighestLimitNumber = (T)Convert.ChangeType(100, typeof(T));
}
catch
{
mvarLowestLimitNumber = default(T);
mvarHighestLimitNumber = default(T);
}
}
}
}
}
My code for NumberLimitRang:
namespace VariableTypes
{
/// <summary>
/// Can contain a number that need to be between or equal to two limit numbers
/// </summary>
public class NumberLimitRang<T> where T : IComparable
{
private Supplement.LimitRang<T> mvarRang;
private T mvarNumber;
/// <summary>
/// The default constructor (between 0 and 100/Number = 0)
/// </summary>
public NumberLimitRang(T mvarRang)
{
mvarRang = new Supplement.LimitRang<T>();
mvarNumber = (T)Convert.ChangeType(0, typeof(T));
}
}
}
Even when I replace the two T's into int, I still receive the error:
- private Supplement.LimitRang<int> mvarRang;
- mvarRang = new Supplement.LimitRang<int>();
Can you tell me what I do wrong?
Thx a lot

Problem is here:
public NumberLimitRang(T mvarRang)
{
mvarRang = new LimitRang<T>();
mvarNumber = (T)Convert.ChangeType(0, typeof(T));
}
You pass mvarRang as an argument and you have field with name mvarRang. But in method NumberLimitRang compiler uses mvarRang as argument, wich is type of T and tryes to replace it's value with new LimitRang<T>();.
So, first of all, rename field or argument. For example:
public class NumberLimitRang<T> where T : IComparable
{
private LimitRang<T> _mvarRang;
private T _mvarNumber;
/// <summary>
/// The default constructor (between 0 and 100/Number = 0)
/// </summary>
public NumberLimitRang(T mvarRang)
{
_mvarRang = new LimitRang<T>();
_mvarNumber = (T)Convert.ChangeType(0, typeof(T));
}
}
It's good practice to name fields with _ in the beginning.
But now, your argument mvarRang is unused. Revise your logic in code.

public NumberLimitRang(T mvarRang)
{
mvarRang = new Supplement.LimitRang<T>();
mvarNumber = (T)Convert.ChangeType(0, typeof(T));
}
The constructor takes a variable mvarRang of type T. This declaration hides the instance member mvarRang of the type LimitRang<T>.
Either change the parameter name, or refer to the instance member explicitly using this:
public NumberLimitRang(T mvarRang)
{
this.mvarRang = new Supplement.LimitRang<T>();
mvarNumber = (T)Convert.ChangeType(0, typeof(T));
}

Pass variable type to a method without enumerating

I want to be able to pass a variable type to a method, mainly so that I can pass an entity framework query to a method that will apply common includes of nested object.
This is what I want to do...
public Person GetPersonByID(int personID)
{
var query = from Perspn p in Context.Persons
where p.PersonID = personID
select p;
ObjectQuery<Person> personQuery = ApplyCommonIncludes<Person>(query);
return personQuery.FirstOrDefault();
}
public ObjectQuery<T> ApplyCommonIncludes<T>(SomeType query)
{
return ((ObjectQuery<T>)query)
.Include("Orders")
.Include("LoginHistory");
}

Seems to be you actually want SomeType to be ObjectQuery<T>, right?
public ObjectQuery<T> ApplyCommonIncludes<T>(ObjectQuery<T> query)
{
return query
.Include("Orders")
.Include("LoginHistory");
}
This is valid syntax. Is there any problem with this?
This ought to work and do delayed execution (I think this is what you mean by "without enumerating") until FirstOrDefault() is called.

I ended up creating a different approach. My repository now has a list of string used for Includes. To retain type safety for creating includes, I created the following class:
/// <summary>
/// Builds Includes
/// </summary>
public class IncludeBuilder
{
/// <summary>
/// List of parts for the Include
/// </summary>
private List<string> Parts;
/// <summary>
/// Creates a new IncludeBuilder
/// </summary>
private IncludeBuilder()
{
this.Parts = new List<string>();
}
/// <summary>
/// Creates a new IncludeBuilder
/// </summary>
public static IncludeBuilder Create()
{
return new IncludeBuilder();
}
/// <summary>
/// Adds a property name to the builder
/// </summary>
public IncludeBuilder AddPart<TEntity, TProp>(Expression<Func<TEntity, TProp>> expression)
{
string propName = ExpressionHelper.GetPropertyNameFromExpression(expression);
this.Parts.Add(propName);
return this;
}
/// <summary>
/// Gets a value of the include parts separated by
/// a decimal
/// </summary>
public override string ToString()
{
return string.Join(".", this.Parts.ToArray());
}
This allows me to do this...
myPersonRepository.AppendInclude(
IncludeBuilder.Create()
.AddPart((Person p) => p.Orders)
.AddPart((Order o) => o.Items));
The above statement passes expressions to the IncludeBuilder class which then translates the above into "Orders.Items".
I then created helper methods in my RepositoryBase that given an ObjectQuery, will apply the includes, execute the query, and return the result. Not quite what I was looking for, but works well.

How do I create dynamic properties in C#?

I am looking for a way to create a class with a set of static properties. At run time, I want to be able to add other dynamic properties to this object from the database. I'd also like to add sorting and filtering capabilities to these objects.
How do I do this in C#?

You might use a dictionary, say
Dictionary<string,object> properties;
I think in most cases where something similar is done, it's done like this.
In any case, you would not gain anything from creating a "real" property with set and get accessors, since it would be created only at run-time and you would not be using it in your code...
Here is an example, showing a possible implementation of filtering and sorting (no error checking):
using System;
using System.Collections.Generic;
using System.Linq;
namespace ConsoleApplication1 {
class ObjectWithProperties {
Dictionary<string, object> properties = new Dictionary<string,object>();
public object this[string name] {
get {
if (properties.ContainsKey(name)){
return properties[name];
}
return null;
}
set {
properties[name] = value;
}
}
}
class Comparer<T> : IComparer<ObjectWithProperties> where T : IComparable {
string m_attributeName;
public Comparer(string attributeName){
m_attributeName = attributeName;
}
public int Compare(ObjectWithProperties x, ObjectWithProperties y) {
return ((T)x[m_attributeName]).CompareTo((T)y[m_attributeName]);
}
}
class Program {
static void Main(string[] args) {
// create some objects and fill a list
var obj1 = new ObjectWithProperties();
obj1["test"] = 100;
var obj2 = new ObjectWithProperties();
obj2["test"] = 200;
var obj3 = new ObjectWithProperties();
obj3["test"] = 150;
var objects = new List<ObjectWithProperties>(new ObjectWithProperties[]{ obj1, obj2, obj3 });
// filtering:
Console.WriteLine("Filtering:");
var filtered = from obj in objects
where (int)obj["test"] >= 150
select obj;
foreach (var obj in filtered){
Console.WriteLine(obj["test"]);
}
// sorting:
Console.WriteLine("Sorting:");
Comparer<int> c = new Comparer<int>("test");
objects.Sort(c);
foreach (var obj in objects) {
Console.WriteLine(obj["test"]);
}
}
}
}

If you need this for data-binding purposes, you can do this with a custom descriptor model... by implementing ICustomTypeDescriptor, TypeDescriptionProvider and/or TypeCoverter, you can create your own PropertyDescriptor instances at runtime. This is what controls like DataGridView, PropertyGrid etc use to display properties.
To bind to lists, you'd need ITypedList and IList; for basic sorting: IBindingList; for filtering and advanced sorting: IBindingListView; for full "new row" support (DataGridView): ICancelAddNew (phew!).
It is a lot of work though. DataTable (although I hate it) is cheap way of doing the same thing. If you don't need data-binding, just use a hashtable ;-p
Here's a simple example - but you can do a lot more...

Use ExpandoObject like the ViewBag in MVC 3.

Create a Hashtable called "Properties" and add your properties to it.

I'm not sure you really want to do what you say you want to do, but it's not for me to reason why!
You cannot add properties to a class after it has been JITed.
The closest you could get would be to dynamically create a subtype with Reflection.Emit and copy the existing fields over, but you'd have to update all references to the the object yourself.
You also wouldn't be able to access those properties at compile time.
Something like:
public class Dynamic
{
public Dynamic Add<T>(string key, T value)
{
AssemblyBuilder assemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(new AssemblyName("DynamicAssembly"), AssemblyBuilderAccess.Run);
ModuleBuilder moduleBuilder = assemblyBuilder.DefineDynamicModule("Dynamic.dll");
TypeBuilder typeBuilder = moduleBuilder.DefineType(Guid.NewGuid().ToString());
typeBuilder.SetParent(this.GetType());
PropertyBuilder propertyBuilder = typeBuilder.DefineProperty(key, PropertyAttributes.None, typeof(T), Type.EmptyTypes);
MethodBuilder getMethodBuilder = typeBuilder.DefineMethod("get_" + key, MethodAttributes.Public, CallingConventions.HasThis, typeof(T), Type.EmptyTypes);
ILGenerator getter = getMethodBuilder.GetILGenerator();
getter.Emit(OpCodes.Ldarg_0);
getter.Emit(OpCodes.Ldstr, key);
getter.Emit(OpCodes.Callvirt, typeof(Dynamic).GetMethod("Get", BindingFlags.Instance | BindingFlags.NonPublic).MakeGenericMethod(typeof(T)));
getter.Emit(OpCodes.Ret);
propertyBuilder.SetGetMethod(getMethodBuilder);
Type type = typeBuilder.CreateType();
Dynamic child = (Dynamic)Activator.CreateInstance(type);
child.dictionary = this.dictionary;
dictionary.Add(key, value);
return child;
}
protected T Get<T>(string key)
{
return (T)dictionary[key];
}
private Dictionary<string, object> dictionary = new Dictionary<string,object>();
}
I don't have VS installed on this machine so let me know if there are any massive bugs (well... other than the massive performance problems, but I didn't write the specification!)
Now you can use it:
Dynamic d = new Dynamic();
d = d.Add("MyProperty", 42);
Console.WriteLine(d.GetType().GetProperty("MyProperty").GetValue(d, null));
You could also use it like a normal property in a language that supports late binding (for example, VB.NET)

I have done exactly this with an ICustomTypeDescriptor interface and a Dictionary.
Implementing ICustomTypeDescriptor for dynamic properties:
I have recently had a requirement to bind a grid view to a record object that could have any number of properties that can be added and removed at runtime. This was to allow a user to add a new column to a result set to enter an additional set of data.
This can be achieved by having each data 'row' as a dictionary with the key being the property name and the value being a string or a class that can store the value of the property for the specified row. Of course having a List of Dictionary objects will not be able to be bound to a grid. This is where the ICustomTypeDescriptor comes in.
By creating a wrapper class for the Dictionary and making it adhere to the ICustomTypeDescriptor interface the behaviour for returning properties for an object can be overridden.
Take a look at the implementation of the data 'row' class below:
/// <summary>
/// Class to manage test result row data functions
/// </summary>
public class TestResultRowWrapper : Dictionary<string, TestResultValue>, ICustomTypeDescriptor
{
//- METHODS -----------------------------------------------------------------------------------------------------------------
#region Methods
/// <summary>
/// Gets the Attributes for the object
/// </summary>
AttributeCollection ICustomTypeDescriptor.GetAttributes()
{
return new AttributeCollection(null);
}
/// <summary>
/// Gets the Class name
/// </summary>
string ICustomTypeDescriptor.GetClassName()
{
return null;
}
/// <summary>
/// Gets the component Name
/// </summary>
string ICustomTypeDescriptor.GetComponentName()
{
return null;
}
/// <summary>
/// Gets the Type Converter
/// </summary>
TypeConverter ICustomTypeDescriptor.GetConverter()
{
return null;
}
/// <summary>
/// Gets the Default Event
/// </summary>
/// <returns></returns>
EventDescriptor ICustomTypeDescriptor.GetDefaultEvent()
{
return null;
}
/// <summary>
/// Gets the Default Property
/// </summary>
PropertyDescriptor ICustomTypeDescriptor.GetDefaultProperty()
{
return null;
}
/// <summary>
/// Gets the Editor
/// </summary>
object ICustomTypeDescriptor.GetEditor(Type editorBaseType)
{
return null;
}
/// <summary>
/// Gets the Events
/// </summary>
EventDescriptorCollection ICustomTypeDescriptor.GetEvents(Attribute[] attributes)
{
return new EventDescriptorCollection(null);
}
/// <summary>
/// Gets the events
/// </summary>
EventDescriptorCollection ICustomTypeDescriptor.GetEvents()
{
return new EventDescriptorCollection(null);
}
/// <summary>
/// Gets the properties
/// </summary>
PropertyDescriptorCollection ICustomTypeDescriptor.GetProperties(Attribute[] attributes)
{
List<propertydescriptor> properties = new List<propertydescriptor>();
//Add property descriptors for each entry in the dictionary
foreach (string key in this.Keys)
{
properties.Add(new TestResultPropertyDescriptor(key));
}
//Get properties also belonging to this class also
PropertyDescriptorCollection pdc = TypeDescriptor.GetProperties(this.GetType(), attributes);
foreach (PropertyDescriptor oPropertyDescriptor in pdc)
{
properties.Add(oPropertyDescriptor);
}
return new PropertyDescriptorCollection(properties.ToArray());
}
/// <summary>
/// gets the Properties
/// </summary>
PropertyDescriptorCollection ICustomTypeDescriptor.GetProperties()
{
return ((ICustomTypeDescriptor)this).GetProperties(null);
}
/// <summary>
/// Gets the property owner
/// </summary>
object ICustomTypeDescriptor.GetPropertyOwner(PropertyDescriptor pd)
{
return this;
}
#endregion Methods
//---------------------------------------------------------------------------------------------------------------------------
}
Note: In the GetProperties method I Could Cache the PropertyDescriptors once read for performance but as I'm adding and removing columns at runtime I always want them rebuilt
You will also notice in the GetProperties method that the Property Descriptors added for the dictionary entries are of type TestResultPropertyDescriptor. This is a custom Property Descriptor class that manages how properties are set and retrieved. Take a look at the implementation below:
/// <summary>
/// Property Descriptor for Test Result Row Wrapper
/// </summary>
public class TestResultPropertyDescriptor : PropertyDescriptor
{
//- PROPERTIES --------------------------------------------------------------------------------------------------------------
#region Properties
/// <summary>
/// Component Type
/// </summary>
public override Type ComponentType
{
get { return typeof(Dictionary<string, TestResultValue>); }
}
/// <summary>
/// Gets whether its read only
/// </summary>
public override bool IsReadOnly
{
get { return false; }
}
/// <summary>
/// Gets the Property Type
/// </summary>
public override Type PropertyType
{
get { return typeof(string); }
}
#endregion Properties
//- CONSTRUCTOR -------------------------------------------------------------------------------------------------------------
#region Constructor
/// <summary>
/// Constructor
/// </summary>
public TestResultPropertyDescriptor(string key)
: base(key, null)
{
}
#endregion Constructor
//- METHODS -----------------------------------------------------------------------------------------------------------------
#region Methods
/// <summary>
/// Can Reset Value
/// </summary>
public override bool CanResetValue(object component)
{
return true;
}
/// <summary>
/// Gets the Value
/// </summary>
public override object GetValue(object component)
{
return ((Dictionary<string, TestResultValue>)component)[base.Name].Value;
}
/// <summary>
/// Resets the Value
/// </summary>
public override void ResetValue(object component)
{
((Dictionary<string, TestResultValue>)component)[base.Name].Value = string.Empty;
}
/// <summary>
/// Sets the value
/// </summary>
public override void SetValue(object component, object value)
{
((Dictionary<string, TestResultValue>)component)[base.Name].Value = value.ToString();
}
/// <summary>
/// Gets whether the value should be serialized
/// </summary>
public override bool ShouldSerializeValue(object component)
{
return false;
}
#endregion Methods
//---------------------------------------------------------------------------------------------------------------------------
}
The main properties to look at on this class are GetValue and SetValue. Here you can see the component being casted as a dictionary and the value of the key inside it being Set or retrieved. Its important that the dictionary in this class is the same type in the Row wrapper class otherwise the cast will fail. When the descriptor is created the key (property name) is passed in and is used to query the dictionary to get the correct value.
Taken from my blog at:
ICustomTypeDescriptor Implementation for dynamic properties

You should look into DependencyObjects as used by WPF these follow a similar pattern whereby properties can be assigned at runtime. As mentioned above this ultimately points towards using a hash table.
One other useful thing to have a look at is CSLA.Net. The code is freely available and uses some of the principles\patterns it appears you are after.
Also if you are looking at sorting and filtering I'm guessing you're going to be using some kind of grid. A useful interface to implement is ICustomTypeDescriptor, this lets you effectively override what happens when your object gets reflected on so you can point the reflector to your object's own internal hash table.

As a replacement for some of orsogufo's code, because I recently went with a dictionary for this same problem myself, here is my [] operator:
public string this[string key]
{
get { return properties.ContainsKey(key) ? properties[key] : null; }
set
{
if (properties.ContainsKey(key))
{
properties[key] = value;
}
else
{
properties.Add(key, value);
}
}
}
With this implementation, the setter will add new key-value pairs when you use []= if they do not already exist in the dictionary.
Also, for me properties is an IDictionary and in constructors I initialize it to new SortedDictionary<string, string>().

I'm not sure what your reasons are, and even if you could pull it off somehow with Reflection Emit (I' not sure that you can), it doesn't sound like a good idea. What is probably a better idea is to have some kind of Dictionary and you can wrap access to the dictionary through methods in your class. That way you can store the data from the database in this dictionary, and then retrieve them using those methods.

Why not use an indexer with the property name as a string value passed to the indexer?

Couldn't you just have your class expose a Dictionary object? Instead of "attaching more properties to the object", you could simply insert your data (with some identifier) into the dictionary at run time.

If it is for binding, then you can reference indexers from XAML
Text="{Binding [FullName]}"
Here it is referencing the class indexer with the key "FullName"

Store generic data in a non-generic class

I have a DataGridView that I want to use to store generic data. I want to keep a typed data list in the DataGridView class so that all of the sorts, etc. can be handled internally. But I don't want to have to set the type on the DataGridView since I won't know the data type until the InitializeData method is called.
public class MyDataGridView : DataGridView {
private List<T> m_data;
public InitializeData<T>(List<T> data) {
m_data = data;
}
... internal events to know when the datagrid wants to sort ...
m_data.Sort<T>(...)
}
Is this possible? If so, how?

If you won't know the type until you call InitializeData, then the type clearly can't be a compile-time part of the object.
Do you know everything you need to know about the sorting when you call InitializeData<T>? If so, how about you do something like:
private IList m_data;
private Action m_sorter;
public InitializeData<T>(List<T> data)
{
m_data = data;
// This captures the data variable. You'll need to
// do something different if that's not good enough
m_sorter = () => data.Sort();
}
Then when you need to sort later, you can just call m_sorter().
If you might sort on different things, you could potentially change it from an Action to Action<string> or whatever you'd need to be able to sort on.

If Jon's answer isn't sufficient, here's a more general (but more involved, and probably somewhat more confusing) approach:
/// <summary>
/// Allows a list of any type to be used to get a result of type TResult
/// </summary>
/// <typeparam name="TResult">The result type after using the list</typeparam>
interface IListUser<TResult>
{
TResult Use<T>(List<T> list);
}
/// <summary>
/// Allows a list of any type to be used (with no return value)
/// </summary>
interface IListUser
{
void Use<T>(List<T> list);
}
/// <summary>
/// Here's a class that can sort lists of any type
/// </summary>
class GenericSorter : IListUser
{
#region IListUser Members
public void Use<T>(List<T> list)
{
// do generic sorting stuff here
}
#endregion
}
/// <summary>
/// Wraps a list of some unknown type. Allows list users (either with or without return values) to use the wrapped list.
/// </summary>
interface IExistsList
{
TResult Apply<TResult>(IListUser<TResult> user);
void Apply(IListUser user);
}
/// <summary>
/// Wraps a list of type T, hiding the type itself.
/// </summary>
/// <typeparam name="T">The type of element contained in the list</typeparam>
class ExistsList<T> : IExistsList
{
List<T> list;
public ExistsList(List<T> list)
{
this.list = list;
}
#region IExistsList Members
public TResult Apply<TResult>(IListUser<TResult> user)
{
return user.Use(list);
}
public void Apply(IListUser user)
{
user.Use(list);
}
#endregion
}
/// <summary>
/// Your logic goes here
/// </summary>
class MyDataGridView
{
private IExistsList list;
public void InitializeData<T>(List<T> list)
{
this.list = new ExistsList<T>(list);
}
public void Sort()
{
list.Apply(new GenericSorter());
}
}

You should define delgates or an interface for any generic operations you need to perform at runtime. As Jon Skeet mentioned, you can't strongly-type your data grid if you don't know the types at compile time.
This is the way the framework does it. For example:
Array.Sort();
Has a few ways it can be used:
Send it an array of objects that implement IComparable or IComparable<T>
Send in a second parameter, which is a class that implements IComparer or IComparer<T>. Used to compare the objects for sorting.
Send in a second parameter, which is a Comparison<T> delegate that can be used to compare objects in the array.
This is an example of how you approach the problem. At its most basic level, your scenario can be solved by a strategy pattern, which is what Array.Sort() does.
If you need to sort by things dynamically at run time, I would create an IComparer class that takes the column you want to sort by as an argument in its constructor. Then in your compare method, use that column as the sort type.
Here is an example of how you would do it using some basic example classes. Once you have these classes set up, then you'd pass both into your data grid and use them where appropriate.
public class Car
{
public string Make { get; set; }
public string Model { get; set; }
public string Year { get; set; }
}
public class CarComparer : IComparer
{
string sortColumn;
public CarComparer(string sortColumn)
{
this.sortColumn = sortColumn;
}
public int Compare(object x, object y)
{
Car carX = x as Car;
Car carY = y as Car;
if (carX == null && carY == null)
return 0;
if (carX != null && carY == null)
return 1;
if (carY != null && carX == null)
return -1;
switch (sortColumn)
{
case "Make":
return carX.Make.CompareTo(carY.Make);
case "Model":
return carX.Model.CompareTo(carY.Model);
case "Year":
default:
return carX.Year.CompareTo(carY.Year);
}
}
}

Can a C# class inherit attributes from its interface?

This would appear to imply "no". Which is unfortunate.
[AttributeUsage(AttributeTargets.Interface | AttributeTargets.Class,
AllowMultiple = true, Inherited = true)]
public class CustomDescriptionAttribute : Attribute
{
public string Description { get; private set; }
public CustomDescriptionAttribute(string description)
{
Description = description;
}
}
[CustomDescription("IProjectController")]
public interface IProjectController
{
void Create(string projectName);
}
internal class ProjectController : IProjectController
{
public void Create(string projectName)
{
}
}
[TestFixture]
public class CustomDescriptionAttributeTests
{
[Test]
public void ProjectController_ShouldHaveCustomDescriptionAttribute()
{
Type type = typeof(ProjectController);
object[] attributes = type.GetCustomAttributes(
typeof(CustomDescriptionAttribute),
true);
// NUnit.Framework.AssertionException: Expected: 1 But was: 0
Assert.AreEqual(1, attributes.Length);
}
}
Can a class inherit attributes from an interface? Or am I barking up the wrong tree here?

No. Whenever implementing an interface or overriding members in a derived class, you need to re-declare the attributes.
If you only care about ComponentModel (not direct reflection), there is a way ([AttributeProvider]) of suggesting attributes from an existing type (to avoid duplication), but it is only valid for property and indexer usage.
As an example:
using System;
using System.ComponentModel;
class Foo {
[AttributeProvider(typeof(IListSource))]
public object Bar { get; set; }
static void Main() {
var bar = TypeDescriptor.GetProperties(typeof(Foo))["Bar"];
foreach (Attribute attrib in bar.Attributes) {
Console.WriteLine(attrib);
}
}
}
outputs:
System.SerializableAttribute
System.ComponentModel.AttributeProviderAttribute
System.ComponentModel.EditorAttribute
System.Runtime.InteropServices.ComVisibleAttribute
System.Runtime.InteropServices.ClassInterfaceAttribute
System.ComponentModel.TypeConverterAttribute
System.ComponentModel.MergablePropertyAttribute

You can define a useful extension method ...
Type type = typeof(ProjectController);
var attributes = type.GetCustomAttributes<CustomDescriptionAttribute>( true );
Here is the extension method:
/// <summary>Searches and returns attributes. The inheritance chain is not used to find the attributes.</summary>
/// <typeparam name="T">The type of attribute to search for.</typeparam>
/// <param name="type">The type which is searched for the attributes.</param>
/// <returns>Returns all attributes.</returns>
public static T[] GetCustomAttributes<T>( this Type type ) where T : Attribute
{
return GetCustomAttributes( type, typeof( T ), false ).Select( arg => (T)arg ).ToArray();
}
/// <summary>Searches and returns attributes.</summary>
/// <typeparam name="T">The type of attribute to search for.</typeparam>
/// <param name="type">The type which is searched for the attributes.</param>
/// <param name="inherit">Specifies whether to search this member's inheritance chain to find the attributes. Interfaces will be searched, too.</param>
/// <returns>Returns all attributes.</returns>
public static T[] GetCustomAttributes<T>( this Type type, bool inherit ) where T : Attribute
{
return GetCustomAttributes( type, typeof( T ), inherit ).Select( arg => (T)arg ).ToArray();
}
/// <summary>Private helper for searching attributes.</summary>
/// <param name="type">The type which is searched for the attribute.</param>
/// <param name="attributeType">The type of attribute to search for.</param>
/// <param name="inherit">Specifies whether to search this member's inheritance chain to find the attribute. Interfaces will be searched, too.</param>
/// <returns>An array that contains all the custom attributes, or an array with zero elements if no attributes are defined.</returns>
private static object[] GetCustomAttributes( Type type, Type attributeType, bool inherit )
{
if( !inherit )
{
return type.GetCustomAttributes( attributeType, false );
}
var attributeCollection = new Collection<object>();
var baseType = type;
do
{
baseType.GetCustomAttributes( attributeType, true ).Apply( attributeCollection.Add );
baseType = baseType.BaseType;
}
while( baseType != null );
foreach( var interfaceType in type.GetInterfaces() )
{
GetCustomAttributes( interfaceType, attributeType, true ).Apply( attributeCollection.Add );
}
var attributeArray = new object[attributeCollection.Count];
attributeCollection.CopyTo( attributeArray, 0 );
return attributeArray;
}
/// <summary>Applies a function to every element of the list.</summary>
private static void Apply<T>( this IEnumerable<T> enumerable, Action<T> function )
{
foreach( var item in enumerable )
{
function.Invoke( item );
}
}
Update:
Here is a shorter version as proposed by SimonD in a comment:
private static IEnumerable<T> GetCustomAttributesIncludingBaseInterfaces<T>(this Type type)
{
var attributeType = typeof(T);
return type.GetCustomAttributes(attributeType, true)
.Union(type.GetInterfaces().SelectMany(interfaceType =>
interfaceType.GetCustomAttributes(attributeType, true)))
.Cast<T>();
}

An article by Brad Wilson about this: Interface Attributes != Class Attributes
To summarise: classes don't inherit from interfaces, they implement them. This means that the attributes are not automatically part of the implementation.
If you need to inherit attributes, use an abstract base class, rather than an interface.

While a C# class does not inherit attributes from its interfaces, there is a useful alternative when binding models in ASP.NET MVC3.
If you declare the view's model to be the interface rather than the concrete type, then the view and the model binder will apply the attributes (e.g., [Required] or [DisplayName("Foo")] from the interface when rendering and validating the model:
public interface IModel {
[Required]
[DisplayName("Foo Bar")]
string FooBar { get; set; }
}
public class Model : IModel {
public string FooBar { get; set; }
}
Then in the view:
#* Note use of interface type for the view model *#
#model IModel
#* This control will receive the attributes from the interface *#
#Html.EditorFor(m => m.FooBar)

This is more for people looking to extract attributes from properties that may exist on an implemented interface. Because those attributes are not part of the class, this will give you access to them. note, I have a simple container class that gives you access to the PropertyInfo - as that is what I needed it for. Hack up as you need. This worked well for me.
public static class CustomAttributeExtractorExtensions
{
/// <summary>
/// Extraction of property attributes as well as attributes on implemented interfaces.
/// This will walk up recursive to collect any interface attribute as well as their parent interfaces.
/// </summary>
/// <typeparam name="TAttributeType"></typeparam>
/// <param name="typeToReflect"></param>
/// <returns></returns>
public static List<PropertyAttributeContainer<TAttributeType>> GetPropertyAttributesFromType<TAttributeType>(this Type typeToReflect)
where TAttributeType : Attribute
{
var list = new List<PropertyAttributeContainer<TAttributeType>>();
// Loop over the direct property members
var properties = typeToReflect.GetProperties();
foreach (var propertyInfo in properties)
{
// Get the attributes as well as from the inherited classes (true)
var attributes = propertyInfo.GetCustomAttributes<TAttributeType>(true).ToList();
if (!attributes.Any()) continue;
list.AddRange(attributes.Select(attr => new PropertyAttributeContainer<TAttributeType>(attr, propertyInfo)));
}
// Look at the type interface declarations and extract from that type.
var interfaces = typeToReflect.GetInterfaces();
foreach (var #interface in interfaces)
{
list.AddRange(#interface.GetPropertyAttributesFromType<TAttributeType>());
}
return list;
}
/// <summary>
/// Simple container for the Property and Attribute used. Handy if you want refrence to the original property.
/// </summary>
/// <typeparam name="TAttributeType"></typeparam>
public class PropertyAttributeContainer<TAttributeType>
{
internal PropertyAttributeContainer(TAttributeType attribute, PropertyInfo property)
{
Property = property;
Attribute = attribute;
}
public PropertyInfo Property { get; private set; }
public TAttributeType Attribute { get; private set; }
}
}

One of the answers worked for attributes placed on the interface itself, but I was looking for interface property member attributes. I could not find it anywhere and had to alter some of the answers here to get what I needed. Going to leave the extension method I used here. This method will get all custom attributes of type T from a PropertyInfo including attributes placed on interfaces.
public static IEnumerable<T> GetAllCustomAttributes<T> (this MemberInfo member) where T: Attribute
{
return member.GetCustomAttributes<T>(true)
.Union(member.DeclaringType.GetInterfaces().SelectMany(i => i.GetProperties())
.Select(p => p.GetCustomAttribute<T>(true)));
}
For my case I have a lot of cards and each card can have a two icons, but those icons refer to different values. All the values were being defined on my interfaces and I didn't want to bring each down to the class implementation since it would be the same across all cards. My usage looks like:
public interface IConsumable
{
[CardIcon(CardIconPosition.Right)]
public int Food { get; }
}
public class FoodCard : IConsumable
{
public bool TryGetCardIcon(CardIconPosition cardIconPosition, out string text)
{
var property = Card.GetType().GetProperties()
.FirstOrDefault(p => p.GetAllCustomAttributes<CardIconAttribute>()?
.Any(c => c.CardIconPosition == cardIconPosition) ?? false);
bool hasProperty = property != null;
text = string.Empty;
if (hasProperty)
{
text = property?.GetValue(Card).ToString() ?? string.Empty;
}
return hasProperty;
}
}

EDIT: this covers inheriting attributes from interfaces on members (incl. properties). There are simple answers above for type definitions. I just posted this because I found it to be an irritating limitation and wanted to share a solution :)
Interfaces are multiple inheritance and behave as inheritance in the type system. There isn't a good reason for this kind of stuff. Reflection is a bit hokey. I've added comments to explain the nonsense.
(This is .NET 3.5 because this it just happens to be what the project I'm doing at the moment is using.)
// in later .NETs, you can cache reflection extensions using a static generic class and
// a ConcurrentDictionary. E.g.
//public static class Attributes<T> where T : Attribute
//{
// private static readonly ConcurrentDictionary<MemberInfo, IReadOnlyCollection<T>> _cache =
// new ConcurrentDictionary<MemberInfo, IReadOnlyCollection<T>>();
//
// public static IReadOnlyCollection<T> Get(MemberInfo member)
// {
// return _cache.GetOrAdd(member, GetImpl, Enumerable.Empty<T>().ToArray());
// }
// //GetImpl as per code below except that recursive steps re-enter via the cache
//}
public static List<T> GetAttributes<T>(this MemberInfo member) where T : Attribute
{
// determine whether to inherit based on the AttributeUsage
// you could add a bool parameter if you like but I think it defeats the purpose of the usage
var usage = typeof(T).GetCustomAttributes(typeof(AttributeUsageAttribute), true)
.Cast<AttributeUsageAttribute>()
.FirstOrDefault();
var inherit = usage != null && usage.Inherited;
return (
inherit
? GetAttributesRecurse<T>(member)
: member.GetCustomAttributes(typeof (T), false).Cast<T>()
)
.Distinct() // interfaces mean duplicates are a thing
// note: attribute equivalence needs to be overridden. The default is not great.
.ToList();
}
private static IEnumerable<T> GetAttributesRecurse<T>(MemberInfo member) where T : Attribute
{
// must use Attribute.GetCustomAttribute rather than MemberInfo.GetCustomAttribute as the latter
// won't retrieve inherited attributes from base *classes*
foreach (T attribute in Attribute.GetCustomAttributes(member, typeof (T), true))
yield return attribute;
// The most reliable target in the interface map is the property get method.
// If you have set-only properties, you'll need to handle that case. I generally just ignore that
// case because it doesn't make sense to me.
PropertyInfo property;
var target = (property = member as PropertyInfo) != null ? property.GetGetMethod() : member;
foreach (var #interface in member.DeclaringType.GetInterfaces())
{
// The interface map is two aligned arrays; TargetMethods and InterfaceMethods.
var map = member.DeclaringType.GetInterfaceMap(#interface);
var memberIndex = Array.IndexOf(map.TargetMethods, target); // see target above
if (memberIndex < 0) continue;
// To recurse, we still need to hit the property on the parent interface.
// Why don't we just use the get method from the start? Because GetCustomAttributes won't work.
var interfaceMethod = property != null
// name of property get method is get_<property name>
// so name of parent property is substring(4) of that - this is reliable IME
? #interface.GetProperty(map.InterfaceMethods[memberIndex].Name.Substring(4))
: (MemberInfo) map.InterfaceMethods[memberIndex];
// Continuation is the word to google if you don't understand this
foreach (var attribute in interfaceMethod.GetAttributes<T>())
yield return attribute;
}
}
Barebones NUnit test
[TestFixture]
public class GetAttributesTest
{
[AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = true)]
private sealed class A : Attribute
{
// default equality for Attributes is apparently semantic
public override bool Equals(object obj)
{
return ReferenceEquals(this, obj);
}
public override int GetHashCode()
{
return base.GetHashCode();
}
}
[AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
private sealed class ANotInherited : Attribute { }
public interface Top
{
[A, ANotInherited]
void M();
[A, ANotInherited]
int P { get; }
}
public interface Middle : Top { }
private abstract class Base
{
[A, ANotInherited]
public abstract void M();
[A, ANotInherited]
public abstract int P { get; }
}
private class Bottom : Base, Middle
{
[A, ANotInherited]
public override void M()
{
throw new NotImplementedException();
}
[A, ANotInherited]
public override int P { get { return 42; } }
}
[Test]
public void GetsAllInheritedAttributesOnMethods()
{
var attributes = typeof (Bottom).GetMethod("M").GetAttributes<A>();
attributes.Should()
.HaveCount(3, "there are 3 inherited copies in the class heirarchy and A is inherited");
}
[Test]
public void DoesntGetNonInheritedAttributesOnMethods()
{
var attributes = typeof (Bottom).GetMethod("M").GetAttributes<ANotInherited>();
attributes.Should()
.HaveCount(1, "it shouldn't get copies of the attribute from base classes for a non-inherited attribute");
}
[Test]
public void GetsAllInheritedAttributesOnProperties()
{
var attributes = typeof(Bottom).GetProperty("P").GetAttributes<A>();
attributes.Should()
.HaveCount(3, "there are 3 inherited copies in the class heirarchy and A is inherited");
}
[Test]
public void DoesntGetNonInheritedAttributesOnProperties()
{
var attributes = typeof(Bottom).GetProperty("P").GetAttributes<ANotInherited>();
attributes.Should()
.HaveCount(1, "it shouldn't get copies of the attribute from base classes for a non-inherited attribute");
}
}

Add interface with properties that have attributes/custom attributes attached to the same properties that class have. We can extract the interface of the class by using Visual studio refactor feature.
Have a partial class implement that interface.
Now Get "Type" object of the class object and get custom attributes from the property info using getProperties on Type object.
This will not give the custom attributes on the class object as the class properties did not had the interface properties' custom attributes attached/inherited.
Now call GetInterface(NameOfImplemetedInterfaceByclass) on the class's Type object retrieved above. This will
provide the interface's "Type" object. we should know the implemented interface's NAME. From Type object get property information and if the interface's property has any custom attributes attached then property information will provide
custom attribute list. The implementing class must have provided implementation of the interface's properties.
Match the class object's specific property name within the list of the interface's property information to get the custom attributes list.
This will work.

Though my answer is late and specific to a certain case, I would like to add some ideas.
As suggested in other answers, Reflection or other methods would do it.
In my case a property (timestamp) was needed in all models to meet certain requirement (concurrency check attribute) in a Entity framework core project.
We could either add [] above all class properties (adding in IModel interface which models implemented, didn't work). But I saved time through Fluent API which is helpful in these cases. In fluent API, I can check for specific property name in all models and set as IsConcurrencyToken() in 1 line !!
var props = from e in modelBuilder.Model.GetEntityTypes()
from p in e.GetProperties()
select p;
props.Where(p => p.PropertyInfo.Name == "ModifiedTime").ToList().ForEach(p => { p.IsConcurrencyToken = true; });
Likewise if you need any attribute to be added to same property name in 100's of classes/models, we can use fluent api methods for inbuilt or custom attribute resolver.
Though EF (both core and EF6) fluent api may use reflection behind the scenes, we can save effort :)