Adding generic extension methods to interfaces like IEnumerable

Question

I've been trying and trying to make my generic extension methods work, but they just refuse to and I can't figure out why. This thread didn't help me, although it should.

Of course I've looked up how to, everywhere I see they say it's simple and it should be in this syntax:
(On some places I read that I need to add "where T: [type]" after the parameter decleration, but my VS2010 just says that's a syntax error.)

using System.Collections.Generic;
using System.ComponentModel;

public static class TExtensions
{
    public static List<T> ToList(this IEnumerable<T> collection)
    {
        return new List<T>(collection);
    }

    public static BindingList<T> ToBindingList(this IEnumerable<T> collection)
    {
        return new BindingList<T>(collection.ToList());
    }
}

But that just doesn't work, I get this error:

The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?)

If I then replace

public static class TExtensions

by

public static class TExtensions<T>

it gives this error:

Extension method must be defined in a non-generic static class

Any help will be much appreciated, I'm really stuck here.

score 15 · Accepted Answer · answered Jun 21 '11 at 09:33

15

I think what you're missing is making the methods generic in T:

public static List<T> ToList<T>(this IEnumerable<T> collection)
{
    return new List<T>(collection);
}

public static BindingList<T> ToBindingList<T>(this IEnumerable<T> collection)
{
    return new BindingList<T>(collection.ToList());
}

Note the <T> after the name of each method, before the parameter list. That says it's a generic method with a single type parameter, T.

answered Jun 21 '11 at 09:33

Jon Skeet

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Oh god I knew it was something stupid like that. Thanks a bunch! Can't believe I missed that > – Rik De Peuter Jun 21 '11 at 09:34

score 1 · Answer 2 · answered Jun 21 '11 at 09:34

Try:

public static class TExtensions
{
  public static List<T> ToList<T>(this IEnumerable<T> collection)
  {
      return new List<T>(collection);
  }

  public static BindingList<T> ToBindingList<T>(this IEnumerable<T> collection)
  {
      return new BindingList<T>(collection.ToList());
  }
}

score 0 · Answer 3 · answered Jun 21 '11 at 09:38

You have not actually created generic methods you have declared non geeneric methods that return List<T> without defining T. You need to change as below:

public static class TExtensions
    {
        public static List<T> ToList<T>(this IEnumerable<T> collection)
        {
            return new List<T>(collection);
        }

        public static BindingList<T> ToBindingList<T>(this IEnumerable<T> collection)
        {
            return new BindingList<T>(collection.ToList());
        }
    }

Adding generic extension methods to interfaces like IEnumerable

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