How to get the type of T from a member of a generic class or method?

Question

Let say I have a generic member in a class or method, so:

public class Foo
{
    public List Bar { get; set; }

    public void Baz()
    {         


        

Answer 1

You can get the type of "T" from any collection type that implements IEnumerable with the following:

public static Type GetCollectionItemType(Type collectionType)
{
    var types = collectionType.GetInterfaces()
        .Where(x => x.IsGenericType 
            && x.GetGenericTypeDefinition() == typeof(IEnumerable<>))
        .ToArray();
    // Only support collections that implement IEnumerable once.
    return types.Length == 1 ? types[0].GetGenericArguments()[0] : null;
}

Note that it doesn't support collection types that implement IEnumerable twice, e.g.

public class WierdCustomType : IEnumerable, IEnumerable { ... }

I suppose you could return an array of types if you needed to support this...

Also, you might also want to cache the result per collection type if you're doing this a lot (e.g. in a loop).