Q:

C# overloading with generics: bug or feature?

Let's have a following simplified example:
void Foo<T>(IEnumerable<T> collection, params T[] items) 
{
    // ...
}

void Foo<C, T>(C collection, T item)
    where C : ICollection<T>
{
    // ...
}

void Main()
{
    Foo((IEnumerable<int>)new[] { 1 }, 2);
}

Compiler says:

The type 'System.Collections.Generic.IEnumerable' cannot be used as type parameter 'C' in the generic type or method 'UserQuery.Foo(C, T)'. There is no implicit reference conversion from 'System.Collections.Generic.IEnumerable' to 'System.Collections.Generic.ICollection'.

If I change Main to:
void Main()
{
    Foo<int>((IEnumerable<int>)new[] { 1 }, 2);
}

It will work ok. Why compiler does not choose the right overload?

A:

Your question is answered here.
http://blogs.msdn.com/ericlippert/archive/2009/12/10/constraints-are-not-part-of-the-signature.aspx
Please also read the approximately one million comments telling me that I am wrong for some interesting additional commentary on this issue.

