C# overloading with generics: bug or feature?

Question

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?

Answer 1

My guess is that the compiler chooses the best match before it uses the generic constraint. In your example the method with the constraint is preferable because it doesn't have a params last parameter.

Edit - Eric Lippert confirms this in his answer.