C#6's Improved overload resolution - clarification?

Question

Among all the new features in C#6, the most mysterious feature (to me) is the \"improved overload resolution\".

Maybe it\'s because I couldn\'t find related

Answer 1

I believe what is meant here is the "better betterness" rules which are documented in the Roslyn github repo.

Sample code:

using System;

class Test
{
    static void Foo(Action action) {}
    static void Foo(Func<int> func) {}
    static int Bar() { return 1; }

    static void Main()
    {
        Foo(Bar);        
    }
}

Using the C# 5 compiler (e.g. in c:\Windows\Microsoft.NET\Framework\v4.0.30319\) this gives two errors:

Test.cs(11,9): error CS0121: The call is ambiguous between the following methods or properties:
     'Test.Foo(System.Action)' and 'Test.Foo(System.Func)'
Test.cs(11,13): error CS0407: 'int Test.Bar()' has the wrong return type

Using the C# 6 compiler, it compiles fine.

Likewise using exact matching for lambda expressions, this generates an ambiguous overload error with the C# 5 compiler, but not for C# 6:

using System;

class Test
{
    static void Foo(Func<Func<long>> func) {}
    static void Foo(Func<Func<int>> func) {}

    static void Main()
    {
        Foo(() => () => 7);
    }
}