using System;

class P
{
    static void Main()
    {
        Foo(x => x.X() as object); // error CS0121: The call is ambiguous between the following methods or properties: 'P.Foo(System.Func<A,object>)' and 'P.Foo(System.Func<B,object>)'
    }

    static extern void Foo(Func<A, object> x);
    static extern void Foo(Func<B, object> x);

}

class A
{
    public Func<int> X { get; set; }
}

class B
{
    public extern string X();
}