Question

Why Extension methods do not use implicit conversions but static methods do? Can anybody explain with a proper example?

Thanks

Answer 1

Because the C# spec states:

An extension method Ci.Mj is eligible if:

· Ci is a non-generic, non-nested class

· The name of Mj is identifier

· Mj is accessible and applicable when applied to the
arguments as a static method as shown above

· An implicit identity, reference or boxing conversion exists
from expr to the type of the first parameter of Mj.

As far as the C# spec is concerned, a user-defined conversion operator is different than an implicit reference conversion, and certainly different than an identity or boxing conversion.

For a hint on why:

public static class Extensions
{
    public static void DoSomething(this Bar b)
    {
        Console.Out.WriteLine("Some bar");
    }

    public static void DoSomething(this Boo b)
    {
        Console.Out.WriteLine("Some boo");
    }
}

public class Foo
{
    public static implicit operator Bar(Foo f)
    {
        return new Bar();
    }
    public static implicit operator Boo(Foo f)
    {
        return new Boo();
    }
}

public class Bar { }
public class Boo { }

public class Application
{
    private Foo f;
    public void DoWork()
    {
        // What would you expect to happen here?
        f.DoSomething();

        // Incidentally, this doesn't compile either:
        Extensions.DoSomething(f);
    }
}

C# could not unambiguously choose which implicit conversion to execute.