Curious null-coalescing operator custom implicit conversion behaviour

This question arose when writing my answer to this one , which talks about the associativity of the null-coalescing operator .

Just as a reminder, the idea of the null-coalescing operator is that an expression of the form

x ?? y

first evaluates x , then:

• If the value of x is null, y is evaluated and that is the end result of the expression
  


• If the value of x is non-null, y is not evaluated, and the value of x is the end result of the expression, after a conversion to the compile-time type of y if necessary
  

Now usually there's no need for a conversion, or it's just from a nullable type to a non-nullable one - usually the types are the same, or just from (say) int? to int . However, you can create your own implicit conversion operators, and those are used where necessary.

For the simple case of x ?? y , I haven't seen any odd behaviour. However, with (x ?? y) ?? z I see some confusing behaviour.

Here's a short but complete test program - the results are in the comments:

using System;



public struct A

{

    public static implicit operator B(A input)

    {

        Console.WriteLine("A to B");

        return new B();

    }



    public static implicit operator C(A input)

    {

        Console.WriteLine("A to C");

        return new C();

    }

}



public struct B

{

    public static implicit operator C(B input)

    {

        Console.WriteLine("B to C");

        return new C();

    }

}



public struct C {}



class Test

{

    static void Main()

    {

        A? x = new A();

        B? y = new B();

        C? z = new C();

        C zNotNull = new C();



        Console.WriteLine("First case");

        // This prints

        // A to B

        // A to B

        // B to C

        C? first = (x ?? y) ?? z;



        Console.WriteLine("Second case");

        // This prints

        // A to B

        // B to C

        var tmp = x ?? y;

        C? second = tmp ?? z;



        Console.WriteLine("Third case");

        // This prints

        // A to B

        // B to C

        C? third = (x ?? y) ?? zNotNull;

    }

}

So we have three custom value types, A , B and C , with conversions from A to B, A to C, and B to C.

I can understand both the second case and the third case... but why is there an extra A to B conversion in the first case? In particular, I'd really have expected the first case and second case to be the same thing - it's just extracting an expression into a local variable, after all.

Any takers on what's going on? I'm extremely hesistant to cry "bug" when it comes to the C# compiler, but I'm stumped as to what's going on...

EDIT: Okay, here's a nastier example of what's going on, thanks to configurator's answer, which gives me further reason to think it's a bug. EDIT: The sample doesn't even need two null-coalescing operators now...

using System;



public struct A

{

    public static implicit operator int(A input)

    {

        Console.WriteLine("A to int");

        return 10;

    }

}



class Test

{

    static A? Foo()

    {

        Console.WriteLine("Foo() called");

        return new A();

    }



    static void Main()

    {

        int? y = 10;



        int? result = Foo() ?? y;

    }

}

The output of this is:

Foo() called

Foo() called

A to int

The fact that Foo() gets called twice here is hugely surprising to me - I can't see any reason for the expression to be evaluated twice.

Source: Tips4all, CCNA FINAL EXAM