Possible to override null-coalescing operator?

Question

Is it possible to override the null-coalescing operator for a class in C#?

Say for example I want to return a default value if an instance is null and return the instance if it's not. The code would look like something like this:

   return instance ?? new MyClass("Default");

But what if I would like to use the null-coalescing operator to also check if the MyClass.MyValue is set?

Of course there is no real need for this (at least I think so) - so before you answer "why would you want to do that" - I am just curious if it's possible.

Answer 1

Simple answer: No

C# design principles do not allow operator overloading that change semantics of the language. Therefore complex operators such as compound assignment, ternary operator and ... can not be overloaded.

Answer 2

Good question! It's not listed one way or another in the list of overloadable and non-overloadable operators and nothing's mentioned on the operator's page.

So I tried the following:

public class TestClass
{
    public static TestClass operator ??(TestClass  test1, TestClass test2)
    {
        return test1;
    }
}

and I get the error "Overloadable binary operator expected". So I'd say the answer is, as of .NET 3.5, a no.

Answer 3

According to ECMA-334, is is not possible to overload the ?? operator. Similarly, you cannot overload the following operators:

• =
• &&
• ||
• ?:
• checked
• unchecked
• new
• typeof
• as
• is

Answer 4

If anyone is here looking for a solution, the closest example would be to do this

return instance.MyValue != null ? instance : new MyClass("Default");