Possible to overload null-coalescing operator?

问题:

Is it possible to overload 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?

回答1:

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.

回答2:

According to the ECMA-334 standard, it is not possible to overload the ?? operator.

Similarly, you cannot overload the following operators:

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

回答3:

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.

回答4:

This is rumored to be part of the next version of C#. From http://damieng.com/blog/2013/12/09/probable-c-6-0-features-illustrated

7. Monadic null checking

Removes the need to check for nulls before accessing properties or methods. Known as the Safe Navigation Operator in Groovy).

Before

if (points != null) {
    var next = points.FirstOrDefault();
    if (next != null && next.X != null) return next.X;
}   
return -1;

After

var bestValue = points?.FirstOrDefault()?.X ?? -1;

回答5:

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

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