If the left operand to the ?? operator is not null, does the right operand get evaluated?

Question

I'm looking at using the ?? operator (null-coalescing operator) in C#. But the documentation at MSDN is limited.

My question: If the left-hand operand is not null, does the right-hand operand ever get evaluated?

Answer 1

As ever, the C# specification is the best place to go for this sort of thing.

From section 7.13 of the C# 5 specification (emphasis mine):

A null coalescing expression of the form a ?? b a ?? b requires a to be of a nullable type or reference type. If a is non-null, the result of a ?? b a ?? b is a ; otherwise, the result is b . The operation evaluates b only if a is null.

There are more details around when any conversions are performed, and the exact behaviour, but that's the main point given your question. It's also worth noting that the null-coalescing operator is right-associative, so a ?? b ?? c a ?? b ?? c a ?? b ?? c is evaluated as a ?? (b ?? c) a ?? (b ?? c) ... which means it will only evaluate c if both a and b are null.

Answer 2

Why not test it :)

void Main()
{
    var leftOrRight = left ?? right;
}

public bool? left 
{
    get
    {
        Console.WriteLine ("Left hit");
        return true;
    }
}

public bool right 
{
    get
    {
        Console.WriteLine ("Right hit");
        return true;
    }   
}

And the answer to the question is no ... The second value doesn't get evaluated

Answer 3

From the specification:

A null coalescing expression of the form a ?? b requires a to be of a nullable type or reference type. If a is non-null, the result of a ?? b is a; otherwise, the result is b. The operation evaluates b only if a is null.

(Emphasis added)

Answer 4

Check this out. If name is not null, console show "MyName". If it's null, console shows "Called" then "Allo"

using System;           
public class Program
{
    public static void Main()
    {
        string name = "MyName";

        Console.WriteLine(name??test());
    }

    private static string test()
    {
        Console.WriteLine("Called");
        return "Allo";
    }
}