The Null Coalescing Operator (??)

Introduction

I'm constantly surprised by the number of developers who aren't aware of this handy piece of syntax. It's my favourite thing to come out of C# 2.0 and no developer should be without it.

Like the conditional (?:) operator's big brother... introducing it for your coding pleasure...

The Null Coalescing Operator (??)

The null-coalescing-operator is a brilliant new terse operator that provides syntax for beautifully concise if statements. Essentially, it returns the left-hand-side of the ?? operator, unless null, in which case it executes and returns the right-hand-side of the operator. This may be a statement or a variable reference. Let's jump straight to some examples:

// used inline outputs the value foo or if null returns Undefined
Console.WriteLine("The value of foo is " + (foo ?? "Undefined") + ".");
    
Input:  foo = "24";
Output: The value of foo is 24.
    
Input:  foo = null;
Output: The value of foo is Undefined.

The operator is right-associative meaning statements can be chained together; thus returning the first non-null instance:

// assigns foo to the first non-null instance, else returns Undefined
string foo = foo1 ?? foo2 ?? foo3 ?? foo4 ?? "Undefined";
    
Console.WriteLine("The value of foo is " + foo + ".");

Input:  foo1 = null;
        foo2 = null;
        foo3 = null;
        foo4 = null;
Output: The value of foo is Undefined.
    
Input:  foo1 = null;
        foo2 = "foo2";
        foo3 = null;
        foo4 = "foo4";
Output: The value of foo is foo2.

Handling null ViewState references:

// try to assign ViewState value as an int, else if null assign 123
int foo = (int?)ViewState["foo"] ?? 123;
    
Response.Write("The value of foo is " + foo + ".");
    
Input:  ViewState["foo"]=1;
Output:  The value of foo is 1.  
    
Input:  ViewState["foo"]=null;
Output:  The value of foo is 123.

And my personal favorite, on demand field instantiation:

private IList<string> foo;

public IList<string> Foo
{
    get
    {
        return foo ?? (foo = new List<string>());
    }
}

Here's an interesting example derived from an idea in the discussions below. It shows how an operator override can be used within an object's definition to enable shorthand syntax for double-null checking. The scenario is checking an object property for null using a null-coalescing-operator, but also defaulting when null-object-reference occurs; which would normally cause a runtime exception. (Note that I don't recommend actually using this approach, I just thought it made an interesting example.)

public class Address
{
    private static Address Empty = new Address();
    public string StreetName = null;
    
    public static Address operator +(Address address)
    {
        return address ?? Empty
    }
}
  
Console.WriteLine("The street name is "+ (+address).StreetName ?? "n/a" + ".");
  
Input:  address = new Address();
Output:  The street name is n/a.  
    
Input:  address = new Address();
            address.StreetName = "Regent St";
Output:  The street name is Regent St.    
    
Input:  address = null;
Output:  The street name is n/a.

The Rules

To use the null-coalescing-operator, there are some compile-time ground rules.

The left-hand-side must evaluate to a reference or nullable type.
All evaluated statements must be of matching type, unless they can be implicitly converted.

Summary

As you can see from the examples above, this little gem is very powerful and the possibilities are endless. Of course the benefits are purely syntactical, but it helps keep the code clean and easier to follow. I hope you enjoy it as much as I do.