Few new operators have been introduced in C#2.0. Null Coalescing operator, which is one of them, is discussed in this post. While coding, we frequently need to perform null checks as follows–

Contact contact = provider.GetContact(contactId);
if (contact == null)
{
   contact = new Contact();
}

Instead of using if block we could just write it –

Contact contact = provider.GetContact(contactId);
contact = contact ?? new Contact();

It is pretty useful in the context of nullable types. It can also be handy while converting nullable type to value type, as shown below–

int? nullableInt = null;
int valueInt = nullableInt ?? default(int);

The valueInt become value type after executing this statement. But if we write something like this–

int? nullableIntAgain = nullableInt ?? 5;

the int literal 5 is automatically converted to a nullable type by CLR and afterwards, being assigned to nullableIntAgain. Subsequently, we can assign it to null now–

nullableIntAgain = null;

Isn’t pretty slick and handy? However, when using this operator we need to keep in mind that–

Null coalescing operator can only be used in nullable or reference type. Otherwise it will raise an InvalidOperationException.

We need to consider thread-safety also. Otherwise it could end up in race condition.

Hope it helps. Thanks!

Revisions

[R-1: 29-03-2013] Updated formatting to make this post more consistent with current CSS.