I came across a very interesting operator today in C#.
At first I thought it was like a ternary operator but it turned out not to be. The operator "??", yes two question marks.
It is known as the "nullable type", "null-coalescing" or just the double question mark operator.
I've only come accross it in C# and according to the Microsoft, help page:
"The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand."
The remarks went on to say:<!-- more -->
"A nullable type can contain a value, or it can be undefined. The ?? operator defines the default value to be returned when a nullable type is assigned to a non-nullable type. If you try to assign a nullable type to a non-nullable type without using the ?? operator, you will generate a compile-time error. If you use a cast, and the nullable type is currently undefined, an InvalidOperationException exception will be thrown."
While its not very common, it can offer very tricky questions in interviews as a friend of mine recently found out.
Some example code of using it, (taken from the help page again):
// nullable_type_operator.cs
using System;
class MainClass
{
static int? GetNullableInt()
{
return null;
}
static string GetStringValue()
{
return null;
}
static void Main()
{
// ?? operator example.
int? x = null;
// y = x, unless x is null, in which case y = -1.
int y = x ?? -1;
// Assign i to return value of method, unless
// return value is null, in which case assign
// default value of int to i.
int i = GetNullableInt() ?? default(int);
string s = GetStringValue();
// ?? also works with reference types.
// Display contents of s, unless s is null,
// in which case display "Unspecified".
Console.WriteLine(s ?? "Unspecified");
}
}
Further reading (References)
http://msdn.microsoft.com/en-us/library/ms173224(VS.80).aspx
http://msdn.microsoft.com/en-us/library/ms173224.aspx
http://www.superjason.com/archive/2007/06/21/the-little-known-c-sharp-question-mark-operator.aspx