What is the "??" operator for?

2018-06-19 02:38:39

This question already has an answer here:

What do two question marks together mean in C#? 16 answers

It's called the "null coalescing operator" and works something like this:

Instead of doing:

int? number = null;
int result = number == null ? 0 : number;

You can now just do:

int result = number ?? 0;

It's the null coalescing operator. It was introduced in C# 2.

The result of the expression a ?? b a ?? b is a if that's not null, or b otherwise. b isn't evaluated unless it's needed.

Two nice things:

The overall type of the expression is that of the second operand, which is important when you're using nullable value types:

int? maybe = ...;
int definitely = maybe ?? 10;

(Note that you can't use a non-nullable value type as the first operand - it would be pointless.)

The associativity rules mean you can chain this really easily. For example:

string address = shippingAddress ?? billingAddress ?? contactAddress;

That will use the first non-null value out of the shipping, billing or contact address.

That is the coalesce operator. It essentially is shorthand for the following

x ?? new Student();
x != null ? x : new Student();

MSDN Documentation on the operator

http://msdn.microsoft.com/en-us/library/ms173224.aspx

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