What does "??" do in C#?

Question

I found this new and interesting code in my project. What does it do, and how does it work?

MemoryStream stream = null;
MemoryStream st = stream ?? new MemoryStream();

?? operator chaekcs if stream is null if null then creates the new memorystream using new keyword . — Mahesh, Jul 31 '13 at 05:27
it is amazing how lazy peopele can be. Google query "?? operator" gives you an answer on first hit. Seriously. — Nikita B, Jul 31 '13 at 05:33

score 8 · Accepted Answer · answered Jul 31 '13 at 05:28

8

A ?? B

is a shorthand for

if (A == null) 
    B
else 
    A

or more precisely

A == null ? B : A

so in the most verbose expansion, your code is equivalent to:

MemoryStream st;
if(stream == null)
    st = new MemoryStream();
else
    st = stream;

answered Jul 31 '13 at 05:28

Sina Iravanian

score 1 · Answer 2 · answered Jul 31 '13 at 05:27

1

Basically it means if MemoryStream stream equals null, create MemoryStream st = new MemoryStream();

so in this case the following:

MemoryStream st = stream ?? new MemoryStream();

means

MemoryStream st;

if (stream == null)
   st = new MemoryStream();
else 
   st = stream;

It's called a null coelesce operator. More info here: http://msdn.microsoft.com/en-us/library/ms173224.aspx

answered Jul 31 '13 at 05:27

mnsr

score 0 · Answer 3 · answered Jul 31 '13 at 05:28

0

It's called a null coalesce operator. See here.

It means that if stream is null, it will create a new MemoryStream object.

answered Jul 31 '13 at 05:28

whastupduck

3 Answers3