What does "??" do in C#?

2018-06-19 02:45:53

This question already has an answer here:

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

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;

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

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

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

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