What do two question marks together mean in C#?

It's the null coalescing operator, and quite like the ternary (immediate-if) operator. See also ?? Operator - MSDN.

FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();

expands to:

FormsAuth = formsAuth != null ? formsAuth : new FormsAuthenticationWrapper();

which further expands to:

if(formsAuth != null)
    FormsAuth = formsAuth;
else
    FormsAuth = new FormsAuthenticationWrapper();

In English, it means "If whatever is to the left is not null, use that, otherwise use what's to the right."

Note that you can use any number of these in sequence. The following statement will assign the first non-null Answer# to Answer (if all Answers are null then the Answer is null):

string Answer = Answer1 ?? Answer2 ?? Answer3 ?? Answer4;

Also it's worth mentioning while the expansion above is conceptually equivalent, the result of each expression is only evaluated once. This is important if for example an expression is a method call with side effects. (Credit to @Joey for pointing this out.)

Just because no-one else has said the magic words yet: it's the null coalescing operator. It's defined in section 7.12 of the C# 3.0 language specification.

It's very handy, particularly because of the way it works when it's used multiple times in an expression. An expression of the form:

a ?? b ?? c ?? d

will give the result of expression a if it's non-null, otherwise try b, otherwise try c, otherwise try d. It short-circuits at every point.

Also, if the type of d is non-nullable, the type of the whole expression is non-nullable too.

It's the null coalescing operator.

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

Yes, nearly impossible to search for unless you know what it's called! :-)

EDIT: And this is a cool feature from another question. You can chain them.

Hidden Features of C#?

Thanks everybody, here is the most succinct explanation I found on the MSDN site:

// y = x, unless x is null, in which case y = -1.
int y = x ?? -1;

?? is there to provide a value for a nullable type when the value is null. So, if formsAuth is null, it will return new FormsAuthenticationWrapper().

It's short hand for the ternary operator.

FormsAuth = (formsAuth != null) ? formsAuth : new FormsAuthenticationWrapper();

Or for those who don't do ternary:

if (formsAuth != null)
{
  FormsAuth = formsAuth;
}
else
{
  FormsAuth = new FormsAuthenticationWrapper();
}

If you're familiar with Ruby, its ||= seems akin to C#'s ?? to me. Here's some Ruby:

irb(main):001:0> str1 = nil
=> nil
irb(main):002:0> str1 ||= "new value"
=> "new value"
irb(main):003:0> str2 = "old value"
=> "old value"
irb(main):004:0> str2 ||= "another new value"
=> "old value"
irb(main):005:0> str1
=> "new value"
irb(main):006:0> str2
=> "old value"

And in C#:

string str1 = null;
str1 = str1 ?? "new value";
string str2 = "old value";
str2 = str2 ?? "another new value";

coalescing operator

it's equivalent to

FormsAuth = formsAUth == null ? new FormsAuthenticationWrapper() : formsAuth

For your amusement only (knowing you are all C# guys ;-).

I think it originated in Smalltalk, where it has been around for many years. It is defined there as:

in Object:

? anArgument
    ^ self

in UndefinedObject (aka nil's class):

? anArgument
    ^ anArgument

There are both evaluating (?) and non-evaluating versions (??) of this.
It is often found in getter-methods for lazy-initialized private (instance) variables, which are left nil until really needed.

Others have described the Null Coalescing Operator quite well. In cases where a single test for null is required, the shortened syntax ??= can add readability.

Legacy null test:

if (myvariable == null)
{
    myvariable = new MyConstructor();
}

Using the Null Coalescing Operator this can be written:

myvariable = myvariable ?? new MyConstructor();

which can also be written with the shortened syntax:

myvariable ??= new MyConstructor();

Some find it more readable and succinct.

In simplest way, two question marks are called "Coalescing Operator", which returns first non null value from the chain.

e.g if you are getting a values from a nullable object, in a variable which is not nullable, then you can use this operator. i.e.

int a = 1;
int? b = null;
a = b??0;

Result of above equation would be zero, because b is null and we have used the ?? operator along with zero, which means that it will return 0 if and only if b is null.

int a = 1;
int? b = 15;
a = b??0;

In above equation, a will get value "15" because b has a valid value and is not null. Also, you can not use ?? operator on a non nullable object.

In above examples, I used ?? 0, however a complete new equation can also be used after ?? operator.

Such as
a = b ?? ( x==1 ? 10 : 15)
I hope it will clear your question.

The ?? operator is called the null-coalescing operator. It returns the left-hand operand if the operand is not null; otherwise it returns the right hand operand.

int? variable1 = null;
int variable2  = variable1 ?? 100;

Set variable2 to the value of variable1, if variable1 is NOT null; otherwise, if variable1 == null, set variable2 to 100.