What does the ?? operator do? [duplicate]

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What is the “??” operator for?

I have recently come across the ??开发者_运维技巧 operator in C#. What does this operator do and when would someone use it?

Example:

string name = nameVariable ?? string.Empty;

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The ?? operator basically means "or if it is null, blah". It is equivalent to:

string name = (nameVariable == null) ? string.Empty : nameVariable;

Which, if you're not familiar with the syntax, is basically:

string name;
if (nameVariable == null)
    name = string.Empty;
else
    name = nameVariable;

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It's a null-coalescing operator It will right part if the left one is null.

The interesting fact is that you can even use it like this:

string temp = (first ?? second).Text

and it will return Text property of the 'second' variable if 'first' is null.

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It has the catchy title of the Null Coalescing Operator. What it does is evaluate an expression and then if the expression is null it returns the right-hand operand, otherwise it returns the left-hand operand (ie. the original value).

Using your example as a basis you'd get these results:

string nameVariable = "Diplodocus";
string name = nameVariable ?? string.Empty;
// assigns name the value "Diplodocus"

And...

string nameVariable = null;
string name = nameVariable ?? string.Empty; 
// assigns name the value String.Empty;

Note you can use it with any reference or nullable type, not just strings.

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It is equivalent to checking for null and setting the value to something if the first one is. Your statement above is equivalent to:

string name = nameVariable == null ? string.Empty : nameVariable;

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It is a null reference check if nameVariable is null it would return an empty string.

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The expression

value1 ?? value2

returns value1 if value1 is a value different from null, and returns value2 if value1 equals null.