what does double question marks mean in c#

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What is the “??” operator for?

Debugging some code and found ?? inside the code. What does this mean?

This question is asked more than ten times in stackoverflow itself. Some of them are here. 1. http://stackoverflow.com/questions/827454/what-is-the-operator-for 2. http://stackoverflow.com/questions/3925726/coalesce-operator-in-c 3. http://stackoverflow.com/questions/770096/what-does-mean. — Bipul, Jan 31 '11 at 07:07

Mitch Wheat · Answer 1 · 2011-01-31T06:22:49.807

19

?? is the null-coalescing operator for nullable types.

object obj = canBeNull ?? alternative;

// equivalent to:
object obj = canBeNull != null ? canBeNull : alternative;

edited Jan 31 '11 at 06:22

answered Jan 31 '11 at 06:17

Mitch Wheat

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score 5 · Answer 2 · edited Mar 22 '12 at 10:05

http://msdn.microsoft.com/en-us/library/ms173224.aspx refer to this for description. it's an operator

The ?? operator defines the default value to be returned when a nullable type is assigned to a non-nullable type.

    // ?? operator example.
    int x = null;

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

    // Assign i to return value of method, unless
    // return value is null, in which case assign
    // default value of int to i.
    int i = GetNullableInt() ?? default(int);

    string s = GetStringValue();
    // ?? also works with reference types. 
    // Display contents of s, unless s is null, 
    // in which case display "Unspecified".
    Console.WriteLine(s ?? "Unspecified");

what does double question marks mean in c#

2 Answers2