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?? Operator (C# Reference)

The ?? operator is called the null-coalescing operator and is used to define a default value for a nullable value types as well as reference types. It returns the left-hand operand if it is not null; otherwise it returns the right operand.

A nullable type can contain a value, or it can be undefined. The ?? operator defines the default value to be returned when a nullable type is assigned to a non-nullable type. If you try to assign a nullable value type to a non-nullable value type without using the ?? operator, you will generate a compile-time error. If you use a cast, and the nullable value type is currently undefined, an InvalidOperationException exception will be thrown.

For more information, see Nullable Types (C# Programming Guide).

The result of a ?? operator is not considered to be a constant even if both its arguments are constants.

class NullCoalesce
    static int? GetNullableInt()
        return null;

    static string GetStringValue()
        return null;

    static void Main()
        // ?? 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");