?: Operator (C# Reference)

 

Updated: July 20, 2015

For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

The conditional operator (?:) returns one of two values depending on the value of a Boolean expression. Following is the syntax for the conditional operator.

condition ? first_expression : second_expression;  

The condition must evaluate to true or false. If condition is true, first_expression is evaluated and becomes the result. If condition is false, second_expression is evaluated and becomes the result. Only one of the two expressions is evaluated.

Either the type of first_expression and second_expression must be the same, or an implicit conversion must exist from one type to the other.

You can express calculations that might otherwise require an if-else construction more concisely by using the conditional operator. For example, the following code uses first an if statement and then a conditional operator to classify an integer as positive or negative.

  
int input = Convert.ToInt32(Console.ReadLine());  
string classify;  
  
// if-else construction.  
if (input > 0)  
    classify = "positive";  
else  
    classify = "negative";  
  
// ?: conditional operator.  
classify = (input > 0) ? "positive" : "negative";  
  

The conditional operator is right-associative. The expression a ? b : c ? d : e is evaluated as a ? b : (c ? d : e), not as (a ? b : c) ? d : e.

The conditional operator cannot be overloaded.

    class ConditionalOp
    {
        static double sinc(double x)
        {
            return x != 0.0 ? Math.Sin(x) / x : 1.0;
        }

        static void Main()
        {
            Console.WriteLine(sinc(0.2));
            Console.WriteLine(sinc(0.1));
            Console.WriteLine(sinc(0.0));
        }
    }
    /*
    Output:
    0.993346653975306
    0.998334166468282
    1
    */

C# Reference
C# Programming Guide
C# Operators
if-else
?. and ?Operators
?? Operator

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