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?: Operator (C# Reference) 

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

condition ? first_expression : second_expression;

Remarks

If condition is true, first expression is evaluated and becomes the result; if false, the second expression is evaluated and becomes the result. Only one of two expressions is ever evaluated.

Calculations that might otherwise require an if-else construction can be expressed more concisely and elegantly with the conditional operator. For example, to avoid a division by zero in the calculation of the sin function you could write either

if(x != 0.0) s = Math.Sin(x)/x; else s = 1.0;

or, using the conditional operator,

s = x != 0.0 ? Math.Sin(x)/x : 1.0;

The conditional operator is right-associative, so an expression of the form

a ? b : c ? d : e

is evaluated as

a ? b : (c ? d : e)

not

(a ? b : c) ? d : e

The conditional operator cannot be overloaded.

Example

// cs_operator_conditional.cs
using System;
class MainClass
{
    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

See Also

Reference

C# Operators
if-else (C# Reference)

Concepts

C# Programming Guide

Other Resources

C# Reference