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?: Operator (C# Reference)

Visual Studio 2010

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

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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.

C#
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int input = Convert.ToInt32(Console.ReadLine());
string classify;

// if-else construction.
if (input < 0)
    classify = "negative";
else
    classify = "positive";

// ?: conditional operator.
classify = (input < 0) ? "negative" : "positive";

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.

C#
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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
*/
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Nullable Types
If you are using this operator with a nullable type and want to be able to use null explicitly, you have to cast it as the nullable type.

Lets say myClass has an integer member id:

int? id = (myClass == null) ? null : myClass.MemberID;

This will give an error. "Type of conditional expression cannot be determined because there is no implicit conversion between '<null>' and 'int'"

This will work:
int? id = (myClass == null) ? (int?)null : myClass.MemberID;

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