C# 0 The Complete Reference

balance

 is 

null

. This causes 

currentBalance

 to be 

assigned the value 0.0 and no exception will be thrown.

double? balance = null;

double currentBalance;

currentBalance = balance ?? 0.0;

In the next sequence, 

balance

 is given the value 123.75:

double? balance = 123.75;

double currentBalance;

currentBalance = balance ?? 0.0;

Now, 

currentBalance

 will contain the value of 

balance

, which is 123.75.

One other point: The right-hand expression of the 

??

 is evaluated only if the left-hand 

expression does not contain a value. The following program demonstrates this fact:

// Using ??

using System;

class NullableDemo2 {

www.freepdf-books.com

---

602

 

P a r t   I :  

T h e   C #   L a n g u a g e

  // Return a zero balance.

  static double GetZeroBal() {

    Console.WriteLine("In GetZeroBal().");

      return 0.0;

  }

  static void Main() {

    double? balance = 123.75;

    double currentBalance;

    // Here, GetZeroBal( ) is not called because balance

    // contains a value.

    currentBalance = balance ?? GetZeroBal();

    Console.WriteLine(currentBalance);

  }

}

In this program, the method