Boxing is an implicit conversion of a value type to the type Object. Boxing a value allocates an instance of Object and copies the value into the new object instance, as shown in Figure 11-4.

Figure 11-4. Boxing value types

Boxing is implicit when you provide a value type where a reference is expected. The runtime notices that you’ve provided a value type and silently boxes it within an object. You can, of course, first cast the value type to a reference type, as in the following:

    int myIntegerValue = 5;
    object myObject = myIntegerValue; // cast to an object
    myObject.ToString();

This is not necessary, however, as the compiler boxes the value for you silently and with no action on your part:

    int myIntegerValue = 5;
    myIntegerValue.ToString(); // myIntegerValue is boxed

To return the boxed object back to a value type, you must explicitly unbox it. For the unboxing to succeed, the object being unboxed must really be of the type you indicate when you unbox it.

You should accomplish unboxing in two steps:

Example 11-5 illustrates boxing and unboxing.

using System;
public class UnboxingTest
{
   public static void Main(  )
   {
      int myIntegerVariable = 123;

      //Boxing
      object myObjectVariable = myIntegerVariable;
      Console.WriteLine( "myObjectVariable: {0}",
      myObjectVariable.ToString(  ) );

      // unboxing (must be explicit)
      int anotherIntegerVariable = (int)myObjectVariable;
      Console.WriteLine( "anotherIntegerVariable: {0}",
      anotherIntegerVariable );
   }
}

Output:
myObjectVariable: 123
anotherIntegerVariable: 123

Figure 11-5 illustrates unboxing.

Example 11-5 creates an integer myIntegerVariable and implicitly boxes it when it is assigned to the object myObjectVariable; then, to exercise the newly boxed object, its value is displayed by calling ToString( ).

The object is then explicitly unboxed and assigned to a new integer variable, anotherIntegerVariable, whose value is displayed to show that the value has been preserved.

Figure 11-5. Unboxing

The most common place that value types were boxed in C# 1.x was in collections that expected Objects. Now that C# supports generics, collections that hold integers need not box and unbox them, and that can increase performance when you have a very large collection. Generics are discussed in more detail in the Chapter 14.