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Unboxing Conversion

Visual Studio .NET 2003

Unboxing is an explicit conversion from the type object to a value type or from an interface type to a value type that implements the interface. An unboxing operation consists of:

  • Checking the object instance to make sure it is a boxed value of the given value type.
  • Copying the value from the instance into the value-type variable.

The following statements demonstrate both boxing and unboxing operations:

int i = 123;          // A value type
object box = i;       // Boxing
int j = (int)box;     // Unboxing

The following figure demonstrates the result of the preceding statements.

Unboxing Conversion

For an unboxing conversion to a given value type to succeed at run time, the value of the source argument must be a reference to an object that was previously created by boxing a value of that value type. If the source argument is null or a reference to an incompatible object, an InvalidCastException is thrown.

Example

The following example demonstrates a case of invalid unboxing, of how incorrect unboxing leads to InvalidCastException. By using try and catch, an error message is displayed when the error occurs.

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

      // Boxing
      object o = intI;

      // Reference to incompatible object produces InvalidCastException
      try 
      {
         int intJ = (short) o;
         Console.WriteLine("Unboxing OK.");
      }

      catch (InvalidCastException e) 
      {
         Console.WriteLine("{0} Error: Incorrect unboxing.",e);
      }
   }
}

Output

System.InvalidCastException
   at UnboxingTest.Main() Error: Incorrect unboxing.

If you change the statement:

int intJ = (short) o;

to:

int intJ = (int) o;

the conversion will be performed, and you will get the output Unboxing OK.

See Also

Boxing and Unboxing | Boxing Conversion

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