Unboxing Conversion (C# Programming Guide) 

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 o = i;     // boxing
int j = (int) o;  // unboxing

The following figure demonstrates the result of the preceding statements.

Unboxing Conversion

For the unboxing of value types to succeed at run time, the item being unboxed must be a reference to an object that was previously created by boxing an instance of that value type. Attempting to unbox null or a reference to an incompatible value type will result in an InvalidCastException.

Example

The following example demonstrates a case of invalid unboxing and the resulting InvalidCastException. Using try and catch, an error message is displayed when the error occurs.

class TestUnboxing
{
    static void Main() 
    {
        int i = 123;
        object o = i;  // implicit boxing

        try 
        {
            int j = (short) o;  // attempt to unbox
            
            System.Console.WriteLine("Unboxing OK.");
        }
        catch (System.InvalidCastException e) 
        {
            System.Console.WriteLine("{0} Error: Incorrect unboxing.", e.Message);
        }
    }
}

Output

Specified cast is not valid. Error: Incorrect unboxing.

If you change the statement:

int j = (short) o;

to:

int j = (int) o;

the conversion will be performed, and you will get the output:

Unboxing OK.

See Also

Concepts

C# Programming Guide
Boxing and Unboxing (C# Programming Guide)
Boxing Conversion (C# Programming Guide)