Boxing & unboxing

【Boxing & unboxing]

Boxing is the process of converting a value type to the type object or to any interface type implemented by this value type. when the CLR boxes a value type, it wraps the value inside a system. object and stores it on the managed heap. unboxing extracts the value type from the object. boxing is implicit; unboxing is explicit. the concept of boxing and unboxing underlies the C # uniied view of the type system in which a value of any type can be treated as an object.

int i = 123;// The following line boxes i.object o = i;  o = 123;i = (int)o;  // unboxing

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In relation to simple assignments, boxing and unboxing are computationally expensive processes. when a value type is boxed, a new object must be allocated and constructed. to a lesser degree, the cast required for unboxing is also expensive computationally. for more information, see performance.

Unboxing can only correspond to the same type; otherwise, an error occurs, as shown in the following example:

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);        }    }}

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Reference: http://msdn.microsoft.com/zh-cn/library/yz2be5wk.aspx