【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# unified 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只能对应同一类型，否则出错，见下例：

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