Unboxing in C#

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 that 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 previous 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 causes a NullReferenceException. Attempting to unbox a reference to an incompatible value type causes an InvalidCastException.