This is not a good and safe practice to call a Dispose. 
In "Dot Net" it is automatically handle by Garbage 
collection. It is the responsibility of GC to call the 
Dispose and clear the memory.. but it is not sure that when 
GC will call the Dispose.
 
so in that case we can forcefuly call it.
 
Consider one case when we forcefully destroy the object and 
after some time i will try to use the same object at that 
time u will get the error msg.
 
so better to object = null;

other material:
http://blogs.msdn.com/irenak/archive/2006/10/05/SYSK-
212_3A00_--Does-.NET-Garbage-Collector-call-Dispose_3F00_--
Dispose-_1320_-Best-Practices_2E00_.aspx

obj.dispose has to be used and not obj=null, bcoz JIT 
compiler is optimizing compiler, and setting a local 
variable or parameter to null is the same as not 
referencing the variable at all.