now you can peek into .net framework code by using one of the most popular decompilers ".Net Reflector",it's a mzaing to use it,u can use it for any .net namespace or any .net dll.
below is the download link :
note : this reflector was released newly,but for people who starting decompile .net framework code since 2002..there's another decompiler called "Anakrino" .
it's good but in some times it throw an exception for some .net dll code.
Enjoy......decompile .net code.................:)