You can covert a .NET assembly (DLL or EXE) into its IL using the ILDASM (ildasm.exe) utility. This is usually installed in: C:\Program Files\Microsoft Visual Studio .NET\FrameworkSDK\Bin.

If you want to run this utility via command line, it would help if you add the above path to your Environment Path variable.

Here is a sample command line that will disassemble an assembly:

ildasm MyFile.exe /output:MyFile.il 

You can reassemble the above IL after making some minor changes via the ILASM utility. This is usually done to make some minor changes to an assembly like modifying the version number of another assembly to which it links.

Contributed from George Shepherd's Windows Forms FAQ