How to patch .NET DLLs?

[Newbie_Cracker]

Hi everybody.
Yesterday I tried to patch a component of ASP .NET. The DLL had a simple check routine. It tested a 30-day license key. If there was no license key, It would show a box in a design page with this message :
To remove this message, please, obtain a 30 day trial key.

I changed some br and brfalse and so on. But at re-compiling time, .NET compiler stoped and said signature not found or something like this.
What's the problem? What's wrong with the patched DLL?
I thought my changes was wrong. So, I changed the place of EXPIRED and NO LICENSE messages in the DLL, but the above failure was occured again.
Is there a signature stored in the DLL and .NET runtime libraries check them? Something like CRC?

One of my friends has seen some crashes in patched DLLs. But patched EXE files work fine.


Best regards.