How to patch .NET DLLs?

There is no need to decompile to IL and recompile, nor is there any need to remove the public key. You can just patch the assembly directly.

Use ILdasm to determine the hex sequence that is to be patched and use a hex editor to search for the unique occurrences of those bytes and modify the IL op-codes directly. Use one of the many MSIL op-code references for a listing of them.

If the assembly is strong named, then you will need to patch that as well.
Yes for a winforms assembly the patching of the size of the strong name field in the COR header will do the trick, but for an asp.net assembly you will need to also patch the strong named attribute which is stored as meta data before the RAS key.

Also note that if there are other strong name assemblies referenced, they may also be required to be patched because strong named assemblies need to call other strong named assemblies.

Additionally if the assembly does some self checking this may also require patching, however it is very rarely implemented.

ZD