Assume I have a .NET assembly which is strong named. Only I have access to the private key. I then distribute the assembly to some client system.
How hard is it for the client to modify the assembly? Ie: what would they need to do to modify my assembly?
Strong-naming does not prevent modifying the assembly, but it does prevent other applications which reference a strong-named assembly from inadvertently using a modified version.
It's no different from modifying a non-strongly typed assembly. The only real difference is that they would have to run the strong name utility (sn.exe) in order to use the modified assembly.
As others have said, its very easy.
One technique you can use is to use the public key (or token) of your assembly to encrypt important information (such as algorithm parameters, connection strings, etc) in your assembly. This way if the public key has been changed or removed, the decryption will fail and your assembly would no longer run correctly. Obfuscators such as Crypto Obfuscator use this technique as one part of the protection.