When a dll or exe file is signed with strong name keys, .NET doesn't allow you to modify it. If you try to use a signed assembly, after you have made some changes with Hex editor or any other tool, you'll get the following exception:
An unhandled exception of type 'System.IO.FileLoadException' occurred in system.windows.forms.dll Additional information: Strong name validation failed for assembly 'MyAssembly.Test'.
If you try to run a signed executable file after you have made some changes in it, you'll get the following error message:
String name validation failed for assembly... The file may have been tampered with or it was partially signed but not fully signed with the correct private key.