Patches

Question

Dear All I am interested in learning more about software and program patches. How are they done? Do you plan them before you start working on the software? I am mainly intested in pactches that change a part of the program without replacing the whole exe like for example I change one line of code and then I make a patch and email it to my client he runs the patch on his computer and his exe is automatically updated. I don't know where to start. I need your help. Best Regards

Alan · Answer

Well I'd start by reading this Wikipedia article for a good overview of the theory and practice of software patching: http://en.wikipedia.org/wiki/Patch_(computing ) I think the only time when you'd plan for a possible patch is if you knew that part of the software was flakey but needed to release it anyway at the client's insistence or at least with the client's knowledge. In general it's much more difficult to alter a piece of a program rather than just replace the whole program. It has to be remembered that the client will usually only have the binary not the source code so you need to find a way of injecting machine code into the binary (in the case of an unmanaged executable) or IL into the binary (in the case of a managed executable). I would say that this is even more difficult for managed rather than unmanaged executables because it's not just the code you have to worry about but its effects on the metadata and security of the .NET assembly. Moreover if the assembly contains code which you wouldn't want your competitors to get their hands on it might have been obfuscated prior to release. For these and other reasons I never attempt to patch .NET assemblies myself - I always provide a replacement for the whole program. Anyway don't let me put you off if you're really interested in pursuing this :) Here's a recent Code Project article I found on writing a .NET patching tool which contains a link to a library someone has written to help with modifying IL: http://www.codeproject.com/useritems/NetDasm.asp