How can I update an in-use COM DLL?

One part of some software I have written is a COM dll. Other software uses this COM dll.

My software has an update function where it will download a newer version of the dll, but the update will fail if the dll is in use because the file cannot be deleted or written to.

The question is, how can I update a COM dll that is in use?

I have considered popping up a message asking the user to close any applications that are using the DLL if it is in use, if this is the best solution how would I go about detecting if the COM dll w开发者_如何学Pythonas in use before popping up the message?

Thanks in advance.

You cannot update it in place for existing applications, but one way to do this would be to save it with a different file name or different folder and call DllRegisterServer on the DLL to register it under the new name. New applications which begin using your object should now use the new version.

If this is just a matter of detecting whether you can replace the file then it is easy. Just try to open it with a share flag that denies reading. That's going to fail if the DLL is loaded in another process. Use _fsopen() or CreateFile(). Beware of the race condition.

Detecting which processes have the file loaded is a harder problem, CreateToolhelp32Snapshot() and Process32First/Next plus Module32First/Next to enumerate processes and the DLLs they have loaded. Still tough to generate a good diagnostic for the user, the process name isn't that helpful.

When you have downloaded the update, you must launch a third program (which you write) that does not have any dependancies on your COM component, or any other piece that is to be updated. This launcher, or bootstrapper, must shut down all your pieces, uninstall them, and install the update. When the update is installed you may then re-launch your application.

If you need also to download updates to the updater itself, your main program can do that.

Here is a simple solution for you. Create a wrapper DLL, which will be used by the other processes. Inside that DLL you explicitly load/unload your DLL, which is subject to updates. Of course you will have to suspend all callers when an update process kicks in.