Sending WM_COPYDATA but getting WM_ACTIVATEAPP on the receiving side

Question

I have the following scenario:

My app gets some data from the command line.

After getting executed by the first time, my app runs always one instance and that instance will be in memory until the use explicitly tells it to shutdown instead of just hiding the form when not needed.

When the user tries to run the app a second time, the process starts, checks if there is another one in memory and if that is true, it sends a WM_COPYDATA message to the process in memory with the data it got from the command line and exits.

That all works well when the it«s the user who runs the app.

I needed to ran it from the Microsoft Word 2003 toolbar so i used a "Add-in" for that. The problem is that when my app is started from that Add-In (using Process class), it seems that the process already in memory gets a WM_ACTIVATEAPP message instead of a WM_COPYDATA one, so i can't get the needed data sent from the process started by the Add-In.

I have no idea on why is that happening and how to fix it. I've googled for hours and nothing helped...

Can anyone help?

Answer 1

Getting a WM_ACTIVATEAPP message is quite normal, part of the usual notifications that Windows sends. Don't assume that the first message you'll get is WM_COPYDATA, keep looking. If you don't get it at all then the window handle that you used to send the message was wrong. Which is a very common problem, it is not that easy to accurately find a window back.

The .NET framework already has very good support for single-instance apps that can retrieve the command line from a second instance. Consider using it instead. Check this blog post.