I've been using events to communicate between my app and my watchdog service, but after I noticed some crash events in the app can sometimes cause it to corrupt the log file, I decided to have it also send error logs to the service which would write them out in case the app crashes.

Except I don't know how to send a string to a service. I only see how to send it control codes, but not data. I couldn't figure out how to use WM_COPYDATA even if I make the service impersonate the user account (which I already do to access the user's registry). What do? I thought of creating a hidden window to get an HWND but I don't even know if that would work, so I didn't want to put in the time before asking...

One way would be to just send it through the message pump.

MSDN - SendMessge

You could just send the message using the HWND_BROADCAST. See the remarks section about creating messages for inter-application communication.

[edit]
Instead you might use MSDN - PostMessage as it doesn't wait for the function call to return, if you don't want any kind of acknowledgement.

You can't send a window message to a service. Services don't have a window handle.

While I've never tried it, the documentation would seem to suggest that it is possible (at least in a similar fashion as I described), because the message pump can receive messages that are just for the thread, not for any particular window.

From the MSDN -

Quote:
MSG structure
Members

hwnd
Type: HWND

A handle to the window whose window procedure receives the message. This member is NULL when the message is a thread message.

So while I'm not guaranteeing that SendMessage/PostMessage are the correct method of sending the message (MSDN - BroadcastSystemMessageEx or MSDN - PostThreadMessage looks like they might be candidates as well), I don't see any reason why it can't receive a message, just because it doesn't have a window handle.