Q:

Background processing on UI thread? ( Winforms)

Is there a (or, do you have your own) preferred way to do background processing in slices on the UI thread in Windows Forms?  Like OnIdle() in MFC? 
In native Windows programming you could roll your own message loop to do this, but Application. Run() doesn't give us access to the message loop. 
The Application. Idle event gives us no way to trigger it repeatedly. 
I guess you could call native PostMessage() with P/Invoke (since there's no managed version) to post yourself a private "WM_IDLE" message, and override WndProc() to catch it.  I don't know how this would get along with Application. Run(). 
So far I've used a short Timer for this, but I'm afraid I may be losing cycles sleeping, especially since the actual Timer resolution is coarser than the nominal 1 ms minimum. 

A:

The best option I've seen is to use a modified version of the Managed DirectX Render Loop designed by Tom Miller.   By adding a call to Thread. Sleep() inside the render loop, you can pull your CPU usage down dramatically. 
This does require a P/Invoke call to track that the application is still idle, but as long as it's idle, you can make a "timer" that fires continuously during the idle phases, and use that to do your processing. 
That being said, on modern systems, you almost always have extra cores.   I would suggest just doing the processing on a true background thread.