With regard to more than one UI thread per process... I've managed to get some added information from the folks at Microsoft, that I'd thought I'd share. From a purely architectural point of view, Windows supports any number of threads per process pumping windows messages for the windows and controls created in their respective thread.
Microsoft advises against multiple UI threads per process for a variety of reasons. One being the complexity that's involved with managing the independent window and control data on each thread and marshaling data between them (if needed), synchronization, etc. From an implementation stand-point, all the above isn't limiting, just *complex*. Another, more limiting reason Microsoft advises against more than one UI thread per process is that there is a limit to five device contexts (DCs) that can be actively written to. This limitation is system-wide. Adding another UI thread to a process is defeating because the application would then be able, and more likely, to have least two of these five DCs open for write at a time (assuming two GUIs and they are visible at the same time). When there are five active DC writes, any additional writes are queued. As far as I know, it's impossible to know how many DCs are actively being written to at a given time, so it would be impossible for application to effectively synchronize access to methods that caused a DC write, if you wanted to. The impression I got was, given this almost "serial" nature of control painting, it would actually be slower and more taxing (thread context switching, thread stack, optional synchronization, etc) to have more than one GUI thread per process. Unlike VB6 and MFC, WinForms limits UI threads no more than the underlying OS does. The folks that have said Windows does not support more than one GUI thread (or windows message pump, or windows message loop) per process have not responded to me with a basis for these comments, so, I cannot include their thoughts. (possibly a hold-over from Win 3.1?). As a side note, I'm looking to use up about 9 hours of free yearly development advisory service from MS, if anyone has anything more on this topic they would like answered (or any other topic you haven't had a acceptable response on), that I can't answer, I can pass it along. -- Peter Microsoft MVP, Visual Developer - Visual C# http://www.peterRitchie.com/blog/ =================================== This list is hosted by DevelopMentor® http://www.develop.com View archives and manage your subscription(s) at http://discuss.develop.com
