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.NET Forum / Languages / C# / October 2007cross - thread problem
if (e.Clicks > 0 && e.Button == MouseButtons.Middle) { f1.NewThread = new Thread(new ThreadStart(f1.ShowMainNodes)); f1.NewThread.Start(); } private void ShowMainNodes() { API.POINTAPI papi = new ALaunch.API.POINTAPI(); API.GetCursorPos(ref papi); Point p = new Point(papi.x /*- this.Location.X*/ , papi.y /*- this.Location.Y - 16*/); this.Activate(); // x-thread error here - why; I thought this was done in a thread-safe manner? } Thank you. //System.InvalidOperationException was unhandled Message="Cross-thread operation not valid: Control 'Form1' accessed from a thread other than the thread it was created on." Source="System.Windows.Forms" StackTrace: at System.Windows.Forms.Control.get_Handle() at System.Windows.Forms.Form.Activate() at ALaunch.Form1.ShowMainNodes() in C:\Documents and Settings\Alistair George\My Documents\Visual Studio 2005\Projects\Launcher\Form1.cs:line 335 at System.Threading.ThreadHelper.ThreadStart_Context(Object state) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart() Alistair, Well, doing it in a thread-safe manner is one thing, but in this case, you are making one (possibly two) calls to the UI on a thred that is not the UI thread. You need to make the call to Activate on the same thread that the control was created on. Also, and I am not sure, the call to GetCursorPos might have to be made on the UI as well (although I can see why it might not be required).  Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > if (e.Clicks > 0 && e.Button == MouseButtons.Middle) > { [quoted text clipped - 28 lines] > executionContext, ContextCallback callback, Object state) > at System.Threading.ThreadHelper.ThreadStart() > Alistair, > [quoted text clipped - 6 lines] > might have to be made on the UI as well (although I can see why it might not > be required). Ok, unfortunately, thats not clear - there is the old thread already running, which I cant call as it conflicts with current mouse events, hence the reason for starting a new thread. I dont call the old thread from the new thread do I? Alistair, Well, yes, you could, by calling the Invoke method on the control, and passing in a delegate which will execute the code on the UI thread. The thing is, the code you have shown doesn't really do much, and shouldn't intefere with the UI thread at all. It's very short, and doesn't do much. Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com >> Alistair, >> [quoted text clipped - 11 lines] > hence the reason for starting a new thread. I dont call the old thread > from the new thread do I? > Alistair, > [quoted text clipped - 4 lines] > shouldn't intefere with the UI thread at all. It's very short, and doesn't > do much. I notice that where the form F1 is instantiated, it also has Single threaded [STAThread] set; is that a likely problem? [STAThread] static void Main() { Application.Run(f1); } Alistair, Well, it is the cause, but it's supposed to be there. It's a fundamental windows programming tennant that you have to make calls to UI components on the same thread that they were created. You are looking to activate your form, so that call has to be made on the main thread. If you remove that attribute, your program is going to stop working in a number of other places really quick. Now, there is no reason you can't get the cursor position and expose those details to the worker thread, and then call Invoke to call a delegate that will call Activate on your form when done. Of course, I still don't see why you have to, as the code you have is very small, and shouldn't take long to run at all. Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com >> Alistair, >> [quoted text clipped - 12 lines] > Application.Run(f1); > } > Alistair, Invoke to call a delegate > that will call Activate on your form when done. > > Of course, I still don't see why you have to, as the code you have is > very small, and shouldn't take long to run at all. Cheers Nicholas. Your help appreciated. I am quite clueless when it comes to this part of programming never having been there before. C# is much more picky than Delphi used to be, but I can figure its for stability. Am trying to get the middle mouse button to activate a form, as well as hotkey combo (which is working well). Catching the middle mouse button, but from there the problem area is. An alternative could be to simulate a hotkey press to activate the form. Hi, You cannot use a Control from a thread other than the UI thread. In these cases you need to use Control.Invoke You can use any control to be used as the "hook" > if (e.Clicks > 0 && e.Button == MouseButtons.Middle) > { [quoted text clipped - 28 lines] > executionContext, ContextCallback callback, Object state) > at System.Threading.ThreadHelper.ThreadStart() Thanks; how about some short examples for clarity. Hi, > Thanks; how about some short examples for clarity. Google Control.Invoke you will get a ton of examples as this is a VERY recurrent question > Hi, > >> Thanks; how about some short examples for clarity. > > Google Control.Invoke you will get a ton of examples as this is a VERY > recurrent question Thanks. Should have thought of that - been up since 4am working on this problem! Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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