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Working on an existing project, I've got to use WinForms (haven't worked with it for a while) and have got an issue synchronizing with the UI thread. The design I've got to integrate with works as follows: A BackgroundWorker gets an Action as a parameter and executes it asynchronously. The action I'm working on has two parts; a core class (containing the business logic) and a GUI part being notified by the core through events if it has to request user interaction. I've added the handle creation to the constructor of the form if (!IsHandleCreated) { //be sure to create the handle in the constructor //to allow synchronization with th GUI thread //when using Show() or ShowDialog() CreateHandle(); } With this, following code works: private DialogResult ShowDialog(Form form) { DialogResult dialogResult = DialogResult.None; Action action = delegate { dialogResult = form.ShowDialog(); }; form.Invoke(action); return dialogResult; } For this example, the startup location has been set to windows default. If I change it to: Action action = delegate { dialogResult = form.ShowDialog(ParentWindow); }; Where ParentWindow is an instance of IWin32Window and the WindowStartupLocation is set to CenterParent . I get a cross-thread exception when calling form.Invoke(action) . Cross-thread operation not valid: Control 'ActivationConfirmationForm' accessed from a thread other than the thread it was created on. Questions: Why is there a cross thread exception only when setting the startup location as CenterParent ? And how do I avoid it? Why is form.InvokeRequired always false ? Both are probably related!?

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