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    <p>I suspect that the question boils down to what you mean when you say:</p> <blockquote> <p>A message box does not block the main application.</p> </blockquote> <p>What I take this to mean is that when you show the message box, your VCL form can still be interacted with. The issue here is unrelated to threads and I suggest we remove them from the equation. Your understanding of what <code>Synchronize</code> does is sound.</p> <p>The issue is entirely related to the concept of a <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms632599.aspx#owned_windows">window's owner</a>, and how modal dialog windows behave with respect to their owners. Note that by owner, I don't mean the Delphi property <code>TComponent.Owner</code>, but I mean the Windows API meaning of <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms632599.aspx#owned_windows">owner</a>.</p> <p>Create a VCL app and drop two buttons on the form. Add the following <code>OnClick</code> handlers.</p> <pre><code>procedure TForm1.Button1Click(Sender: TObject); begin MessageBox(0, 'Not owned', nil, MB_OK); end; procedure TForm1.Button2Click(Sender: TObject); begin MessageBox(Handle, 'Owned by the VCL form', nil, MB_OK); end; </code></pre> <p>Now observe what happens when you click on <code>Button1</code>. The message box shows, but you can still click on the VCL form. And compare with <code>Button2</code>. When it shows the message box, the VCL form cannot be interacted with.</p> <p>When a modal dialog window is shown, the dialog window disables its owner. In the case of <code>Button2</code>, the owner is the VCL form. And once the form is disabled, you cannot interact with it. In the case of <code>Button1</code>, there is no owner and so the modal dialog window does not disable any other window. That's why the VCL form can be interacted with.</p> <p>Raymond Chen has a long series on modality at his Old New Thing blog:</p> <ul> <li><a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/02/18/376080.aspx">Modality, part 1: UI-modality vs code-modality</a></li> <li><a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/02/21/377337.aspx">Modality, part 2: Code-modality vs UI-modality</a></li> <li><a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/02/22/378018.aspx">Modality, part 3: The WM_QUIT message</a></li> <li><a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/02/23/378866.aspx">Modality, part 4: The importance of setting the correct owner for modal UI</a></li> <li><a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/02/24/379635.aspx">Modality, part 5: Setting the correct owner for modal UI</a></li> <li><a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/02/28/381591.aspx">Modality, part 6: Interacting with a program that has gone modal</a></li> <li><a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/03/01/382380.aspx">Modality, part 7: A timed MessageBox, the cheap version</a></li> <li><a href="http://blogs.msdn.com/b/oldnewthing/archive/2005/03/04/385100.aspx">Modality, part 8: A timed MessageBox, the better version</a></li> <li><a href="http://blogs.msdn.com/b/oldnewthing/archive/2011/01/21/10118482.aspx">Modality, part 9: Setting the correct owner for modal UI, practical exam</a></li> </ul>
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    1. POWhy does a MessageBox not block the Application on a synchronized thread?
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    1. USDavid Heffernan
    1. USDavid Heffernan
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