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Task.Factory.StartNew and sync context

Simple question. Here is part of the WinForms application:

void Form1_Load(object sender, EventArgs e)
{
    var task2 = Task.Factory.StartNew(() => MessageBox.Show("Task!"),
        CancellationToken.None,
        TaskCreationOptions.None,
        TaskScheduler.FromCurrentSynchronizationContext());

    Debug.WriteLine("Before Exit");
    MessageBox.Show("Exit!");
}

Can someone explain why I see "Task!". first and then "Exit" when I run this?

And when I see "Task!" the "Before Exit" message box has already been printed at the debug output.

EDIT: simpler, same effect:

void Form1_Load(object sender, EventArgs e)
{
    SynchronizationContext.Current.Post((_) => 
        MessageBox.Show("Task!"), null);

    Debug.WriteLine("Before Exit");
    MessageBox.Show("Exit!");
}

EDIT: if I replace MessageBox.Show("Exit!")with Form { Text = "Exit!" }.ShowDialog(), I see "Exit" and then "Task", as expected. Why is this?

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3 answers

Exact information about forwarding Win32 messages in the .NET Framework is undocumented. Note that the Win32 API has historically allowed repeated behavior.

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In fact, you would run your task outside of the user interface thread, and only a continuation requiring access to the user interface would be in the user interface thread. However, it is very interesting how it MessageBoxcaptures what is happening - it can have unpleasant consequences if your background task gets stuck, causing something in the user interface thread, which is effectively transmitted in the message window; your asynchrony goes there :))

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