Dispatcher processing has been suspended, but messages are still being processed.

I was playing with TabControl, and I tried to implement SelectionChange event of TabControl.

So I did something like this:

XAML:

<Window x:Class="WpfApplication20.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TabControl SelectionChanged="TabControl_SelectionChanged">
            <TabItem Header="Test1"></TabItem>
            <TabItem Header="Test2"></TabItem>
        </TabControl>
    </Grid>
</Window>

Code behind:

private void TabControl_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
  MessageBox.Show("test");
}

With that code I was receiving error:

Dispatcher processing has been suspended, but messages are still being processed.

Solution:

private void TabControl_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
  Dispatcher.BeginInvoke(DispatcherPriority.Normal, new Action(this.MyTest));
}

private void MyTest()
{
  MessageBox.Show("test");
}

It seems that problem is in multi thread... Because for example code like this:

private void TabControl_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
  // ... Get TabControl reference.
  var item = sender as TabControl;
  // ... Set Title to selected tab header.
  var selected = item.SelectedItem as TabItem;
  this.Title = selected.Header.ToString();
}

Works without problem.