Strange behaviour with TabControl in WPF on SelectionChanged

Question

I made a TabControl and three TabItems inside. My Code looks as follows:

XAML:

<TabControl Name="ConfigTabs" HorizontalAlignment="Left" VerticalAlignment="Top" SelectionChanged="TabControlSelectionChanged">
  <TabItem Header="Allgemeines">
    ...
  </TabItem>
  <TabItem Header="Monitorbelegung">
    ...
  </TabItem>
  <TabItem Header="Produkt-Konfigurationen">
    ...
  </TabItem>
</TabControl>

C# (Code-Behind):

private void TabControlSelectionChanged(object sender, SelectionChangedEventArgs e)
{
  if (e.Source is TabControl && this.IsLoaded)
  {
    TabControl MyTabControl = (TabControl)sender;
    if (MyTabControl.SelectedIndex == 0)
    {
      MessageBox.Show("Allgemeines");
    }
    else if (MyTabControl.SelectedIndex == 1)
    {
      MessageBox.Show("Monitor");
    }
    else if (MyTabControl.SelectedIndex == 2)
    {
      MessageBox.Show("Configs");
    }
  }
}

When I change the TabItem, a MessageBox with the Text pops up, as intended. BUT when I now click another Item I get the MessageBox of the next Item AND afterwards a MessageBox of the past one. Where I am not completly sure of the logic behind that. When I delete the MessageBoxes everything works fine, but I need them, because of some logic I want to implement at a later time.

The Question would be now "How can I prevend the event from firing twice?";

Answer 1

Ok, I found it. The MessageBox interrupts the change-event, so we have to use some other way. When you use a style, you can catch the change-event of the TabItems:

XAML:

    <TabControl Name="ConfigTabs" HorizontalAlignment="Left" VerticalAlignment="Top">
        <TabControl.Resources>
            <Style TargetType="TabItem">
                <EventSetter Event="Selector.Selected" Handler="OnNewTabSelected"/>
            </Style>
        </TabControl.Resources>
        <TabItem Header="Allgemeines">
        ...           

        </TabItem>
        <TabItem Header="Monitorbelegung">
        ...
        </TabItem>
        <TabItem Header="Produkt-Konfigurationen">
        ...
        </TabItem>
    </TabControl>

C# (Code-Behind):

    private void OnNewTabSelected(object sender, RoutedEventArgs e)
    {
        if (e.Source is TabItem && this.IsLoaded)
        {
            TabItem MyTab = (TabItem)sender;
            TabControl MyControl = (TabControl)MyTab.Parent;
            if (MyControl.SelectedIndex == 0)
            {
                MessageBox.Show("Beep" + MyControl.SelectedIndex);
            }
            else if (MyControl.SelectedIndex == 1)
            {
                MessageBox.Show("Beep" + MyControl.SelectedIndex);
            }
            else if (MyControl.SelectedIndex == 2)
            {
                MessageBox.Show("Beep" + MyControl.SelectedIndex);
            }
        }
    }