Question

The following behaviour is occurring with the .net 4.0 code set out further below:

Click on textbox so it gains focus, and then click button:

1. As code stands, lostfocus handler is called, but not buttonclick handler
2. Comment out MessageBox.Show(“handlelostfocus”) and then click handler is called
3. Set breakpoint in handlelostfocus and breakpoint is hit, but click handler is not called

Are these bugs or behaviour by design – if later, is there any further explanation?

<Window x:Class="WpfApplication4.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>
            <Button Content="Button" Height="23" HorizontalAlignment="Left" Margin="216,194,0,0" Name="button1" VerticalAlignment="Top" Width="75" Click="button1_Click" />
            <TextBox Height="23" HorizontalAlignment="Left" Margin="197,108,0,0" Name="textBox1" VerticalAlignment="Top" Width="120" />
        </Grid>
    </Window>

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            textBox1.LostFocus += new RoutedEventHandler(handlelostfocus);
        }

        private void handlelostfocus(object sender, RoutedEventArgs e)
        {
            MessageBox.Show("handlelostfocus");
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            MessageBox.Show("click");
        }
    }

Answer 1

The ‘click’ in this case never occurs because as H.B. indicated you are interrupting the UI/event logic by showing the modal messagebox so there is never a mouse down event on the button.

Try replacing the messagebox with a non-modal wndow such as this:
new Window() { Width = 300, Height = 100, Title = “handlelostfocus” }.Show();

and you will see that the events still occur because you are not drawing the focus away from the main window in the middle of the event logic.