WPF值更改事件在值更改之前运行

Question

i have the following datepicker:

<xctk:DateTimePicker HorizontalAlignment="Right" Margin="0,23,671,0" Name="datepicker" VerticalAlignment="Top" Width="120" Height="49" BorderBrush="Black" Cursor="Hand" ValueChanged="dateTimePicker1_ValueChanged" />

Now as you can see component has a valueChanged event. The event code looks like this:

    private void dateTimePicker1_ValueChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{

    DateTime date = (DateTime)datepicker.Value;
    datepicker.Text = date.Date.ToString();
    UpdateDateLabels(date);
}

private void UpdateDateLabels(DateTime date) 
{
    dstart.Content = date.Date.ToShortDateString();

    switch (datebox.SelectedIndex) 
    {
        case 0:
            date.AddDays(1);
            break;
        case 1:
            date = date.AddDays(7);
            break;
        case 2:
            date = date.AddMonths(1);
            break;
        case 3:
            date = date.AddYears(1);
            break;
        default:
            break; 
    }
    MessageBox.Show(datebox.SelectedValue.ToString());
    dend.Content = date.Date.ToShortDateString();

}

Now as some of you might know a datepicker looks like this:

Now when i click the arrow keys (up and down) everything works as it should. But if i however click the button to view the calendar then the changedevent is already fired without actually changing (i don't even get to see the calendar before i get the following error):

Can anyone tell me why this is happening?

Answer 1

You should check OriginalSource .

Try this:

private void dateTimePicker1_ValueChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
    if (e.OriginalSource is Xceed.Wpf.Toolkit.DateTimePicker)
    {
        DateTime date = (DateTime)datepicker.Value;
        datepicker.Text = date.Date.ToString();
        UpdateDateLabels(date);
    }
}

Answer 2

I'll go for the "teach a man to fish" answer.

The best way to answer a "why is this happening?" question is often to look at your call stack. If it shows a line saying [] you [b]NEED[/b] to turn off Just My Code in the debugging options (Options->Debugging->Uncheck "Enable Just My Code".

After doing this, you should see a much more informative call stack where the exception is occurring.

What I'm going to guess you'll see in there is a bunch of Binding calls, probably for the new editor that is being displayed when you click that button. There will often be value changed events thrown when the binding is established, even if the value has not changed.

So, as per @kmatyaszek's answer, you need to be sure you're only handling the event when you need to.