Question

I have a databound TextBox in my application like so: (The type of Height is decimal?)

    <TextBox Text="{Binding Height, UpdateSourceTrigger=PropertyChanged,                              ValidatesOnExceptions=True,                              Converter={StaticResource NullConverter}}" /> 

    public class NullableConverter : IValueConverter {         public object Convert(object o, Type type, object parameter, CultureInfo culture) {             return o;         }          public object ConvertBack(object o, Type type, object parameter, CultureInfo culture) {             if (o as string == null || (o as string).Trim() == string.Empty)                 return null;             return o;         }     } 

Configured this way, any non-empty strings which cannot be converted to decimal result in a validation error which will immediately highlight the textbox. However, the TextBox can still lose focus and remain in an invalid state. What I would like to do is either:

1. Not allow the TextBox to lose focus until it contains a valid value.
2. Revert the value in the TextBox to the last valid value.

What is the best way to do this?

Update:

I’ve found a way to do #2. I don’t love it, but it works:

private void TextBox_LostKeyboardFocus(object sender, RoutedEventArgs e) {     var box = sender as TextBox;     var binding = box.GetBindingExpression(TextBox.TextProperty);     if (binding.HasError)         binding.UpdateTarget(); } 

Does anyone know how to do this better? (Or do #1.)

Answer 1

You can force the keyboard focus to stay on the TextBox by handling the PreviewLostKeyBoardFocus event like this:

     <TextBox PreviewLostKeyboardFocus="TextBox_PreviewLostKeyboardFocus" />  

     private void TextBox_PreviewLostKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e) {          e.Handled = true;      }