How to Handle validation within a custom TextBox - wpf

Question

I've got a custom TextBox to handle measurements (feet, inches, mm) that has a couple Dependency Properties that determine what the formatting of the box should be when the box loses focus. I got all the conversions happening in the OnLostFocus function because conversions mid-input would not work. In OnLostFocus, I convert the value to a number depending on some other DP properties and set a Measurement property. All this works great.
My question is, how do I handle validation? When someone inputs an invalid value, I want the textbox to go red, much like you could do with a Binding that has ValidatesOnExceptions=true. I tried something like below in the catch block of OnLostfocus
protected override void OnLostFocus(RoutedEventArgs e)
{
try
{
if (string.IsNullOrWhiteSpace(Text))
{
Text = "0";
}
if (IsMetric)
{
var measurement = convertStuffHere();
Text = measurement.Text;
Measurement = measurement.Value;
}
else
{
var measurement = convertOtherStuffHere();
// convert and formatting stuff here...
Text = measurement.Text;
Measurement = measurement.Value;
}
var binding = this.GetBindingExpression(TextBox.TextProperty);
if (binding != null)
Validation.ClearInvalid(this.GetBindingExpression(TextBox.TextProperty));
}
catch (Exception)
{
var rule = new DataErrorValidationRule();
var binding = this.GetBindingExpression(TextBox.TextProperty);
if (binding != null)
{
ValidationError validationError = new ValidationError(rule, this.GetBindingExpression(TextBox.TextProperty));
validationError.ErrorContent = "This is not a valid input";
Validation.MarkInvalid(this.GetBindingExpression(TextBox.TextProperty), validationError);
}
}
finally
{
base.OnLostFocus(e);
}
}
This almost works, but the validation error shows up late. I have to lose focus, get focus, and lose focus again before a red box shows around the textbox.
I'm using it like <myns:MeasurementTextBox Text="{Binding MeasurementText1, ValidatesOnExceptions=True}" Margin="10" IsMetric="True"></myns:MeasurementTextBox>

Answer 1

You can use TextBoxBase.TextChanged event instead of UIElement.LostFocus.