Cancel TextBox input on validation errors - wpf

Question

The standard behavior in TextBoxes when a input string is not valid is to show a red square (for example user introduces a letter in numeric TextBox). This happens when TextBox loses focus.
I want implement this behavior:
The textBox loses focus.
TextBox do internal validation (date, numeric, etc).
If input user string is not valid, old value is restored and TextBox don't show any error.

Answer 1

you have an exemple of validation in textbox here in this link: http://www.codeproject.com/Tips/690130/Simple-Validation-in-WPF
<ControlTemplate x:Key="validationErrorTemplate">
<DockPanel>
<TextBlock Foreground="Red"
DockPanel.Dock="Top">!</TextBlock>
<AdornedElementPlaceholder
x:Name="ErrorAdorner"
></AdornedElementPlaceholder>
</DockPanel>
</ControlTemplate>
public class NameValidator : ValidationRule
{
public override ValidationResult Validate
(object value, System.Globalization.CultureInfo cultureInfo)
{
if (value == null)
return new ValidationResult(false, "value cannot be empty.");
else
{
if (value.ToString().Length > 3)
return new ValidationResult
(false, "Name cannot be more than 3 characters long.");
}
return ValidationResult.ValidResult;
}
}
<TextBox Height="23" HorizontalAlignment="Left"
Grid.Column="1" Grid.Row="0" Name="textBox1"
VerticalAlignment="Top" Width="120"
Validation.ErrorTemplate="{StaticResource validationErrorTemplate}"
>
<TextBox.Text>
<Binding Path="Name" Mode="TwoWay"
UpdateSourceTrigger="LostFocus">
<Binding.ValidationRules>
<local:NameValidator></local:NameValidator>
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>

Answer 2

Reverting to the old value is quite easy if you use MVVM. In your ViewModel's prop setter you can simply not set the new value to the model if it's invalid and invoke PropertyChanged instead. This will tell the bound view element to call your property getter, which will return the old value and thus revert the view element's content to the old value.
Example (validating that user input is an int value):
public string Number
{
get { return _model.Number.ToString(); }
set
{
if (_model.Number.ToString() != value)
{
int number;
if (int.TryParse(value, out number))
{
_model.Number = number;
}
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Number));
}
}
}