Setting Validation error template from code in WPF

Question

I have a TextBox in my WPF app. I have defined a ControlTemplate for validation error as follows:

Answer 1

To set "Validation.HasError" in code behind you can use the Validation.MarkInvalid method

private void blockTextBox_PreviewLostKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e) 
{ 
    TextBox txtBox = sender as TextBox;
    //...
    BindingExpression bindingExpression =
        BindingOperations.GetBindingExpression(txtBox, TextBox.TextProperty);

    BindingExpressionBase bindingExpressionBase = 
        BindingOperations.GetBindingExpressionBase(txtBox, TextBox.TextProperty);

    ValidationError validationError =
        new ValidationError(new ExceptionValidationRule(), bindingExpression);

    Validation.MarkInvalid(bindingExpressionBase, validationError);
}

To unset the value you use

Validation.ClearInvalid