Using WPF Validation rules and disabling a 'Save' button

I have a page where a few textboxes cannot be empty before clicking a Save button.

<TextBox...

                <TextBox.Text>
                    <Binding Path ="LastName" UpdateSourceTrigger="PropertyChanged">

                        <Binding.ValidationRules>
                            <local:StringRequiredValidationRule />
                        </Binding.ValidationRules>                              
                    </Binding>
                </TextBox.Text>

My rule works. I have a red border around my textbox until I enter a value. So now I want to add this validation rule to my other text boxes.

Now, how do I disable the Save button until the page has no errors? I do not know what to check to see if there are any validation errors.

From stackoverflow

• You want to use Validation.HasError attached property.

  Along the same lines Josh Smith has an interesting read on Binding to (Validation.Errors)[0] without Creating Debug Spew.

• On the codebehind for the view you could wireup the Validation.ErrorEvent like so;

  this.AddHandler(Validation.ErrorEvent,new RoutedEventHandler(OnErrorEvent));
  

  And then

  private int errorCount;
  private void OnErrorEvent(object sender, RoutedEventArgs e)
  {
      var validationEventArgs = e as ValidationErrorEventArgs;
      if (validationEventArgs  == null)
          throw new Exception("Unexpected event args");
      switch(validationEventArgs.Action)
      {
          case ValidationErrorEventAction.Added:
              {
                  errorCount++; break;
              }
          case ValidationErrorEventAction.Removed:
              {
                  errorCount--; break;
              }
          default:
              {
                  throw new Exception("Unknown action");
              }
      }
      Save.IsEnabled = errorCount == 0;
  }
  

  This makes the assumption that you will get notified of the removal (which won't happen if you remove the offending element while it is invalid).