private string _name;
[Required(ErrorMessage = "Name is required.")]
public string Name
{
  get { return _name; }
  set  {
    if (_canValidate)
    {
       Validator.ValidateProperty(value,
                  new ValidationContext(this, null, null) { MemberName = "Name" });
    }
    _name = value;
    OnPropertyChanged("Name");
   }
}

<TextBox Name="tbxName" Grid.Row="0" Grid.Column="1"   Margin="4"  MinWidth="160">
<TextBox.Text>
 <Binding Path="Name" ValidatesOnExceptions="True" ValidatesOnDataErrors="True" NotifyOnValidationError="True" ValidatesOnNotifyDataErrors="True"  Mode="TwoWay">
 </Binding>
</TextBox.Text>
</TextBox>

private void OKButton_Click(object sender, RoutedEventArgs e)
{
  var mm = this.DataContext as ViewModel;
  try
  {
    mm.Validate();
    mm.UpdateModel();
    this.DialogResult = true;
  }
  catch ( ValidationException ex)
  {
   MessageBox.Show(ex.Message);
  }
}

public void Validate()
{
  //Validator.ValidateObject(this, new ValidationContext(this, null, null), true);
  var errors = new List<ValidationResult>();
  var isValid = Validator.TryValidateObject(this, new ValidationContext(this, null, null), errors, true);
  if(!isValid)
  {
    throw new ValidationException( string.Join( Environment.NewLine,  errors.Select((e) => e.ErrorMessage )));
  }
}

Name = Name; // дало нужные рюшечки
if ( HasErrors) { // реализовало защиту.
  MessageBox.Show("Validation failed" );
  return false;
}
return true;

private void OKButton_Click(object sender, RoutedEventArgs e)
{
  var mm = this.DataContext as ViewModel;
  if ( !mm.Validate() ) return;
  mm.UpdateModel();
  this.DialogResult = true;
}