WPF

private static bool IsEmailAllowed(string text)
        {
            bool blnVallidEmail = false;
            Regex regEMail = new Regex(@"^[a-zA-Z][\w\.-]{2,28}[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$");
            if (text.Length > 0)
            {
                if (!regEMail.IsMatch(text))
                {
                    MessageBox.Show("E-Mail expected", "Error", MessageBoxButton.OK, MessageBoxImage.Error);                    
                    blnVallidEmail = !regEMail.IsMatch(text);
                }
            }

            return blnVallidEmail;
        }

        private void txtEmail_PreviewTextInput(object sender, TextCompositionEventArgs e)
        {
            e.Handled = !IsEmailAllowed(e.Text);                    
        }

arkiboys wrote:

However, I am trying to use the following method instead which is due to the
requirements...

UpdateSourceTrigger=LostFocus, ValidatesOnDataErrors=true, NotifyOnValidationError=true

public string Error
{
    get { throw new NotImplementedException(); }
}

public string this[string columnName]
{
    get
    {
      string result = null;
      if (columnName == "Email") // Assuming your property is called Email.
      {
        Regex regEMail = new Regex(@"^[a-zA-Z][\w\.-]{2,28}[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$");
        bool isValid = regEmail.IsMatch(Email);
        if (!isValid)
        {
          result="You must enter an email";

        }
      }
      return result;
    }
}

ObservableCollection<DADirective> listdata;
        DADirective SelectDirective;
 public void LoadDirective()
        {
            //DerivedAttributeEntity oDAEEntity;
            listdata = new ObservableCollection<DADirective>();
            
            SelectDirective = (DADirective)lbDirectiveList.SelectedItem;
            
            listdata.Add(new DADirective { DirectiveName = "Filter trading account", DirectiveDescription = "Rows will Trading account value to be filtered" });
            listdata.Add(new DADirective { DirectiveName = "Filter Counter party rows", DirectiveDescription = "Rows where counter party is null will be filtered" });
            listdata.Add(new DADirective { DirectiveName = "Filter USD currency records", DirectiveDescription = "Filter those rows where currency is USD" });

            lbDirectiveList.ItemsSource = listdata;           
        }