public virtual void CustomTextBox_MaskInputRejected(object sender, MaskInputRejectedEventArgs e)
{
System.Text.StringBuilder messageBoxCS = new System.Text.StringBuilder();
messageBoxCS.AppendFormat("{0} = {1}", "Character Position", e.Position);
messageBoxCS.AppendLine();
messageBoxCS.AppendFormat("{0} = {1}", "Reason Rejected", e.RejectionHint);
messageBoxCS.AppendLine();
MessageBox.Show(messageBoxCS.ToString(), "Input Mask Invalid...");
}