//First, place an error provider component onto the form. It will show up in the components tray.
  
  
  private void txtCustomer_Validating(object sender, CancelEventArgs e)
        {
            if (txtCustomer.Text.Trim().Length == 0) 
            {
                errorProvider1.SetError(txtCustomer,"Please enter customer name.");
                e.Cancel= true;
            }
                        
            else
                errorProvider1.SetError(txtCustomer, "" );
        }