Question

I have a dialog that I show with <class>.ShowDialog(). It has an OK button and a Cancel button; the OK button also has an event handler.

I want to do some input validation in the event handler and, if it fails, notify the user with a message box and prevent the dialog from closing. I don’t know how to do the last part (preventing the close).

Answer 1

Given that you’ve specified you want a pop error dialog, one way of doing this is to move your validation into a Form.OnFormClosing event handler. In this example the form close is aborted if the user answers yes to the question in the dialog.

private void Form1_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
   // Determine if text has changed in the textbox by comparing to original text.
   if (textBox1.Text != strMyOriginalText)
   {
      // Display a MsgBox asking the user to save changes or abort.
      if(MessageBox.Show("Do you want to save changes to your text?", "My Application",
         MessageBoxButtons.YesNo) ==  DialogResult.Yes)
      {
         // Cancel the Closing event from closing the form.
         e.Cancel = true;
         // Call method to save file...
      }
   }
}

By setting e.Cancel = true you will prevent the form from closing.

However, it would be a better design/user experience to display the validation errors inline (via highlighting the offending fields in some way, displaying tooltips, etc.) and prevent the user from selecting the OK button in the first place.