Whenever you show a Modal Dialog a DialogResult is expected. This DialogResult can be OK, Cancel, Yes, No and so forth. Now, imagine this scenario:

You show a Modal Dialog from inside another Form. The parent form expects a result, whatever that may be. Imagine further that a user forgot to fill in all the necessary details and click OK. By not sending the correct information through the Modal dialog box, your app might end up crashing or you may not get the proper result you expected. You need to validate your Modal dialog first.

Validating means to ensure that all the data is valid. Now if validation fails, the Modal dialog still sends an OK result to the calling form. This is wrong and troublesome.

What you should do in cases such as this is to revert the Modal window’s Dialog Result back to DialogResult.None before attempting to send information, and only once all the information is correct, reset the Dialog result to OK. There is one caveat though: you cannot set the Dialog result to None, and then OK in the same method. If you do, it may cause the dialog to remain open even after you have clicked OK so that you have to click on it again, irrespective of the dialog result. Below is a small example on the logic involved ensuring you get the correct DialogResult:

private void btnOK_Click(object sender, EventArgs e) //btnOK has a DialogResult of None        {   intOption            if (ComboBox1.SelectedIndex  -1)            {                btnOK.DialogResult = DialogResult.OK; //Set Result to OK                int.TryParse(ComboBox1.SelectedValue.ToString(), out intOption);            }            else            {                MessageBox.Show("Please Select an Option!", "Attention", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);                btnOK.DialogResult = DialogResult.None; //Set Result to None            }                    }        private void ComboBox1_DropDownClosed(object sender, EventArgs e)        {            if (ComboBox1.SelectedIndex  -1)            {                btnOK.DialogResult = DialogResult.OK; //Set result to OK, before entering btnOK_Click event//Thus bypassing DialogResult logic            }        }