Using DialogResult Correctly

When you open a modal dialog with ShowDialog, the calling code is blocked until the form called closes or hides. If you want to read some public properties of the called form and want to do things (for example save data to a database or to a file) based on the click on the OK or Cancel button, then you need to know if the user wants to do the action or not. The DialogResult returned by the ShowDialog() method allows you to take the appropriate actions…

So for example

using (Form1 form = new Form1())
{
    DialogResult dr = form.ShowDialog();
    if(dr == DialogResult.OK)
    {
        string custName = form.CustomerName;
        SaveToFile(custName);
    }
    
}

An important thing to add to this answer is the fact that the DialogResult property exists both on the Form class and in the Button class. Setting the button’s DialogResult property (both via code or designer) to a value different from DialogResult.None is the key to activate an important behavior for forms. If you click a button with that property set then the Forms Engine transfers the value of the Buttons property to the Forms one and triggers the automatic closure of the form reactivating the caller code. If you have an event handler on the button click then you can run code to validate the form’s inputs and force the form to stay open overriding the form’s DialogResult property setting it back to DialogResult.None

For example, in the modally showed form you can have:

// Event handler for the OK button set with DialogResult.OK
public void cmdOK_Click(object sender, EventArgs e)
{
     // Your code that checks the form data and
     // eventually display an error message.
     bool isFormDataValid = ValidateFormData();

     // If data is not valid force the form to stay open
     if(!isFormDataValid)
        this.DialogResult = DialogResult.None;
}