In windows form, if you don't want user to close the form by pressing Alt+F4 or Cross (x) button accidently and without saving existing form status, you may need to capture the form close event.
In the FormClosing event of form, FormClosingEventArg has a property called CloseReason. This is very usefull because if you want the Form to stop closing if the closing reason was CloseReason.UserClosing or some thing else.
You can do e.Cancel = true to prevent form being closed, then do something in your code and finally close the form programatically.
private void Form1_FormClosing(object sender, FormClosingEventArgs e)
e.Cancel = true;
//Check the status of form, Save form status if required or other logic