How do I catch the event of exiting a Winforms application?

Question

If the user wants to exit the application by clicking on the exit icon or by ALT+F4, I'd like to make a dialog box questioning the user if he/she is really sure about exiting.

How do I capture this event before the application is actually closed?

Answer 1

You should subscribe to the Form_Closing event
Post a dialog box there, and if user abort the closing set the FormCloseEventArgs.Cancel to true.

For example in the Form_Load or using the designer, subscribe the event

Form1.FormClosing += new FormClosingEventHandler(Form1_Closing);

....
private void Form1_FormClosing(Object sender, FormClosingEventArgs e) 
{
    DialogResult d = MessageBox.Show("Confirm closing", "AppTitle", MessageBoxButtons.YesNo );
    if(d == DialogResult.No)
        e.Cancel = true;
}

Depending on the situation is not always a good thing to annoy the user with this kind of processing.
If you have valuable modified data and don't want the risk to loose the changes, then it is always a good thing to do, but if you use this only as a confirm of the close action then it's better to do nothing.