You could try it this way:
Declare a private variable inside a form:
private bool _closedFromMyButton;
Then on FormClosing event check that property:
private void MainForm_FormClosing(object sender, FormClosingEventArgs e)
{
if (_closedFromMyButton) // If closed from MyButton, don't do anything. let the form close.
return;
Hide(); // Hide the form (or not, it's up to you; this is useful if application has an icon in the tray)
e.Cancel = true; // Cancel form closing
}
Then on some button click (if desired), put this code to close the form only from that button (or menuitem or toolbar button, etc.):
private void MyButtonClick(object sender, EventArgs e)
{
_closedFromMyButton = true;
Application.Exit(); // Or this.Close() if you just want to close the form.
}