How do I make the close button of a Windows Form not close the form but make it invisible?
protected override void OnFormClosing(FormClosingEventArgs e) {
    if (IActuallyWantToCloseFlag)
        return;

    var ni = new NotifyIcon(this.components)
    {
        Icon = someIcon,
        Text = "My text",
        Visible = true
    };
    ni.DoubleClick += (sender, args) => { this.Show(); };

    this.Hide();
    e.Cancel = true;
}

FormClosing += (sender, args) => { args.Cancel = true; Visible = false; };