Disable the Close box on a form

I could be missing the point here, but I believe you could also override the OnClosing event.

protected override void OnClosing(CancelEventArgs e)
{
    // allow default behavior..
    base.OnClosing(e);

    // test for whatever...
    // on fail, e.Cancel = true;
    // set some message, etc.

}

This method allows you to disable the action of closing, but leaves the "Close" button enabled - the user can then click it - but nothing happens. This may be confusing, unless you provide a MessageBox or similar to remind the user why he cannot close.