// Recursive access of controls of a root control
// Source: https://social.msdn.microsoft.com/Forums/vstudio/en-US/04d08bfb-2214-4636-b12d-b98f33caa4c4/recursive-search-of-all-controls-on-a-form
private void ProcessAllControls(Control rootControl, Action<Control> action)
{
    foreach (Control childControl in rootControl.Controls)
    {
        ProcessAllControls(childControl, action);
        action(childControl);
    }
}
// Use as: ProcessAllControls(this, ctrl => ctrl.Visible = false);