PLC DataManager is a DLL developed by us that can be loaded and linked by any application such as TwinCAT HMI, windows application, and so on. This is a helper class that implements reading, writing, and subscription mechanism. See how the events work in C# at https://www.hemelix.com/software/csharp-tutorial/

private TcAdsClient adsClient;
adsClient.AdsNotificationEx += new AdsNotificationExEventHandler(adsClient_AdsNotificationEx);
private void adsClient_AdsNotificationEx(object sender, AdsNotificationExEventArgs e)
{
	string tag = null;
	int handle = (int)e.UserData;
	tag = variableHandles.FirstOrDefault(x => x.Value == handle).Key;
	if (tag != null && ChangeUpdate != null)
	{
		ChangeUpdate(tag, e.Value);  //notify to upper level of the application
	}
}

If we use TwinCAT.Ads.dll  library in our visual studio project and we add a reference to the DLL then we can declare a variable type  TcAdsClient. We need to create an event handler to get notification from the DLL. We pass a handler function when some thing happens that interests us ( see event section of https://www.hemelix.com/software/csharp-tutorial/).

If we use APP(WindowsFormsApp1) then we need to subscribe to the needed variables by ourselves and on the other hand, if we use APP (WindowsFormTestWithDLL) then the subscription is done in the PLC DataManager module.

If we use the ADS library directly then we need to subscribe to the change notification by ourselves.

            notificationHandles.Clear();
            try
            {
                //register notification
                notificationHandles.Add(adsClient.AddDeviceNotificationEx("MAIN.testBool", AdsTransMode.OnChange, 100, 0, handleBool, typeof(Boolean)));
                notificationHandles.Add(adsClient.AddDeviceNotificationEx("MAIN.testInt", AdsTransMode.OnChange, 100, 0, handleInteger, typeof(Int16)));
                notificationHandles.Add(adsClient.AddDeviceNotificationEx("MAIN.testReal", AdsTransMode.OnChange, 100, 0, handleReal, typeof(float)));
                notificationHandles.Add(adsClient.AddDeviceNotificationEx("MAIN.testString", AdsTransMode.OnChange, 100, 0, handleString, typeof(String), new int[] { 80 }));
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }