Calling events on PLC Tag datachage in C#

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C#

event

I need to read the value from PLC and display it in a form whenever the PLC Tag value changes.

There will list of tags which I need to monitor. Whenever a TAG value changes i need to call a function(different functions for each tag).

This is what I have done so far for capturing the Tag value change..

After connecting to PLC, i ll create the LIST of tags.

Read TAG values in timer.

While reading i ll check with OLDVALUES tag, if there is any change in value I ll raise an event.

This is working fine with 4 or 5 tags. When the Tag count is more, say 100, some of the Tag change events are not firing..

This is what so far I have done..

public delegate void DataChangedEventHandler(string TagName, string NewValue);
   private Timer tmr = new Timer();

   public event DataChangedEventHandler OnDataChanged;
   private void StartTimer(DataTable dt)
   {
       AddTagList(dt);
       SetInitialVales();
       tmr.Tick += timerticks;
       tmr.Interval = 250;
       tmr.Enabled = true;
       tmr.Start();
   }
   private void StopTimer()
   {
       tmr.Enabled = false;
       tmr.Stop();
   }

I ll add the list of tags..

private List<string> TagValues = new List<string>();
   private List<string> oldValues = new List<string>();
   private List<string> newValues = new List<string>();
   private void AddTagList(DataTable dt)
   {
       int ILoop = 0;

       foreach (DataRow row in dt.Rows)
       {
           TagValues.Add((string)row["Path"]);
           ILoop = ILoop + 1;
       }
   }

To set the initial values of the Tags.

private void SetInitialVales()
    {
        int iLoop = 0;
        foreach (string vals in TagValues)
        {
            var rd = ReadTag(vals);
            oldValues.Add(rd.ToString());
            newValues.Add(rd.ToString());
            iLoop = iLoop + 1;
        }
        //newValues = oldValues
    }

and the main datachange part.

private void timerticks(object sender, EventArgs eventArgs)
 {
     int iLoop = 0;
     foreach (string vals in TagValues)
     {
         oldValues[iLoop] = ReadTag(vals).ToString();
         if (oldValues[iLoop] != newValues[iLoop])
         {
             newValues[iLoop] = oldValues[iLoop];
             if (OnDataChanged != null)
             {
                 OnDataChanged(vals, newValues[iLoop]);
             }
         }
         iLoop = iLoop + 1;
     }
 }

My Queries:

1.What will happen if a event is raised while already raised event is still in progress(sub procedure is not completed)?? Is because of this reason I am missing some datachange events??

2.How to raise a raise a event automatically whenever the member of LIST value changes??

3.Any other better method to handle the timer-read-raiseevent?