Question: I have this timer function, it gives me following exception.  

Collection was modified; enumeration operation may not execute
once i remove the object from hashtable.

what is the solution to implement similar functionality

```
void timerFunction(object data)
    {
    lock (tMap.SyncRoot)
    {
      foreach (UInt32 id in tMap.Keys)
      {
         MyObj obj=(MyObj) runScriptMap[id];
         obj.time = obj.time -1;
         if (obj.time <= 0)
         {                      
            tMap.Remove(id);
         }
       }
    }

```

Answer: 
The traditional way is, in your foreach loop, to collect the items to be deleted into, say, a List. Then once the foreach loop has finished, do another foreach over that List (*not* over tMap this time), calling tMap.Remove:

```
void timerFunction(object data)
{
  lock (tMap.SyncRoot)
  {
    List<UInt32> toRemove = new List<UInt32>();

    foreach (UInt32 id in tMap.Keys)
    {
      MyObj obj=(MyObj) runScriptMap[id];
      obj.time = obj.time -1;
      if (obj.time <= 0)
      {                      
        toRemove.Add(id);
      }
    }

    foreach (UInt32 id in toRemove)
    {
      tMap.Remove(id);
    }
  }
}

```
You could make a temporary list, and add the Ids to remove to that list. Then after your loop finishes, remove all items in that list:

```
void timerFunction(object data)
    {
    lock (tMap.SyncRoot)
    {
      IList<UInt32> toDelete = new List<UInt32>();
      foreach (UInt32 id in tMap.Keys)
      {

         MyObj obj=(MyObj) runScriptMap[id];
         obj.time = obj.time -1;
         if (obj.time <= 0)
         {                      
            toDelete.add(id);
         }
       }
      foreach(UInt32 i in toDelete)
      {
          tMap.Remove(i);
      }
    }

```