C# multithreading some threads seems to not run

Question

I have an item processing application, and I want to process a number of items in parallel. But, it appears sometimes it doesn't process one item at all, and sometimes it processes more than once. My code below

On a timer tick, a new thread is created which prepares an item list from database, then locks every item so that it won't be picked in next loop.

SqlConnection connection = GetConnection();
string selectCommand = my_select_command_where_IsLocked_is_null;
List<Item> itemList = new List<Item>();
using(SqlDataAdapter adapter = new SqlDataAdapter(selectCommand, connection))
{
  using(SqlCommandBuilder cbCommandBuilder = new SqlCommandBuilder(adapter))
  {
    DataTable dtItemStore = new DataTable();
    adapter.Fill(dtItemStore);
    int totalRows = dtItemStore.Rows.Count;
    if(totalRows > 0)
    {
      adapter.UpdateCommand = cbCommandBuilder.GetUpdateCommand();
      foreach(DataRow row in dtItemStore.Rows)
      {
        Item pickedItem = new Item();
        pickedItem.Key = row["Key"].ToString();
        // Set all Item properties
        itemList.Add(pickedItem);
        row["IsLocked"] = 1;
      }
    }
    adapter.Update(dtItemStore);
  }
}

Then it iterates through the list, and fires a new thread for each element to process the item.

var keyList = "";
foreach(Item pickedItem in itemList)
{
  Thread.Sleep(15); // Just to delay a bit
  var newThread = new Thread(delegate() { ProcessItem(pickedItem); });
  newThread.Start();
  Logger.Log(" Spawn thread: " + pickedItem.Key + ", ThreadState: " + newThread.ThreadState);
  keyList = keyList + pickedItem.Key;
}
Logger.Log("Keys picked: " + keyList);

Item processor function unlocks the item at the end of the loop.

function ProcessItem(Item pickedItem)
{
  Logger.Log("Let's process !!! " + pickedItem.Key);
  // Item processing code

  UnlockItem(pickedItem); // unlock and write log
}

The problem is, after a while I realized there is too many locked items in my database, and the number is increasing. Logging at specific points I got something like below

- Spawn thread: 20779205, ThreadState: Running
- Let's process !!! 20779205
-  Spawn thread: 20779206, ThreadState: Running
- Let's process !!! 20779206
-  Spawn thread: 20779207, ThreadState: Running
- Let's process !!! 20779207
-  Spawn thread: 20779208, ThreadState: Running <---Not processed
- Let's process !!! 20779209                    <---Duplicate 
-  Spawn thread: 20779209, ThreadState: Running
- Let's process !!! 20779209
-  Spawn thread: 20779211, ThreadState: Running <---Not processed
- Let's process !!! 20779213
-  Spawn thread: 20779213, ThreadState: Running
- Let's process !!! 20779228
-  Spawn thread: 20779228, ThreadState: Running
- Let's process !!! 20779231                   
- Let's process !!! 20779231                    <---Duplicate
-  Spawn thread: 20779231, ThreadState: Running
-  Spawn thread: 20779237, ThreadState: Running
- Keys picked: 20779205, 20779206, 20779207, 20779208, 20779209, 20779211,   20779213, 20779228, 20779231, 20779237, 
- Let's process !!! 20779237
- STOP processing. Possible duplicate for 20779209
- STOP processing. Possible duplicate for 20779231
- Unlock Key: 20779209, Row count:1
- Unlock Key: 20779231, Row count:1
- Unlock Key: 20779209, Row count:1
- Unlock Key: 20779237, Row count:1
- Unlock Key: 20779228, Row count:1
- Unlock Key: 20779206, Row count:1
- Unlock Key: 20779207, Row count:1
- Unlock Key: 20779205, Row count:1
- Unlock Key: 20779231, Row count:1

Here I could see that sometimes the ProcessItem is not being hit (20779208, 20779211). I couldn't understand why this could be... after spending two days on this, I am totally lost.

Is this something typical in multi-threaded environment? How can I better handle this? I have to make sure that every Item is processed once and only once.

Also, I realized sometimes same Item is being processed more than once (20779209, 20779231), however, I handled it in the ProcessItem function to avoid duplication. Is there a link between those two scenarios ?

What am I doing wrong?

Answer 1

from output line Keys picked: 20779205, 20779206, 20779207, 20779208, 20779209, 20779211, 20779213, 20779228, 20779231, 20779237 i would conclude that all items were hit (one time each)

may be you should modify your code to:

var keyList = "";
foreach(Item pickedItem in itemList)
{
  Thread.Sleep(15); // Just to delay a bit
  Item tmpItem = pickedItem; -- the same variable should not be accessed by many threads
  var newThread = new Thread(delegate() { ProcessItem(tmpItem); });
  newThread.Start();
  Logger.Log(" Spawn thread: " + pickedItem.Key + ", ThreadState: " + newThread.ThreadState);
  keyList = keyList + pickedItem.Key;
}
Logger.Log("Keys picked: " + keyList);