How to handle this error: Collection was modified; enumeration operation may not execute.

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5.00/5 (1 vote)

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Hello Experts

In my Project, I create many transaction and store them in dataGridView with their details.
When press startbtn this code will execute:

C#

Manager manager = new Manager(GridView);

ANd here is the Manager class:

C#

public class Manager 
{
  public static Dictionary<Thread, int> ThreadList;
  public static DataTable DisposableThreads;

  
  public Manager(DataGridView dgv)
  {
     ThreadList = new Dictionary<Thread, int>();
     DisposableThreads = new DataTable();
     DisposableThreads.Columns.Add(new DataColumn("Thread", typeof(Thread)));

     DisposableThreads.RowChanged += RowChanged;
   
     foreach (DataGridViewRow row in dgv.Rows)
     {
        Thread thread = new Thread(() => Transaction(dgv, (row as GridRow).Row));
   
        ThreadList.Add(thread, (row as GridRow).Row);
        thread.Start();
     }            
  }

  private void RowChanged(object sender, DataRowChangeEventArgs e)
  {
     if (e.Action == DataRowAction.Add)
     {
        ThreadList.Remove((Thread)e.Row.ItemArray[0]);
        DisposableThreads.Rows.Remove(e.Row);
     }
  }

  public void Transaction(DataGridView dataGridView, int row)
  {
     // Some Code
     DisposableThreads.Rows.Add(new object[] { Thread.CurrentThread });
  }
}

Every thing goes fine.
But sometimes when I press stop button to cancel the running threads the below exeception occurs.

Collection was modified; enumeration operation may not execute.

The code in Stop button is:

C#

private void Stop_Click(object sender, EventArgs e)
{
  try
  {
     foreach (Thread t in Manager.ThreadList.Keys)
     {
        if (t.ThreadState == System.Threading.ThreadState.Running)
           t.Abort(); // they really do not abort by this code !!??

     } 
     MessageBox.Show("All operations canceled successfully.");
  }
  catch (Exception ex)
  {
     MessageBox.Show(ex.Message);
  }
}

Posted 12-Apr-14 20:33pm

Meysam Toluie

Updated 12-Dec-18 18:32pm

v2

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Comments

johannesnestler 14-Apr-14 10:03am

Sounds like a little confusion cause threads are involved etc. But the cause of the error is quite simple. You are changing a collection while you are enumerating it - this is not allowed in C#. Ashok Rathod showed you a way to circumenvent this problem by creating a temp list... I just want to add the answer to your commented question: No, Thread.Abort just throws an exception on the aborted thread, you have to handle the exception in your threads code for yourself. So nothing will be aborted (but it will may fail due unhandled excpetion) if you don't "react" to it in your code. The best advice if in doubt: have a look at MSDN! (Thread.Abort is well documented, as most parts of .NET Framework are)

Meysam Toluie 15-Apr-14 2:55am

I know the cause of the error is simple. What is not simple is the answer.

johannesnestler 15-Apr-14 7:05am

Hi Meysam,
Now I had a closer look at Ashoks solution I saw the problem. You have to create an independend(!) collection with the threads to Abort (kind of snapshot at that time). After that try to abort all threads without enumerating the new independend collection (use a for-Loop not foreach).

Meysam Toluie 16-Apr-14 4:53am

See my solution.

2 solutions

Solution 1

Its seems like in your stop button click you are accessing Manager.ThreadList.Keys
and its static object (it means handled by System) so when you are using dictionary object its modifiable. SO here situation is that one function is continuously modifying that collection and meanwhile stop_Click is using it...
So here you can do one thing ...

C#

private void Stop_Click(object sender, EventArgs e)
{
  try
  {
    Dictionary<thread,> TempThreadList = new Dictionary<thread,>();
    while(Manager.ThreadList.Keys.Count > 0) //will run upto there is any thread is running.
{
  TempThreadList=Manager.ThreadList; //added one more container for continuosly changing collection.

     foreach (Thread t in TempThreadList.Keys)
     {
        if (t.ThreadState == System.Threading.ThreadState.Running)
           t.Abort(); // they really do not abort by this code !!??

     }
}
     MessageBox.Show("All operations canceled successfully.");
  }
  catch (Exception ex)
  {
     MessageBox.Show(ex.Message);
  }
}

Posted 14-Apr-14 0:13am

ashok rathod

Comments

Meysam Toluie 15-Apr-14 2:53am

I had tried this one too; The same exception.

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Meysam Toluie · Accepted Answer · 2014-04-15T22:50:00

I change stop_Click method by this way:

C#

try
{
   while (Manager.ThreadList.Count != 0)
   {
      var t = Manager.ThreadList.FirstOrDefault(x => (x.Key as Thread).IsAlive == true);
                   
      Manager.DisposableThreads.Rows.Add(t.Key);
   }
  
   MessageBox.Show("All operation canceled successfully.");
}
catch (Exception ex)
{
   MessageBox.Show(ex.Message);
}

Also the RowChanged method changes like this:

C#

private void RowChanged(object sender, DataRowChangeEventArgs e)
{
  if (e.Action == DataRowAction.Add)
  {
      if (((Thread)e.Row.ItemArray[0]).IsAlive)
          ((Thread)e.Row.ItemArray[0]).Abort();

      ThreadList.Remove((Thread)e.Row.ItemArray[0]);
      DisposableThreads.Rows.Remove(e.Row);
    }
}