Question

I need help. I am working with an arraylist, and suddenly I get this error.

An unhandled exception of type ‘System.InvalidOperationException’ occurred in mscorlib.dll

Additional information: Collection was modified; enumeration operation may not execute.

This is the code where it shows the exception…

foreach (PC_list x in onlinelist) {
  if ((nowtime.Subtract(x.time)).TotalSeconds > 5) {
    Invoke(new MethodInvoker(delegate {
      index = Main_ListBox.FindString(x.PcName);
      if(index != ListBox.NoMatches)
      Main_ListBox.Items.RemoveAt(index);
    }));
    onlinelist.Remove(x);
    //Thread.Sleep(500);
  }
}

Where

public class PC_list {
    public string PcName;
    public string ip;
    public string status;
    public string NickName;
    public DateTime time;

}

Notes:

• onlinelist is an arraylist
• nowtime and x.time are DateTime.

Call Stack

mscorlib.dll!System.Collections.ArrayList.ArrayListEnumeratorSimple.MoveNext() + 0x122 bytes    
BlueBall.exe!BlueBall.BlueBall.clean_arraylist() Line 74 + 0x1a8 bytes  C#
BlueBall.exe!BlueBall.BlueBall.server() Line 61 + 0x8 bytes C#
mscorlib.dll!System.Threading.ThreadHelper.ThreadStart_Context(object state) + 0x63 bytes   
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool ignoreSyncCtx) + 0xb0 bytes    
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) + 0x2c bytes    
mscorlib.dll!System.Threading.ThreadHelper.ThreadStart() + 0x44 bytes   
[Native to Managed Transition]

Answer 1

foreach (PC_list x in onlinelist)
{            
     onlinelist.Remove(x); // cannot do this
}

This is the heart of the problem. You cannot remove an item from a collection as you are iterating over it in a foreach. Your options are to make a local copy of the list prior to the loop, loop over the copy, and remove from the original. Or you can keep a separate list of items to remove after you finish the original loop. Or you can switch to a for loop and iterate over it backwards, which allows you to remove items from the end as you go.

While you’re here, if you are not stuck working with C# 1 / .NET 1.1 / Visual Studio 2003, you might want to consider switching from ArrayList to the stronger List<T>, where T is the type of the object in the collection. In your case, that would be a List<PC_list>. You can find it at System.Collections.Generic.List<T>.

And since your question is tagged multithreading, it would also be a smart idea to consult the collections built with concurrency in mind.