Fill ListView using Parallel.For/ Parallel.Foreach and ConcurrentBag

Avoid thread safe problem using Parallel.For/ Parallel.Foreach and ConcurrentBag to fill the ListView.

If we have small list of items to add into ListView then we can easily use simple looping but if the List of items is large then it will take time to add the items into ListView. In this case we can use multithreading to process the items so that we can speed up the process.

Problem-

Before explaining the problem know about thread safe. Thread safe means multiple thread can access the common data without any problem.

As per the definition of ListView class, it is not thread safe means any instance members of ListVIew class are not guaranteed to be thread safe. So, while using ListView with multithreading it may create the problem.  To explain this I have just written the following code-

First add the following namespaces-

using System.Threading.Tasks;

private void btnFillList_Click(object sender, EventArgs e)
         {
             try
            {
                 lstView.Items.Clear();
                 Parallel.For(0, 1000, i =>
                 {
                      lstView.Items.Add(i.ToString());
                });
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }


In above code, I have used Parallel.For to process the items which will run parallel thread. But I got this error-

“Cross-thread operation not valid: Control 'lstView' accessed from a thread other than the thread it was created on.”





The reason of this error -  ListView Class is not thread safe and in above code more than one thread are trying to add the items to ListView because of that it is generating error.

Workaround-

ListView class is not thread safe it doesn’t mean that we cannot use multithreading. Here we have workaround for this. For this we can use ConcurrentBag<T> class which represents a thread-safe, unordered collection of objects. So I have modified my code like this-

First add the following namespaces-

using System.Collections.Concurrent;

private void btnFillListConcurrentBag_Click(object sender, EventArgs e)
         {
             try
            {
                 lstView.Items.Clear();  
                ConcurrentBag<ListViewItem> cbItems = new ConcurrentBag<ListViewItem>();
                 //Using multithreading to process the item
                 Parallel.For(0, 1000, i =>
                {
                    ListViewItem item = new ListViewItem(i.ToString());
                     cbItems.Add(item);
                 });

                 //Finally adding items to ListView
                lstView.Items.AddRange((ListViewItem[])cbItems.ToArray());
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

In above code, I have taken the array object of ConcurrentBag<ListViewItem>. In the loop I am adding items to cbItems object. Here ConcurrentBag is thread safe so it will not create any problem. Finally I am adding complete cbItems object to ListView using lstView.Items.AddRange() method.

Note -

Same workaround can be used with Parallel.Foreach also.

This code is working without any error but ListView items will be unordered.

You can download the complete code from here- ListView