Cross-Threading issue with listView

Question

OK before everyone post duplicate let me inform you I have looked at all those other post and im still lost some say use delegates or background worker etc... but how would I make this thread safe i want to delete the files on its own thread.

here is the code that i am working with.

private void button1_Click(object sender, EventArgs e)
{
    cleanFiles.RunWorkerAsync();
}

private void cleanFiles_DoWork(object sender, DoWorkEventArgs e)
{
    if (listView1.CheckedItems.Count != 0)
    {
        // If so, loop through all checked files and delete.
        for (int x = 0; x <= listView1.CheckedItems.Count - 1; x++)
        {
            string tempDirectory = Path.GetTempPath();
            foreach (ListViewItem item in listView1.CheckedItems)
            {
                string fileName = item.Text;
                string filePath = Path.Combine(tempDirectory, fileName);

                try
                {
                    File.Delete(filePath);
                }
                catch (Exception)
                {
                    //ignore files being in use
                }
            }
        }
        PaintListView(tFile);
        MessageBox.Show("Files removed");
        toolStripStatusLabel1.Text = ("Ready");
    }
    else
    {
        MessageBox.Show("Please put a check by the files you want to delete");
    }
}

Answer 1

As Reed mentioned, you cannot access UI elements from a thread other than the UI thread itself. So, you'll have to pass on a delegate Control.Invoke() to be executed with the UI Thread, like this

Try

    private void cleanFiles_DoWork(object sender, DoWorkEventArgs e)
    {
        if (listView1.CheckedItems.Count != 0)
        {
            // If so, loop through all checked files and delete.
            for (int x = 0; x <= listView1.CheckedItems.Count - 1; x++)
            {
                string tempDirectory = Path.GetTempPath();
                foreach (ListViewItem item in listView1.CheckedItems)
                {
                    string fileName = item.Text;
                    string filePath = Path.Combine(tempDirectory, fileName);

                    try
                    {
                        File.Delete(filePath);
                    }
                    catch (Exception)
                    {
                        //ignore files being in use
                    }
                }
            }

            PaintListViewAndSetLabel();
        }
        else
        {
            ShowMessageBox();
        }
    }

    private void ShowMessageBox()
    {
        if(InvokeRequired)
        {
            this.Invoke(new Action(ShowMessageBox), new object[0]);
            return;
        }
        MessageBox.Show("Please put a check by the files you want to delete");
    }

    private void PaintListViewAndSetLabel()
    {
        if (InvokeRequired)
        {
            this.Invoke(new Action(PaintListViewAndSetLabel),new object[0]);
            return;
        }
        PaintListView(tFile);
        MessageBox.Show("Files removed");
        toolStripStatusLabel1.Text = ("Ready");
    }

Answer 2

The problem is that you can't access any properties of your ListView ( listView1 ) from the background thread (ie: anything inside of the cleanFiles_DoWork method) directly. User interface controls can't be accessed on any thread other than the user interface thread.

You should, instead, make a list of the items to "clean" before calling DoWork, and pass these into the method via the RunWorkerAsync overload taking an object , and retrieve it in your method via DoWorkEventArgs.Argument .

This will allow you to pass a list of items to process, process them in the background, then update your list when you are complete.

Answer 3

Its bad idea to use control from background worker, I had the same problem recently with TreeView control. So the solution for Thread-Safe Calls to Windows Forms Controls is How-to article from microsoft. The main idea it checking safty using InvokeRequired property of your control and, if nessesary, run method invocation through Invoke method which is thread safe.