Question

I am currently working on a home project for myself.
The program is written in C# using winforms.

The problem I’m currently experiencing is as followed:

I have a listview in my mainform called lvwGames
When I run the program without debugging, it runs fine.
However when I start with a debug, I get an error. This has something to do with the background worker thread.

Allow me to post some code to assist me.

    private void MainViewLoad(object sender, EventArgs e)
    {
        RefreshGamesListView();
    }

Nothing special here.
The reason I am calling RefreshGamesListView() is because I have to refresh on several occasions.

The method being called looks like this.

    public void RefreshGamesListView()
    {
        pbRefreshGamesList.Value = 0;
        bgwRefreshList.RunWorkerAsync();
    }

So when the method is called, the background worker is called and runs the dowork method.
This one is quite big.

    private void BgwRefreshListDoWork(object sender, DoWorkEventArgs e)
    {
        List<Game> games = _mainController.RetrieveAllGames();

        int count = 1;
        foreach (Game game in games)
        {
            string id = game.id.ToString();
            var li = new ListViewItem(id, 0);
            li.SubItems.Add(game.title);
            li.SubItems.Add(game.Genre.name);
            li.SubItems.Add(game.Publisher.name);
            li.SubItems.Add(game.Platform.name);
            li.SubItems.Add(game.CompletionType.name);
            li.SubItems.Add(game.gameNotice);
            lvwGames.Items.Add(li);

            double dIndex = (double)(count);
            double dTotal = (double)games.Count;
            double dProgressPercentage = (dIndex / dTotal);
            int iProgressPercentage = (int)(dProgressPercentage * 100);

            count++;
            bgwRefreshList.ReportProgress(iProgressPercentage);
        }
    }

When i run the code in debug, when the code is on lvwGames.Items.Add(li);
It gives me the following error:

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

I have absolutely no clue why.
I think it is code specific. But it can also mean I don’t get the background worker completely, and specifically when to use it properly.

The reason I’m using it is because I’m loading a large large list from the database, I want to keep responsiveness in the UI when the list is loaded, and inform the users how far it is, using a progress bar.

If any code is missing, or you actually understand why this is happening PLEASE explain me why in this case its causing the error. You don’t need to fix it for me. I just want to know WHY it’s caused.

Thanks for taking the time to read this post. I hope to be able to continue using the debugger soon. 🙂

Answer 1

This is because you’re attempting to access a UI control (lvwGames) from a background thread. The way to make it work requires you to marshal the information back to the main UI thread and update the control from there:

private void BgwRefreshListDoWork(object sender, DoWorkEventArgs e)
{
    List<Game> games = _mainController.RetrieveAllGames();

    int count = 1;
    foreach (Game game in games)
    {
        string id = game.id.ToString();
        var li = new ListViewItem(id, 0);
        li.SubItems.Add(game.title);
        li.SubItems.Add(game.Genre.name);
        li.SubItems.Add(game.Publisher.name);
        li.SubItems.Add(game.Platform.name);
        li.SubItems.Add(game.CompletionType.name);
        li.SubItems.Add(game.gameNotice);

        // This is the new line you need:
        lvwGames.Invoke(new MethodInvoker(delegate { lvwGames.Items.Add(item) }));

        double dIndex = (double)(count);
        double dTotal = (double)games.Count;
        double dProgressPercentage = (dIndex / dTotal);
        int iProgressPercentage = (int)(dProgressPercentage * 100);

        count++;
        bgwRefreshList.ReportProgress(iProgressPercentage);
    }
}

Normally you would check the InvokeRequired property first as mentioned in other answers, but there is really no need if you are always calling it from the background thread. Your DoWork method will always require an invoke call, so you might as well just go ahead and write it like that.