2 Issues with BackgroundWorker component

Question

Hi there!

Firstly, I know I should be using proper Threading techniques (Threadpool, BeginInvoke, etc.) to accomplish this, but thats a bit over my head currently and will call for some time to read over material and understand it (if you have any URL references for my scenario, please feel free to post it).

In the interim I am using the backgroundWorker to pull a very large resultset and populate a DatagridView with it. I successfully create a SortableBindingList<TEntities> in my DoWork event and pass that out in the result. And in the RunWorkerCompleted event, I cast and bind that SortableBindingList<TEntities> to my Grid. My 2 main areas of concern are as follows:

1) Access to private variables. I want to pass one of two parameters List<long> into my DoWork event, but run a different query depending on which list was passed to it. I can get around this by declaring a class-level private boolean variable that acts a flag of sorts. This seems silly to ask, but in my DoWork, am I allowed to access that private variable and route the query accordingly? (I've tested this and it does work, without any errors popping up)

private bool SearchEngaged = false;

private void bgw_DoWork(object sender, DoWorkEventArgs e) {
    BackgroundWorker worker = sender as BackgroundWorker;
    e.Result = GetTasks((List<long>)e.Argument, worker, e);
}
SortableBindingList<Task> GetTasks(List<long> argsList, BackgroundWorker worker, DoWorkEventArgs e) {
    SortableBindingList<Task> sbl = null;
    if (worker.CancellationPending) {
        e.Cancel = true;
    }
    else {
        if (SearchEngaged) {
            sbl = DU.GetTasksByKeys(argsList);
        }
        else {
            sbl = DU.GetTasksByDivision(argsList);
        }
    }
    return sbl;
}

2) UI Thread freezes on beginning of RunWorkerCompleted. Ok, I know that my UI is responsive during the DoWork event, 'cos it takes +/- 2seconds to run and return my SortableBindingList<Task> if I don't bind the List to the Grid, but merely populate it. However my UI freezes when I bind that to the Grid, which I am doing in the RunWorkerCompleted event. Keep in mind that my Grid has 4 image columns which I handle in CellFormatting. This process takes an additional 8 seconds to accomplish, during which, my UI is completely non-interactive. Im aware of the cross-thread implications of doing so, but is there any way I can accomplish the Grid population and formatting either in the background or without causing my UI to freeze? RunWorkeCompleted looks like so:

private void bgw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) {
    if (e.Cancelled) {
        lblStatus.Text = "Operation was cancelled";
    }
    else if (e.Error != null) {
        lblStatus.Text = string.Format("Error: {0}", e.Error.Message);
    }
    else {
        SortableBindingList<Task> sblResult = (SortableBindingList<Task>)e.Result;
        dgv.DataSource = sblResult;
        dgv.Enabled = true;
        TimeSpan Duration = DateTime.Now.TimeOfDay - DurationStart;
        lblStatus.Text = string.Format("Displaying {0} {1}", sblResult.Count, "Tasks");
        lblDuration.Visible = true;
        lblDuration.Text = string.Format("(data retrieved in {0} seconds)", Math.Round(Duration.TotalSeconds, 2));
        cmdAsyncCancel.Visible = false;
        tmrProgressUpdate.Stop();
        tmrProgressUpdate.Enabled = false;
        pbStatus.Visible = false;
    }
}

Sorry for the lengthy query, but I will truly appreciate your responses! thank you!

Answer 1

Your code appears to be doing exactly the right thing.

As for the 8 seconds that it takes for the UI thread to update the screen, there's not much you can do about that. See my answer to this question.

To optimise the UI part, you could try calling SuspendLayout and ResumeLayout on the grid or its containing panel.

You could also look at trying to reduce the amount of processing that is done during the data binding. For example:

• Calculations done in the grid could be moved into the data model (thereby doing them in the worker thread).
• If the grid auto-calculates its columns based on the data model, then try hard-coding them instead.
• EDIT: Page the data in the Business Layer and make the grid only show a small number of rows at a time.

Answer 2

I think the easiest solution for your problem is setting the datasource of your grid in DoWork instead of RunWorkerCompleted using Dispatcher.BeginInvoke which you have mentioned yourself. Something like this:

private bool SearchEngaged = false;

private void bgw_DoWork(object sender, DoWorkEventArgs e)
{
    BackgroundWorker worker = sender as BackgroundWorker;
    SortableBindingList<Task> sblResult = GetTasks((List<long>)e.Argument, worker, e);

    BeginInvoke((Action<object>)(o => dataGridView1.DataSource = o), sblResult);
}

private void bgw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    if (e.Cancelled) {
        lblStatus.Text = "Operation was cancelled";
    }
    else if (e.Error != null) {
        lblStatus.Text = string.Format("Error: {0}", e.Error.Message);
    }
    else
    {
        dgv.Enabled = true;
        TimeSpan Duration = DateTime.Now.TimeOfDay - DurationStart;
        lblStatus.Text = string.Format("Displaying {0} {1}", sblResult.Count, "Tasks");
        lblDuration.Visible = true;
        lblDuration.Text = string.Format("(data retrieved in {0} seconds)", Math.Round(Duration.TotalSeconds, 2));
        cmdAsyncCancel.Visible = false;
        tmrProgressUpdate.Stop();
        tmrProgressUpdate.Enabled = false;
        pbStatus.Visible = false;
    }
}

As far as the private variable issue is concerned, I don't think it will be of any problem in your case. In case you are changing it using some UI event, just mark the private field as volatile. The documentation of the volatile keyword can be found here:

http://msdn.microsoft.com/en-us/library/x13ttww7.aspx