Processing Parallel loop with UI Updates

Question

THERE'S AN UPDATE BELOW THIS INITIAL QUESTION

I have a query that pulls in about 90,000 header records. I then want to iterate through that result set to get detail data for each header retrieved. If I process it linearly it take close to an hour and a half. If I parallelize it, I can get it done in 11 minutes. However, there's no screen updates. I have done multi-threaded applications lots of times, and have always been successful with doing things like:

this.lblStatus.Invoke(new MethodInvoker(() => this.lblStatus.Text = "Updating Standards Docs"));

However, this appears to really screen up a Parallel loop. Using that method for some screen updates, the Parallel loop never actually finished. So I need another method.

I've been trying:

Task.Factory.StartNew(() =>
            {
                OrderablePartitioner<PayrollHeader> partitioner = Partitioner.Create(query, EnumerablePartitionerOptions.NoBuffering);
                Parallel.ForEach(partitioner, thisCheck =>
                {
                    Interlocked.Increment(ref iChckNo);

                    lock (_status)
                    {
                        _status.ProcessMsg = "Voucher " + thisCheck.VoucherNumber;
                        _status.ProcessName = thisCheck.EmployeeName;
                        _status.CurrentRec = iChckNo;
                        dtSpan = DateTime.Now.Subtract(dtSpanStart);
                        _status.TimeMsg = string.Format("Elapsed {0}:{1}:{2}", dtSpan.Hours, dtSpan.Minutes, dtSpan.Seconds);
                    }

                    BeginInvoke((Action) (() =>
                    {
                        lblVoucher.Text = _status.ProcessMsg;
                        lblName.Text = _status.ProcessName;
                        lblCount.Text = string.Format("Record {0} of {1}", _status.CurrentRec, _status.TotalRecs);
                        lblTime.Text = _status.TimeMsg;
                        Application.DoEvents();
                    }));

                    thisCheck.GetDetails();
                });
            }).Wait();

The wait on the Task is because afterwards I need to do something else with the query afterwards, which I'll put into a ContinueWith statement eventually, I just really need to get the screen update to work.

I know all about cross thread corruption, which is why I'm trying to use the Invoker method... I firmly believe long running processes still need to keep the user informed, which is why I'm attempting this.

BTW, it's a WinForms app, not a WPF app. Any help at all would be greatly appreciated...

UPDATE:So someone wanted to see the updated code, with the IProgress into it.

Status _status = new Status {TotalRecs = query.Count};

            var progress = new Progress<Status>(msg => WriteStatusUpdate(msg));

            Task.Run(() =>
            {
                OrderablePartitioner<PayrollHeader> partitioner = Partitioner.Create(query, EnumerablePartitionerOptions.NoBuffering);
                Parallel.ForEach(partitioner, thisCheck =>
                {
                    lock (_status)
                    {
                        _status.ProcessMsg = "Voucher " + thisCheck.VoucherNumber;
                        _status.ProcessName = thisCheck.EmployeeName;
                        _status.CurrentRec = ++iChckNo;
                        dtSpan = DateTime.Now.Subtract(dtSpanStart);
                        _status.TimeMsg = string.Format("Elapsed {0}:{1}:{2}", dtSpan.Hours, dtSpan.Minutes, dtSpan.Seconds);
                    }

                    ((IProgress<Status>) progress).Report(_status);
                    thisCheck.GetDetails();
                });
            }).Wait();

 private void WriteStatusUpdate(Status _status)
    {
        lblVoucher.Text = _status.ProcessMsg;
        lblVoucher.Refresh();

        lblName.Text = _status.ProcessName;
        lblName.Refresh();

        lblCount.Text = string.Format("Records {0} of {1}", _status.CurrentRec, _status.TotalRecs);
        lblCount.Refresh();

        lblTime.Text = _status.TimeMsg;
        lblTime.Refresh();
    }

The code to update the screen never gets called...

Answer 1

Don't try to update the UI from inside the parallel loop. It's not just that you can't update the UI from inside a background thread, it results in ugly and unmaintainable code. The parallel loop should do processing. Reporting should be performed by someone else.

The .NET Framework provides the IProgress< T> interface to report progress and the default implementation Progress< T> raises an event or calls a delegate on its creator thread, eg the UI thread. This results in much simpler code, eg:

var stopwatch = Stopwatch.StartNew();
var progressImpl=new Progress<Tuple<int,string,string>>(
                     msg=>ReportProgress(msg,stopwatch))
IProgress<Tuple<int,string,string>> progress=progressImpl;

var  partitioner = Partitioner.Create(query, EnumerablePartitionerOptions.NoBuffering);
Task.Run(()=> Parallel.ForEach(partitioner, thisCheck =>
            {
             .... 
             var msg=Tuple.Create(iChckNo,thisCheck.VoucherNumber,thisCheck.EmployeeName);
             progress.Report(msg);
             ...
            })
        );

 ...
private void ReportProgress(Tuple<int,string,string> msg,Stopwatch stopwatch)
{
    _status.ProcessMsg = "Voucher " + msg.Item2;
    _status.ProcessName = msg.Item3;
    _status.CurrentRec = msg.Item1;
    _status.TimeMsg = string.Format("Elapsed {0:c}", stopwatch.Elapsed);
};

I'm being very lazy here by using a Tuple<int,string,string> instead of a more specific class.

Messages sent from inside the parallel loop will be marshaled on the UI thread by Progress<T> and the ReportProgress function will be called on the UI thread itself.

The cast to IProgress< T> is necessary because the Publish method is explicitly implemented. This is a safety measure to prevent programmers from coding against the implementation itself.

Answer 2

I couldn't get the IProgress thing to actually work, so what I ended up doing, which is probably not the best approach is I put the Parallel.ForEach in a Task, and in the loop I update a public object. When the Task actually starts I sit in a while loop until it's done, and in that while loop I'm updating the UI...

bool blDone = false;
int iChckNo = 0;
_status.TotalRecs = query.Count;
Task.Run(() =>
{
    OrderablePartitioner<PayrollHeader> partitioner = Partitioner.Create(query, EnumerablePartitionerOptions.NoBuffering);
    Parallel.ForEach(partitioner, thisCheck =>
    {
        lock (_status)
        {
            iChckNo++;
            _status.ProcessMsg = "Voucher " + thisCheck.VoucherNumber;
            _status.ProcessName = thisCheck.EmployeeName;
            _status.CurrentRec = iChckNo;
            dtSpan = DateTime.Now.Subtract(dtSpanStart);
            _status.TimeMsg = string.Format("Elapsed {0}:{1}:{2}", dtSpan.Hours, dtSpan.Minutes, dtSpan.Seconds);
        }

        thisCheck.GetDetails();
    });

    blDone = true;
});

while (!blDone)
{
    WriteStatusUpdate();
}

further down in the code is

private void WriteStatusUpdate()
{
    lock (_status)
    {
        lblVoucher.Text = _status.ProcessMsg;
        lblName.Text = _status.ProcessName;
        lblCount.Text = string.Format("Records {0} of {1}", _status.CurrentRec, _status.TotalRecs);
        lblTime.Text = _status.TimeMsg;
        Application.DoEvents();
    }
}

Again, most likely not the best approach, but whatever gets it done...