WPF UI updating threading issue

Question

UPDATE

I have solved this problem. I found I wasn't properly synchronising access to the line and chart objects. Instead of locking them for the duration of the handler, I locked them only for the moment they were used. I think something inside the Dynamic Data Display library was using deferred execution and trying to access one of the objects when I wasn't expecting it.

TL;DR: Properly synchronise locks.

I'm currently working on a GUI using WPF to visualise an algorithm in some class library. All of the controls I'm using within the GUI are either standard WPF or from the Microsoft Dynamic Data Display library.

I'm getting the very common "The calling thread cannot access this object because a different thread owns it" error. I have done a fair amount of searching around but cannot find a solution to this myself.

Right now, I'm a little confused as to how to add lines to the chart if the thread doesn't own it. I've tried using the dispatcher as many other answers on this site have suggested, but it hasn't solved this.

The summary window is intended to be a child window of the main window, which opens before the simulation runs and display data in real time.

The simulator publishes a number of events which are all handled asynchronously (internally, it uses BeginInvoke and EndInvoke). Could this cause a problem in this scenario?

The summary view model registers a number of visualisations, which in turn create their own controls and add them to a canvas provided by the summary window.

A very brief outline of the code is below

public class MainWindowModel
{
    //...
    public async void RunSimulation()
    {
        WindowService.OpenWindowFor<SimulationSummaryViewModel>(SimSummaryVM);
        await Simulator.Run();
    }
}

public class SimulatorSummaryViewModel
{
    //...
    public SimulatorSummaryViewModel()
    {
        Visualisations.Add(new RealTimeChart());
    }
    //...
}

public class RealTimeChart : IVisualisation
{
    // ...
    private ChartPlotter _Chart; //From Dynamic Data Display library

    //Handles adding information to the current line 
    private void OnSimulatorStepHandler(Args a)
    { 
      /* The exception occurs in here, when adding a data point, 
         whilst another handler is adding a line to the chart.
         I have tried adding a lock to the chart and line objects, but it had no effect
      */ 
    }

    //Handles adding the current line to the chart
    private void OnSimulatorRepetitionComplete(Args a)
    {
        //lineToAdd is an EnumerableDataSource<DataPoint>.
        //DataPoint is a simple class with two primitive properties.
        _Chart.Dispatcher.Invoke(new Action(()=>
        {
            _Chart.AddLineGraph(lineToAdd);
        }));
    }
}

public class SummaryWindow : Window
{
    // ...
    public SummaryWindow()
    {
        // ...
        foreach(IVisualisation Vis in ViewModel.Visualisations)
        {
            Vis.Draw(this.VisCanvas);
        }
    }
}

It's an EnumerableDataSource<DataPoint> where data point is some internal class with two primitive properties. I'll add it to my question. — Andy Hunt
– Andy Hunt, Commented Mar 7, 2013 at 13:53
can you be more specific on the contents of OnSimulatorStepHandler? — sim1
– sim1, Commented Mar 7, 2013 at 14:04
Thanks for your enquiries, take a look at my update :) I'll post that an an answer also. — Andy Hunt
– Andy Hunt, Commented Mar 7, 2013 at 14:05

AkselK · Accepted Answer · 2013-03-07 14:08:15Z

2

I assume that OnSimulatorRepetitionComplete is called from another thread, making the call to the chart happening in that thread.

In WPF, all GUI operations need to be done in the Dispatcher Thread. Add this code around the contents in your method:

Dispatcher.BeginInvoke(new Action(() =>
{

}));

So it becomes this:

private void OnSimulatorStepHandler(Args a)
{ 
    Dispatcher.BeginInvoke(new Action(() =>
    {
        // Add your code here
    }));
}

answered Mar 7, 2013 at 14:08

AkselK

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Andy Hunt · Accepted Answer · 2013-03-07 14:19:48Z

I have solved this problem, so I will post my solution here.

Due to the asynchronously handled events being published by the simulator and the implied synchronous handling in the visualisation (a number of steps happen, then a repetition end, then repeat), better synchronisation of access to the chart and line was needed. Below is my (real) original code.

The Chart control I'm using defers use of the seriesToAdd collection (which references the line data) until the chart is added. Since the line data (and chart) weren't locked for the lifetime of the handlers, there was opportunity for the line data to be accessed when it wasn't locked (by me) but in use in the chart control.

I won't post the resulting code, because it simply moves the locks to encapsulate the full method.

[STAThread]
private void OnRepetitionComplete(Evolve.Core.Simulation.RepetitionResult Result)
{
    //Convert the data series to something usable for the chart
    EnumerableDataSource<DataPoint> seriesToAdd = new EnumerableDataSource<DataPoint>(_obCurrentRepetitionDataSeries);

    //Translate the data point properties in to X&Y coords on the graph
    seriesToAdd.SetXYMapping(DP => new System.Windows.Point(DP.Step, DP.Number));

    //Get the line colour and add the line to the chart
    System.Windows.Media.Color lineColor = GetLineColor(Result.ReasonForCompletion);

    lock (chartLockObject)
    {
        lock (lineLockObject)
        {
            _obChart.Dispatcher.Invoke(new Action(() =>
            {
                _obChart.AddLineGraph(seriesToAdd, lineColor, 0.5d);
            }));

            //Renew the data series
            _obCurrentRepetitionDataSeries = new DataSeries();
        }
    }
}

private void OnStep(int StepNumber, int HealthyNodes, int MutantNodes)
{
    //Make sure there's a data series to add to
    if (_obCurrentRepetitionDataSeries == null)
    {
        _obCurrentRepetitionDataSeries = new DataSeries();
    }

    lock (lineLockObject)
    {
        //Add the step to the series
        _obCurrentRepetitionDataSeries.Add(new DataPoint() { Number = MutantNodes, Step = StepNumber });
    }
}

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