Question

I have a problem setting a Property on the main thread from another thread.

This is what i do at the moment:

XAML:

<TabControl Name="tab"/>

Code Behind:

TabItem tabItem = new TabItem();

Binding myBinding = new Binding("ReportView") { Source = ViewModel, Mode =    BindingMode.TwoWay, UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged };
BindingOperations.SetBinding(tabItem, TabItem.ContentProperty, myBinding);

tab.Items.Add(tabItem);

Thread t = new Thread(DoWork);
t.SetApartmentState(ApartmentState.STA);
t.Priority = ThreadPriority.Normal;
t.Start();

DoWork Method:

When i create the ReportPreviewView inside the body of the Dispatcher.BeginInvoke, i don’t get an exception:

    public void DoWork(object sender)
        {
            //I create the relavant ViewModel
            ReportViewModel reportViewModel = new ReportViewModel(SessionContext, Mediator, Parent.ReportRequest, false);

            Dispatcher.BeginInvoke((Action)(() =>
                        {
                            //Create the relevant View
                            ReportPreviewView reportPreviewView = new ReportPreviewView(reportViewModel) { ReportName = Parent.ReportRequest.Report.ReportName };
                            ViewModel.ReportView = reportPreviewView;
                        }));
        }

But this is not correct. It freezes up my UI thread. The creation of the ReportPreviewView takes quit a long time.

So then i move the creation of the ReportPreviewView outside the Dispatcher.BeginInvoke:

public void DoWork(object sender)
    {
        ReportViewModel reportViewModel = new ReportViewModel(SessionContext, Mediator, Parent.ReportRequest, false);

        ReportPreviewView reportPreviewView = new ReportPreviewView(reportViewModel) { ReportName = Parent.ReportRequest.Report.ReportName };

        Dispatcher.BeginInvoke((Action)(() =>
                    {
                        reportLoaderViewModel.ReportView = reportPreviewView;
                    }));
    }

As soon as PropertyChanged fires i get the following exception:

The calling thread cannot access this object because a different thread owns it

Any idea how i can get around this?

Answer 1

You have to create UI elements on UI dispatcher thread. There is no escape. So when you say

The creation of the ReportPreviewView takes quit a long time.

Did you anaylyse why is that? Does it have drawings, animations, heavy styles, non-virtualized panels (such as canvas)? Does it have some code in its constructor that takes time to complete?

If so, please reconsider your UI design.

In WPF the UI always freezes for heavy views. Even if you deserialize views using XAMLReader/Writer, the creation and hosting of the view object has to take place on UI thread.