C＃我从Task或Dispatcher得到null

Question

I have this code:

public void OpenReport(XtraReport report, int zoomMode)
    {
        //Show loading animation.
        MainInterfaceViewModel.State = States.Busy;
        //Load the report, then navigate to the ReportViewer
        Task tsk = Task.Factory.StartNew(() => LoadReport(report, zoomMode));
        //After completing the task hide the loading animation
        tsk.ContinueWith(obj => MainInterfaceViewModel.State = States.Idle);
     }

private void LoadReport(XtraReport report, int zoomMode)
    {
        ReportViewerViewModel reportViewerViewModel;
        reportViewerViewModel = new ReportViewerViewModel(report, zoomMode);
        ReportViewer.Dispatcher.Invoke(new Action(() => ReportViewer.DataContext = reportViewerViewModel)) ;
        Frame.Navigate(ReportViewer);
    }

But when the OpenReport method is executed I get an empty ReportViewer, why I'm getting this problem while I passed the Report to the method ? The image bellow shows the result I get.

The results I'm getting

EDIT:

When I change the OpenReport method to the following code I get the report shown inside the report viewer but the UI freezes when the report is preparing (about 3 seconds).

public void OpenReport(XtraReport report, int zoomMode)
    {
        //Show loading animation.
        MainInterfaceViewModel.State = States.Busy;
        //Load the report
        ReportViewerViewModel reportViewerViewModel;
        reportViewerViewModel = new ReportViewerViewModel(report, zoomMode);
        ReportViewer.DataContext = reportViewerViewModel;
        //Navigate to the ReportViewer
        Frame.Navigate(ReportViewer);
        //Hide the loading animation.
        MainInterfaceViewModel.State = States.Idle;
    }

Answer 1

As @TotaloDotoNeto tried to point out, Task.StartNew is sometimes dangerous method to use, as it uses the TaskScheduler.Current not TaskScheduler.Default property, which is quite uneasy to deal with. Stephen Cleary has a great article regarding differences between StartNew and Run methods ., and if you are interested, here is another one from Stephen Toub . So, first of all I suggest you to switch to Run method:

Task tsk = Task.Run(() => LoadReport(report, zoomMode));

Second, let's see what your code is doing: you start a Task , and inside that task your are using some background logic ( Dispatcher.Invoke ) and foreground logic ( Frame.Navigate ). I think that that problem you have is caused by your code for Frame being executed not in UI-thread. So, you have two options:

// add the Navigate method to the invoking delegate
ReportViewer.Dispatcher.Invoke(new Action(() => 
{
    ReportViewer.DataContext = reportViewerViewModel;
    Frame.Navigate(ReportViewer);
}));

or make your code async , something like this:

public async Task OpenReport(XtraReport report, int zoomMode)
{
    // Show loading animation.
    MainInterfaceViewModel.State = States.Busy;
    // Asynchronously wait for the model to load
    var task = await Task.Run(() => LoadReport(report, zoomMode));
    // process the model after it's being loaded
    ReportViewer.DataContext = task.Result;
    // Navigate to the ReportViewer
    Frame.Navigate(ReportViewer);
    // After completing the task hide the loading animation
    MainInterfaceViewModel.State = States.Idle;
}

private async Task<ReportViewerViewModel> LoadReport(XtraReport report, int zoomMode)
{
    return new ReportViewerViewModel(report, zoomMode);
}

Note that in second case you have to rewrite your code to async all the way . Another article about async/await keywords and some useful article about how to do not block on async code , if you are interested in this theme.