Application.Current and App.Current is null

Question

I'm creating an application (an Office Add-in for Outlook)

The issue I have is updating my screen. I know I need to use invoke the Dispatcher but, it's always null in my ViewModel

    private ObservableCollection<string> _updates;
    public ObservableCollection<string> Updates
    {
        get { return this._updates; }
        set
        {
            this._updates = value;
            OnPropertyChanged("Updates");
        }
    }


        BackgroundWorker bw = new BackgroundWorker();
        bw.DoWork += ((s, e) =>
        {
            //logic
            UpdateProgress("Finished");
        });

        bw.RunWorkerAsync();


    private void UpdateProgress(string s)
    {
        //Application.Current.Dispatcher.Invoke(() =>
       // {
        App.Current.Dispatcher.Invoke(() =>
        {            
            this.Updates.Add(s);
        //});
        });
    }

As you can see, I've tried 2 approaches, but Current is always null.

Oddly, if I use the same code in the code behind of my MainWindow then the following works fine

    private void UpdateProgress(string s)
    {
        Dispatcher.Invoke(() =>
        {
            this.Update = s;
        });
    }

I've read up, the reason is because the MainWindow code behind inherits from Window.

My question is, do I have to create a new Dispatcher object or is there something I'm missing. All I'm trying to do is update my GUI whilst the thread is running.

Application.Current should be null if you attempt to access it in a non-UI thread. You can save a reference to dispatcher in constructor as a field member then use it in a non-UI thread. — Mehrzad Chehraz, Sep 22 '16 at 16:58
The normal entrypoint for a WPF is Main(). Which is auto-generated to create your App.xaml class instance, hard to see. The entrypoint is no longer Main() in an add-in, it is now you Startup event handler. So you have to create your app instance yourself. Boilerplate code [is here](http://stackoverflow.com/a/2694710/17034). — Hans Passant, Sep 22 '16 at 17:09
@HansPassant, is there any way you can move this comment to an answer? — MyDaftQuestions, Sep 23 '16 at 07:34
Just type up the answer in your own post, show what your Startup event handler looks like, and mark it as the answer. — Hans Passant, Sep 23 '16 at 07:39

score 3 · Accepted Answer · answered Jun 11 '18 at 12:42

Answered in comments by Hans Passant

The normal entrypoint for a WPF is Main(). Which is auto-generated to create your App.xaml class instance, hard to see. The entrypoint is no longer Main() in an add-in, it is now you Startup event handler. So you have to create your app instance yourself. Boilerplate code is http://stackoverflow.com/a/2694710/17034

Application.Current and App.Current is null

score 2 · Answer 2 · answered Sep 22 '16 at 18:51

As a workaround you could maybe do the following:

public MainWindow()
{
  InitializeComponent();
  AppWindow = this; // Here you set the static member to reference this MainWindow.
}

public static MainWindow AppWindow
{
  get;
  private set;
}

And then in your viewmodel:

private void UpdateProgress(string s)
{
  MainWindow.AppWindow.Dispatcher.Invoke(() =>
  {
    this.Updates.Add(s);
    //});
  });
}

score 2 · Answer 3 · answered Jun 11 '18 at 12:33

2

If you are trying to open a WPF application from a Winforms appliation, Application.Current is null. Application.Current is a feature of WPF applications only and not Winforms. You may try the below link:

Why does Application.Current == null in a WinForms application?

answered Jun 11 '18 at 12:33

shruti singh

146
11

1

Don't ignore tags, winforms != wpf. Also on SO we are closing questions as duplicates rather than creating new answers with link to duplicated questions. – Sinatr Jan 14 '19 at 15:22
You need to read the reply completely. I was trying to to help this person from something I had earlier faced; and it was in WPF, since I was creating a WPF window from another Winforms app. Don't be keen to just down vote. Also, as far as closing is concerned, not everyone can close a question. You have the required points, hence you are able to. – shruti singh Aug 26 '19 at 11:40

Application.Current and App.Current is null

3 Answers3