How to update WPF(MVVM) UI from WCF callback method

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Hi all,

I am using wcf duplex communication in my project and i have implemented callback method in my Viewmodel. Data is coming to my viewmodel but it is not adding to Observable collection. it is always showing count ass Zero.

Application.Current.Dispatcher.BeginInvoke(
 new Action(() =>
 {
     FromCallBack = new LogEvents()
     {
         DateAndTime = DateTime.Now.ToString()
     };
     clctionlogs.Add(FromCallBack);

 }));

What I have tried:

I have tried with Dispatcher and SendOrPostCallback threading concepts but still data is not adding to collection.

If i try to popup a message box with same incoming data it is working fine.

Posted 8-Jan-19 0:44am

Member 13940788

Updated 8-Jan-19 2:40am

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TABiSH777 8-Jan-19 8:00am

Hi, its really difficult to answer you from the detail that you have provided, but there can be following issues,
1) please check names & their convention, & whether you are adding in the same collection or not.
2) please check if you are even initializing that observable collection or not, other wise it may be giving object reference... error.

Basically you need to put a debugger over your observable collection property & check whether or not its been even called or not at the time of adding.

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Graeme_Grant · Answer 1 · 2019-01-08T02:40:00

It's difficult to tell from the code posted but a common mistake made that breaks Data Binding is to reinitialize an ObservableCollection rather than clearing it after the data binding is set.

So the mistake is to do this:

C#

public class MyViewModel
{
    public ObservableCollection<string> Messages { get; }

    public MyViewModel()
    {
        Messages = new ObservableCollection<string>();
    }

    public void GetData()
    {
        Messages = new ObservableCollection<string>();  // mistake
        //Retrieve data code goes here...
    }
}

The correct method is as follows:

C#

public class MyViewModel
{
    public ObservableCollection<string> Messages { get; }

    public MyViewModel()
    {
        Messages = new ObservableCollection<string>();
    }

    public void GetData()
    {
        Messages.Clear(); // correct
        //Retrieve data code goes here...
    }
}

Dirk Bahle · Answer 2 · 2019-01-08T01:38:00

Hi,

you need to ensure that your binding is working correctly - for that, look at the output window - is there a BindingExpression Error that you might need to fix?

If thats not the case, review your binding in the UI control eg. ListBox should look something like this:

XML

<ListBox ItemsSource="{Binding WCFItemsList}"/>

where WCFItemsList should be your implementation of the observable Collection in the bound ViewModel:

C#

public property ObservableCollection<MyWCFItems> WCFItemsList
{
  get
  {
    return _WCFItemsList;
  }
}

Beyond from this, you should also test with debugger breakpoints in the viewmodel to ensure that your observable collection is loaded correctly.

You might not see the count in the UI because its not an INotifyPropertyChanged property?

Its realy hard to say anything specific since you give no useful source code sample but I hope these checkpoints help you investigate the issue ...

How to update WPF(MVVM) UI from WCF callback method

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