c# - How to update only a property in an observable collection from thread other than dispatcher thread in WPF MVVM?

Question

I am having a class named Employee. I need to update only the property Status from thread other than current dispatcher thread:

class Employee
{
    public string Name { get; set; }
    public string Status { get; set; }
}

Viewmodel (ViewModelBase implements INotifyPropertyChanged interface):

    public abstract class ViewModelBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }


public class DisplayViewModel : ViewModelBase
{

    public static event EventHandler OnStatusChanged;

    ObservableCollection<Employee> _dailyEmployees;

    public ObservableCollection<Employee> DailyEmployees
    {
        get { return _dailyEmployees; }
        set
        {
            _dailyEmployees = value;
            OnPropertyChanged();
        }
    }

    public DisplayViewModel()
    {
        OnStatusChanged += DisplayViewModel_OnStatusChanged;
    }

    //invoked in other thread
    private void DisplayViewModel_OnStatusChanged(object sender, EventArgs e)
    {
        var d = sender as Employee;
        if (d == null)
            return;
        var em = DailyEmployees.FirstOrDefault(a => a.Name == d.Name);

        if(em == null)
        {
            DailyEmployees.Add(em);
        }
        else
        {
            em.Status = d.Status;
        }
    }
}

When i tried adding values to observable collection it throws exception as

"This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread"

How shall I proceed? Thanks.

Answer 1

Either use the dispatcher to call the Add method on the UI thread:

Application.Current.Dispatcher.BeginInvoke(() => DailyEmployees.Add(em)); 

Or call the BindingOperations.EnableCollectionSynchronization method on the UI thread to enable the collection to be used on multiple threads:

public class DisplayViewModel
{
    private readonly ObservableCollection<Employee> _dailyEmployees = new ObservableCollection<Employee>();
    private readonly object _lock = new object();

    public ObservableCollection<Employee> DailyEmployees
    {
        get { return _dailyEmployees; }
    }

    public DisplayViewModel()
    {
        System.Windows.Data.BindingOperations.EnableCollectionSynchronization(_dailyEmployees, _lock);
        OnStatusChanged += DisplayViewModel_OnStatusChanged;
    }

    //invoked in other thread
    private void DisplayViewModel_OnStatusChanged(object sender, EventArgs e)
    {
        var d = sender as Employee;
        if (d == null)
            return;
        var em = DailyEmployees.FirstOrDefault(a => a.Name == d.Name);

        if (em == null)
        {
            DailyEmployees.Add(em);
        }
        else
        {
            em.Status = d.Status;
        }
    }
}

Also note that the Employee class should implement the INotifyPropertyChanged interface and raise the PropertyChanged event for the Status property if you want the value of this one to be reflected in the view when you set it.