Question

My scenario: I have a background thread that polls for changes and periodically updates a WPF DataGrid’s ObservableCollection (MVVM-style). The user can click on a row in the DataGrid and bring up the “details” of that row in an adjacent UserControl on the same main view.

When the background thread has updates, it cycles through the objects in the ObservableCollection and replaces individual objects if they have changed (in other words, I am not rebinding a whole new ObservableCollection to the DataGrid, but instead replacing individual items in the collection; this allows the DataGrid to maintain sorting order during updates).

The problem is that after a user has selected a specific row and the details are displayed in the adjacent UserControl, when the background thread updates the DataGrid the DataGrid loses the SelectedItem (it gets reset back to index of -1).

How can I retain the SelectedItem between updates to the ObservableCollection?

Answer 1

If your grid is single-selection, my suggestion is that you use the CollectionView as the ItemsSource instead of the actual ObservableCollection. Then, make sure that Datagrid.IsSynchronizedWithCurrentItem is set to true. Finally, at the end of your “replace item logic”, just move the CollectionView’s CurrentItem to the corresponding new item.

Below is a sample that demonstrates this. (I’m using a ListBox here though. Hope it works fine with your Datagrid).

EDIT – NEW SAMPLE USING MVVM:

XAML

<Window x:Class="ContextTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        x:Name="window"
        Title="MainWindow" Height="350" Width="525">
    <DockPanel>
        <ListBox x:Name="lb" DockPanel.Dock="Left" Width="200" 
                 ItemsSource="{Binding ModelCollectionView}"
                 SelectionMode="Single" IsSynchronizedWithCurrentItem="True">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Path=Name}"/>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

        <TextBlock Text="{Binding ElementName=lb, Path=SelectedItem.Description}"/>

    </DockPanel>
</Window>

Code-Behind:

using System;
using System.Windows;
using System.Windows.Data;
using System.Collections.ObjectModel;
using System.Windows.Threading;

namespace ContextTest
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = new ViewModel();
        }
    }

    public class ViewModel
    {
        private DataGenerator dataGenerator;
        private ObservableCollection<Model> modelCollection;
        public ListCollectionView ModelCollectionView { get; private set; }

        public ViewModel()
        {
            modelCollection = new ObservableCollection<Model>();
            ModelCollectionView = new ListCollectionView(modelCollection);

            //Create models
            for (int i = 0; i < 20; i++)
                modelCollection.Add(new Model() { Name = "Model" + i.ToString(), 
                    Description = "Description for Model" + i.ToString() });

            this.dataGenerator = new DataGenerator(this);
        }

        public void Replace(Model oldModel, Model newModel)
        {
            int curIndex = ModelCollectionView.CurrentPosition;
            int n = modelCollection.IndexOf(oldModel);
            this.modelCollection[n] = newModel;
            ModelCollectionView.MoveCurrentToPosition(curIndex);
        }
    }

    public class Model
    {
        public string Name { get; set; }
        public string Description { get; set; }
    }

    public class DataGenerator
    {
        private ViewModel vm;
        private DispatcherTimer timer;
        int ctr = 0;

        public DataGenerator(ViewModel vm)
        {
            this.vm = vm;
            timer = new DispatcherTimer(TimeSpan.FromSeconds(5), 
                DispatcherPriority.Normal, OnTimerTick, Dispatcher.CurrentDispatcher);
        }

        public void OnTimerTick(object sender, EventArgs e)
        {
            Random r = new Random();

            //Update several Model items in the ViewModel
            int times = r.Next(vm.ModelCollectionView.Count - 1);
            for (int i = 0; i < times; i++)
            {   
                Model newModel = new Model() 
                    { 
                        Name = "NewModel" + ctr.ToString(),
                        Description = "Description for NewModel" + ctr.ToString()
                    };
                ctr++;

                //Replace a random item in VM with a new one.
                int n = r.Next(times);
                vm.Replace(vm.ModelCollectionView.GetItemAt(n) as Model, newModel);
            }
        }
    }
}

OLD SAMPLE:

XAML:

<Window x:Class="ContextTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <ListBox x:Name="lb" SelectionMode="Single" IsSynchronizedWithCurrentItem="True" SelectionMode="Multiple">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Path=Name}"/>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

        <TextBlock Text="{Binding ElementName=lb, Path=SelectedItem.Name}"/>
        <Button Click="Button_Click">Replace</Button>


    </StackPanel>
</Window>

Code-behind:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Collections.ObjectModel;
using System.ComponentModel;

namespace ContextTest
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        ObservableCollection<MyClass> items;
        ListCollectionView lcv;

        public MainWindow()
        {
            InitializeComponent();

            items = new ObservableCollection<MyClass>();
            lcv = (ListCollectionView)CollectionViewSource.GetDefaultView(items);
            this.lb.ItemsSource = lcv;
            items.Add(new MyClass() { Name = "A" });
            items.Add(new MyClass() { Name = "B" });
            items.Add(new MyClass() { Name = "C" });
            items.Add(new MyClass() { Name = "D" });
            items.Add(new MyClass() { Name = "E" });

        }

        public class MyClass
        {
            public string Name { get; set; }
        }

        int ctr = 0;
        private void Button_Click(object sender, RoutedEventArgs e)
        {
            MyClass selectedItem = this.lb.SelectedItem as MyClass;
            int index = this.items.IndexOf(selectedItem);
            this.items[index] = new MyClass() { Name = "NewItem" + ctr++.ToString() };
            lcv.MoveCurrentToPosition(index);
        }

    }
}