WPF DataGrid multiselect binding

Question

I have a datagrid that is multi-select enabled. I need to change the selection in the viewmodel. However, the SelectedItems property is read only and can't be directly bound to a property in the viewmodel. So how do I signal to the view that the selection has changed?

Answer 1

I haven't worked with the DataGrid much, but one technique that works for the ListView is to bind to the IsSelected property of the individual ListViewItem. Just set this to true for each object in your list, and then it will get selected.

Maybe the object that represents a row in the DataGrid also has an IsSelected property, and can be used in this way as well?

Answer 2

Andy is correct. DataGridRow.IsSelected is a Dependency Property that can be databound to control selection from the ViewModel. The following sample code demonstrates this:

<Window x:Class="DataGridMultiSelectSample.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:tk="clr-namespace:Microsoft.Windows.Controls;assembly=WPFToolkit"
    Title="Window1" Height="300" Width="300">
    <StackPanel>
        <tk:DataGrid AutoGenerateColumns="False" ItemsSource="{Binding}">
            <tk:DataGrid.Columns>
                <tk:DataGridTextColumn Header="Value" Binding="{Binding Value}" />
            </tk:DataGrid.Columns>
            <tk:DataGrid.RowStyle>
                <Style TargetType="tk:DataGridRow">
                    <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
                </Style>
            </tk:DataGrid.RowStyle>
        </tk:DataGrid>
        <Button Content="Select Even" Click="Even_Click" />
        <Button Content="Select Odd" Click="Odd_Click" />
    </StackPanel>
</Window>

---

using System.ComponentModel;
using System.Windows;

namespace DataGridMultiSelectSample
{
    public partial class Window1
    {
        public Window1()
        {
            InitializeComponent();
            DataContext = new[]
                              {
                                  new MyViewModel {Value = "Able"},
                                  new MyViewModel {Value = "Baker"},
                                  new MyViewModel {Value = "Charlie"},
                                  new MyViewModel {Value = "Dog"},
                                  new MyViewModel {Value = "Fox"},
                              };
        }

        private void Even_Click(object sender, RoutedEventArgs e)
        {
            var array = (MyViewModel[]) DataContext;
            for (int i = 0; i < array.Length; ++i)
                array[i].IsSelected = i%2 == 0;
        }

        private void Odd_Click(object sender, RoutedEventArgs e)
        {
            var array = (MyViewModel[])DataContext;
            for (int i = 0; i < array.Length; ++i)
                array[i].IsSelected = i % 2 == 1;
        }
    }

    public class MyViewModel : INotifyPropertyChanged
    {
        public string Value { get; set; }

        private bool mIsSelected;
        public bool IsSelected
        {
            get { return mIsSelected; }
            set
            {
                if (mIsSelected == value) return;
                mIsSelected = value;
                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs("IsSelected"));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}

Answer 3

Guys, thanks for the help. My problem was solved. I think the problem is pretty common for new WPF developers, so I will restate my problem and as well as the solution in more details here just in case someone else runs into the same kind of problems.

The problem: I have a multi-select enabled datagrid of audio files. The grid has multiple column headers. The user can multi-select several row. When he clicks the Play button, the audio files will be played in the order of one the columns headers (say column A). When playback starts, the multi-select is cleared and only the currently playing file is highlighted. When playback is finished for all files, the multi-selection will be re-displayed. The playback is done in the viewmodel. As you can see, there are two problems here: 1) how to select the currently playing file from the viewmodel, and 2) how to signal to the view from the viewmodel that playback is finished and re-display the multi-selection.

The solution: To solve the first problem, I created a property in the viewmodel that is bound to the view's SelectedIndex property to select the currently playing file. To solve the second problem, I created a boolean property in the view model to indicate playback is finished. In the view's code behind, I subscribed the the boolean property's PropertyChanged event. In the event handler, the view's SelectedItems property is re-created from the saved multi-selection (the contents of SelectedItems was saved into a list and SelectedItems was cleared when playback started). At first, I had trouble re-creating SelectedItems. It turned out the problem was due to the fact that re-creation was initiated through a second thread. WPF does not allow that. The solution to this is to use the Dispatcher.Invoke() to let the main thread do the work. This may be a very simple problem for experienced developers, but for newbies, it's a small challenge. Anyway, a lot of help from different people.