Bind to SelectedItems from DataGrid or ListBox in

Question

SEE MY ANSWER AT THE BOTTOM

Just doing some light reading on WPF where I need to bind the selectedItems from a DataGrid but I am unable to come up with anything tangible. I just need the selected objects.

DataGrid:

<DataGrid Grid.Row="5" 
    Grid.Column="0" 
    Grid.ColumnSpan="4" 
    Name="ui_dtgAgreementDocuments"
    ItemsSource="{Binding Path=Documents, Mode=TwoWay}"
    SelectedItem="{Binding Path=DocumentSelection, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
    HorizontalAlignment="Stretch" 
    VerticalAlignment="Stretch" 
    Background="White"
    SelectionMode="Extended" Margin="2,5" 
    IsReadOnly="True" 
    CanUserAddRows="False" 
    CanUserReorderColumns="False" 
    CanUserResizeRows="False"
    GridLinesVisibility="None" 
    HorizontalScrollBarVisibility="Hidden"
    columnHeaderStyle="{StaticResource GreenTea}" 
    HeadersVisibility="Column" 
    BorderThickness="2" 
    BorderBrush="LightGray" 
    CellStyle="{StaticResource NonSelectableDataGridCellStyle}"
    SelectionUnit="FullRow" 
    HorizontalContentAlignment="Stretch" AutoGenerateColumns="False">

Answer 1

My original answer was wrong (you cannot bind to SelectedItems because it is a read-only property).

One fairly MVVM-friendly way to work around this is to bind to the IsSelected property of DataGridRow.

You can set up the binding like this:

<DataGrid ItemsSource="{Binding DocumentViewModels}"
          SelectionMode="Extended">
    <DataGrid.Resources>
        <Style TargetType="DataGridRow">
            <Setter Property="IsSelected"
                    Value="{Binding IsSelected}" />
        </Style>
    </DataGrid.Resources>
</DataGrid>

Then you need to create a DocumentViewModel that inherits from ViewModelBase (or whatever MVVM base class you are using) and has the properties of your Document you want to present in the DataGrid, as well as an IsSelected property.

Then, in your main view model, you create a List(Of DocumentViewModel) called DocumentViewModels to bind your DataGrid to. (Note: if you will be adding/removing items from the list, use an ObservableCollection(T) instead.)

Now, here's the tricky part. You need to hook into the PropertyChanged event of each DocumentViewModel in your list, like this:

For Each documentViewModel As DocumentViewModel In DocumentViewModels
    documentViewModel.PropertyChanged += DocumentViewModel_PropertyChanged
Next

This allows you to respond to changes in any DocumentViewModel.

Finally, in DocumentViewModel_PropertyChanged, you can loop through your list (or use a Linq query) to grab the info for each item where IsSelected = True.

Answer 2

What about using a converter?

public class SelectedItemsConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var dg = value as DataGrid;
        return dg?.SelectedItems;
    }
...

Use it in a DataGrid's ContextMenu like this:

       <DataGrid.ContextMenu>
            <ContextMenu>
                <MenuItem
                    CommandParameter="{Binding Path=MySelectedItems, Converter={StaticResource selectedItemsConverter}, Mode=OneWay, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ContextMenu}}}"
                    Command="{Binding MyDoSomethingWithMySelectedItemsCommand}"
                    Header="Do Something...">
                </MenuItem>

Answer 3

This will work:

MultiSelectorBehaviours.vb

Imports System.Collections
Imports System.Windows
Imports System.Windows.Controls.Primitives
Imports System.Windows.Controls
Imports System

Public NotInheritable Class MultiSelectorBehaviours
    Private Sub New()
    End Sub

    Public Shared ReadOnly SynchronizedSelectedItems As DependencyProperty = _
        DependencyProperty.RegisterAttached("SynchronizedSelectedItems", GetType(IList), GetType(MultiSelectorBehaviours), New PropertyMetadata(Nothing, New PropertyChangedCallback(AddressOf OnSynchronizedSelectedItemsChanged)))

    Private Shared ReadOnly SynchronizationManagerProperty As DependencyProperty = DependencyProperty.RegisterAttached("SynchronizationManager", GetType(SynchronizationManager), GetType(MultiSelectorBehaviours), New PropertyMetadata(Nothing))

    ''' <summary>
    ''' Gets the synchronized selected items.
    ''' </summary>
    ''' <param name="dependencyObject">The dependency object.</param>
    ''' <returns>The list that is acting as the sync list.</returns>
    Public Shared Function GetSynchronizedSelectedItems(ByVal dependencyObject As DependencyObject) As IList
        Return DirectCast(dependencyObject.GetValue(SynchronizedSelectedItems), IList)
    End Function

    ''' <summary>
    ''' Sets the synchronized selected items.
    ''' </summary>
    ''' <param name="dependencyObject">The dependency object.</param>
    ''' <param name="value">The value to be set as synchronized items.</param>
    Public Shared Sub SetSynchronizedSelectedItems(ByVal dependencyObject As DependencyObject, ByVal value As IList)
        dependencyObject.SetValue(SynchronizedSelectedItems, value)
    End Sub

    Private Shared Function GetSynchronizationManager(ByVal dependencyObject As DependencyObject) As SynchronizationManager
        Return DirectCast(dependencyObject.GetValue(SynchronizationManagerProperty), SynchronizationManager)
    End Function

    Private Shared Sub SetSynchronizationManager(ByVal dependencyObject As DependencyObject, ByVal value As SynchronizationManager)
        dependencyObject.SetValue(SynchronizationManagerProperty, value)
    End Sub

    Private Shared Sub OnSynchronizedSelectedItemsChanged(ByVal dependencyObject As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
        If e.OldValue IsNot Nothing Then
            Dim synchronizer As SynchronizationManager = GetSynchronizationManager(dependencyObject)
            synchronizer.StopSynchronizing()

            SetSynchronizationManager(dependencyObject, Nothing)
        End If

        Dim list As IList = TryCast(e.NewValue, IList)
        Dim selector As Selector = TryCast(dependencyObject, Selector)

        ' check that this property is an IList, and that it is being set on a ListBox
        If list IsNot Nothing AndAlso selector IsNot Nothing Then
            Dim synchronizer As SynchronizationManager = GetSynchronizationManager(dependencyObject)
            If synchronizer Is Nothing Then
                synchronizer = New SynchronizationManager(selector)
                SetSynchronizationManager(dependencyObject, synchronizer)
            End If

            synchronizer.StartSynchronizingList()
        End If
    End Sub

    ''' <summary>
    ''' A synchronization manager.
    ''' </summary>
    Private Class SynchronizationManager
        Private ReadOnly _multiSelector As Selector
        Private _synchronizer As TwoListSynchronizer

        ''' <summary>
        ''' Initializes a new instance of the <see cref="SynchronizationManager"/> class.
        ''' </summary>
        ''' <param name="selector">The selector.</param>
        Friend Sub New(ByVal selector As Selector)
            _multiSelector = selector
        End Sub

        ''' <summary>
        ''' Starts synchronizing the list.
        ''' </summary>
        Public Sub StartSynchronizingList()
            Dim list As IList = GetSynchronizedSelectedItems(_multiSelector)

            If list IsNot Nothing Then
                _synchronizer = New TwoListSynchronizer(GetSelectedItemsCollection(_multiSelector), list)
                _synchronizer.StartSynchronizing()
            End If
        End Sub

        ''' <summary>
        ''' Stops synchronizing the list.
        ''' </summary>
        Public Sub StopSynchronizing()
            _synchronizer.StopSynchronizing()
        End Sub

        Public Shared Function GetSelectedItemsCollection(ByVal selector As Selector) As IList
            If TypeOf selector Is MultiSelector Then
                Return TryCast(selector, MultiSelector).SelectedItems
            ElseIf TypeOf selector Is ListBox Then
                Return TryCast(selector, ListBox).SelectedItems
            Else
                Throw New InvalidOperationException("Target object has no SelectedItems property to bind.")
            End If
        End Function

    End Class
End Class

IListItemConverter.vb

''' <summary>
''' Converts items in the Master list to Items in the target list, and back again.
''' </summary>
Public Interface IListItemConverter
    ''' <summary>
    ''' Converts the specified master list item.
    ''' </summary>
    ''' <param name="masterListItem">The master list item.</param>
    ''' <returns>The result of the conversion.</returns>
    Function Convert(ByVal masterListItem As Object) As Object

    ''' <summary>
    ''' Converts the specified target list item.
    ''' </summary>
    ''' <param name="targetListItem">The target list item.</param>
    ''' <returns>The result of the conversion.</returns>
    Function ConvertBack(ByVal targetListItem As Object) As Object
End Interface

TwoListSynchronizer.vb

Imports System.Collections
Imports System.Collections.Specialized
Imports System.Linq
Imports System.Windows

''' <summary>
''' Keeps two lists synchronized. 
''' </summary>
Public Class TwoListSynchronizer
    Implements IWeakEventListener

    Private Shared ReadOnly DefaultConverter As IListItemConverter = New DoNothingListItemConverter()
    Private ReadOnly _masterList As IList
    Private ReadOnly _masterTargetConverter As IListItemConverter
    Private ReadOnly _targetList As IList


    ''' <summary>
    ''' Initializes a new instance of the <see cref="TwoListSynchronizer"/> class.
    ''' </summary>
    ''' <param name="masterList">The master list.</param>
    ''' <param name="targetList">The target list.</param>
    ''' <param name="masterTargetConverter">The master-target converter.</param>
    Public Sub New(ByVal masterList As IList, ByVal targetList As IList, ByVal masterTargetConverter As IListItemConverter)
        _masterList = masterList
        _targetList = targetList
        _masterTargetConverter = masterTargetConverter
    End Sub

    ''' <summary>
    ''' Initializes a new instance of the <see cref="TwoListSynchronizer"/> class.
    ''' </summary>
    ''' <param name="masterList">The master list.</param>
    ''' <param name="targetList">The target list.</param>
    Public Sub New(ByVal masterList As IList, ByVal targetList As IList)
        Me.New(masterList, targetList, DefaultConverter)
    End Sub

    Private Delegate Sub ChangeListAction(ByVal list As IList, ByVal e As NotifyCollectionChangedEventArgs, ByVal converter As Converter(Of Object, Object))

    ''' <summary>
    ''' Starts synchronizing the lists.
    ''' </summary>
    Public Sub StartSynchronizing()
        ListenForChangeEvents(_masterList)
        ListenForChangeEvents(_targetList)

        ' Update the Target list from the Master list
        SetListValuesFromSource(_masterList, _targetList, AddressOf ConvertFromMasterToTarget)

        ' In some cases the target list might have its own view on which items should included:
        ' so update the master list from the target list
        ' (This is the case with a ListBox SelectedItems collection: only items from the ItemsSource can be included in SelectedItems)
        If Not TargetAndMasterCollectionsAreEqual() Then
            SetListValuesFromSource(_targetList, _masterList, AddressOf ConvertFromTargetToMaster)
        End If
    End Sub

    ''' <summary>
    ''' Stop synchronizing the lists.
    ''' </summary>
    Public Sub StopSynchronizing()
        StopListeningForChangeEvents(_masterList)
        StopListeningForChangeEvents(_targetList)
    End Sub

    ''' <summary>
    ''' Receives events from the centralized event manager.
    ''' </summary>
    ''' <param name="managerType">The type of the <see cref="T:System.Windows.WeakEventManager"/> calling this method.</param>
    ''' <param name="sender">Object that originated the event.</param>
    ''' <param name="e">Event data.</param>
    ''' <returns>
    ''' true if the listener handled the event. It is considered an error by the <see cref="T:System.Windows.WeakEventManager"/> handling in WPF to register a listener for an event that the listener does not handle. Regardless, the method should return false if it receives an event that it does not recognize or handle.
    ''' </returns>
    Public Function ReceiveWeakEvent(ByVal managerType As Type, ByVal sender As Object, ByVal e As EventArgs) As Boolean Implements System.Windows.IWeakEventListener.ReceiveWeakEvent
        HandleCollectionChanged(TryCast(sender, IList), TryCast(e, NotifyCollectionChangedEventArgs))

        Return True
    End Function

    ''' <summary>
    ''' Listens for change events on a list.
    ''' </summary>
    ''' <param name="list">The list to listen to.</param>
    Protected Sub ListenForChangeEvents(ByVal list As IList)
        If TypeOf list Is INotifyCollectionChanged Then
            CollectionChangedEventManager.AddListener(TryCast(list, INotifyCollectionChanged), Me)
        End If
    End Sub

    ''' <summary>
    ''' Stops listening for change events.
    ''' </summary>
    ''' <param name="list">The list to stop listening to.</param>
    Protected Sub StopListeningForChangeEvents(ByVal list As IList)
        If TypeOf list Is INotifyCollectionChanged Then
            CollectionChangedEventManager.RemoveListener(TryCast(list, INotifyCollectionChanged), Me)
        End If
    End Sub

    Private Sub AddItems(ByVal list As IList, ByVal e As NotifyCollectionChangedEventArgs, ByVal converter As Converter(Of Object, Object))
        Dim itemCount As Integer = e.NewItems.Count

        For i As Integer = 0 To itemCount - 1
            Dim insertionPoint As Integer = e.NewStartingIndex + i

            If insertionPoint > list.Count Then
                list.Add(converter(e.NewItems(i)))
            Else
                list.Insert(insertionPoint, converter(e.NewItems(i)))
            End If
        Next
    End Sub

    Private Function ConvertFromMasterToTarget(ByVal masterListItem As Object) As Object
        Return If(_masterTargetConverter Is Nothing, masterListItem, _masterTargetConverter.Convert(masterListItem))
    End Function

    Private Function ConvertFromTargetToMaster(ByVal targetListItem As Object) As Object
        Return If(_masterTargetConverter Is Nothing, targetListItem, _masterTargetConverter.ConvertBack(targetListItem))
    End Function

    Private Sub HandleCollectionChanged(ByVal sender As Object, ByVal e As NotifyCollectionChangedEventArgs)
        Dim sourceList As IList = TryCast(sender, IList)

        Select Case e.Action
            Case NotifyCollectionChangedAction.Add
                PerformActionOnAllLists(AddressOf AddItems, sourceList, e)
                Exit Select
            Case NotifyCollectionChangedAction.Move
                PerformActionOnAllLists(AddressOf MoveItems, sourceList, e)
                Exit Select
            Case NotifyCollectionChangedAction.Remove
                PerformActionOnAllLists(AddressOf RemoveItems, sourceList, e)
                Exit Select
            Case NotifyCollectionChangedAction.Replace
                PerformActionOnAllLists(AddressOf ReplaceItems, sourceList, e)
                Exit Select
            Case NotifyCollectionChangedAction.Reset
                UpdateListsFromSource(TryCast(sender, IList))
                Exit Select
            Case Else
                Exit Select
        End Select
    End Sub

    Private Sub MoveItems(ByVal list As IList, ByVal e As NotifyCollectionChangedEventArgs, ByVal converter As Converter(Of Object, Object))
        RemoveItems(list, e, converter)
        AddItems(list, e, converter)
    End Sub

    Private Sub PerformActionOnAllLists(ByVal action As ChangeListAction, ByVal sourceList As IList, ByVal collectionChangedArgs As NotifyCollectionChangedEventArgs)
        If sourceList Is _masterList Then
            PerformActionOnList(_targetList, action, collectionChangedArgs, AddressOf ConvertFromMasterToTarget)
        Else
            PerformActionOnList(_masterList, action, collectionChangedArgs, AddressOf ConvertFromTargetToMaster)
        End If
    End Sub

    Private Sub PerformActionOnList(ByVal list As IList, ByVal action As ChangeListAction, ByVal collectionChangedArgs As NotifyCollectionChangedEventArgs, ByVal converter As Converter(Of Object, Object))
        StopListeningForChangeEvents(list)
        action(list, collectionChangedArgs, converter)
        ListenForChangeEvents(list)
    End Sub

    Private Sub RemoveItems(ByVal list As IList, ByVal e As NotifyCollectionChangedEventArgs, ByVal converter As Converter(Of Object, Object))
        Dim itemCount As Integer = e.OldItems.Count

        ' for the number of items being removed, remove the item from the Old Starting Index
        ' (this will cause following items to be shifted down to fill the hole).
        For i As Integer = 0 To itemCount - 1
            list.RemoveAt(e.OldStartingIndex)
        Next
    End Sub

    Private Sub ReplaceItems(ByVal list As IList, ByVal e As NotifyCollectionChangedEventArgs, ByVal converter As Converter(Of Object, Object))
        RemoveItems(list, e, converter)
        AddItems(list, e, converter)
    End Sub

    Private Sub SetListValuesFromSource(ByVal sourceList As IList, ByVal targetList As IList, ByVal converter As Converter(Of Object, Object))
        StopListeningForChangeEvents(targetList)

        targetList.Clear()

        For Each o As Object In sourceList
            targetList.Add(converter(o))
        Next

        ListenForChangeEvents(targetList)
    End Sub

    Private Function TargetAndMasterCollectionsAreEqual() As Boolean
        Return _masterList.Cast(Of Object)().SequenceEqual(_targetList.Cast(Of Object)().[Select](Function(item) ConvertFromTargetToMaster(item)))
    End Function

    ''' <summary>
    ''' Makes sure that all synchronized lists have the same values as the source list.
    ''' </summary>
    ''' <param name="sourceList">The source list.</param>
    Private Sub UpdateListsFromSource(ByVal sourceList As IList)
        If sourceList Is _masterList Then
            SetListValuesFromSource(_masterList, _targetList, AddressOf ConvertFromMasterToTarget)
        Else
            SetListValuesFromSource(_targetList, _masterList, AddressOf ConvertFromTargetToMaster)
        End If
    End Sub




    ''' <summary>
    ''' An implementation that does nothing in the conversions.
    ''' </summary>
    Friend Class DoNothingListItemConverter
        Implements IListItemConverter

        ''' <summary>
        ''' Converts the specified master list item.
        ''' </summary>
        ''' <param name="masterListItem">The master list item.</param>
        ''' <returns>The result of the conversion.</returns>
        Public Function Convert(ByVal masterListItem As Object) As Object Implements IListItemConverter.Convert
            Return masterListItem
        End Function

        ''' <summary>
        ''' Converts the specified target list item.
        ''' </summary>
        ''' <param name="targetListItem">The target list item.</param>
        ''' <returns>The result of the conversion.</returns>
        Public Function ConvertBack(ByVal targetListItem As Object) As Object Implements IListItemConverter.ConvertBack
            Return targetListItem
        End Function
    End Class

End Class

Then for the XAML:

<DataGrid ..... local:MultiSelectorBehaviours.SynchronizedSelectedItems="{Binding SelectedResults}" />

And finally the VM:

Public ReadOnly Property SelectedResults As ObservableCollection(Of StatisticsResultModel)
    Get
        Return _objSelectedResults
    End Get
End Property

Credit Goes to: http://blog.functionalfun.net/2009/02/how-to-databind-to-selecteditems.html

Answer 4

For me the easiest route was to populate the ViewModel property in the SelectionChanged event.

private void MyDataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    (DataContext as MyViewModel).SelectedItems.Clear();
    foreach (var i in MyDataGrid.SelectedItems) (DataContext as MyViewModel).SelectedItems.Add(i as ItemType);
}