Handling DataGrid.SelectedItems in an MVVM-friendly manner

Handling DataGrid.SelectedItems in an MVVM-friendly manner: "

An interesting question from one of the MVVM Light users today:

Is there an MVVM-friendly way to get a DataGrid’s SelectedItems into the ViewModel?

The issue there is as old as the DataGrid (that’s not very old but still): SelectedItem (singular) is a DependencyProperty and can be databound to a property in the ViewModel. SelectedItems (plural) is not a DependencyProperty.

Thankfully the answer is very simple: Use EventToCommand to call a Command in the ViewModel, and pass the SelectedItems collection as parameter. For example, if the command in the ViewModel is declared as follows:

public RelayCommand<IList> SelectionChangedCommand
{
  get;
  private set;
}

and (in the MainViewModel constructor):

SelectionChangedCommand = new RelayCommand<IList>(
  items =>
  {
      if (items == null)
      {
          NumberOfItemsSelected = 0;
          return;
      }

      NumberOfItemsSelected = items.Count;
  });

Then the XAML markup becomes:

<sdk:DataGrid x:Name="MyDataGrid"
              ItemsSource="{Binding Items}">
  <i:Interaction.Triggers>
      <i:EventTrigger EventName="SelectionChanged">
          <cmd:EventToCommand
              Command="{Binding SelectionChangedCommand}"
              CommandParameter="{Binding SelectedItems,
                  ElementName=MyDataGrid}" />
      </i:EventTrigger>
  </i:Interaction.Triggers>
</sdk:DataGrid>

I slapped a quick sample and published it here (VS2010, SL4 but the concept works in SL3 and WPF too).

Cheers!

Laurent

Laurent Bugnion (GalaSoft)
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