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;
}

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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