MVVM and the WPF DataGrid

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:toolkit="http://schemas.microsoft.com/wpf/2008/toolkit"
    xmlns:vc="clr-namespace:DataGridOpsDemo.ValueConverters" 
    x:Class="DataGridOpsDemo.MainWindow"
    Title="DataGrid Operations Demo" Height="500" Width="300" WindowStartupLocation="CenterScreen">
    <Window.Resources>
        
        <!-- The WPF DataGrid (WPF Toolkit version) has a bug that throws a format exception
        when a WPF DataGrid is bound to a SelectedItem property, as we do here. The Ignore New
        Item Plave Holder Converter works around this bug. For more information on the bug and
        the converter, see Nigel Spencer's Blog (2009-04-30), which discusses the bug and provides 
        the fix. Link to blog post can be found in the remarks to the value converter.  -->
        
        <!-- Value Converters -->
        <vc:IgnoreNewItemPlaceHolderConverter x:Key="ignoreNewItemPlaceHolderConverter"/>

        <!-- DataGrid Background -->
        <LinearGradientBrush x:Key="BlueLightGradientBrush" StartPoint="0,0" EndPoint="0,1">
            <GradientStop Offset="0" Color="#FFEAF3FF"/>
            <GradientStop Offset="0.654" Color="#FFC0DEFF"/>
            <GradientStop Offset="1" Color="#FFC0D9FB"/>
        </LinearGradientBrush>

        <!-- DatGrid style -->
        <Style TargetType="{x:Type toolkit:DataGrid}">
            <Setter Property="Margin" Value="5" />
            <Setter Property="Background" Value="{StaticResource BlueLightGradientBrush}" />
            <Setter Property="BorderBrush" Value="#FFA6CCF2" />
            <Setter Property="RowBackground" Value="White" />
            <Setter Property="AlternatingRowBackground" Value="#FDFFD0" />
            <Setter Property="HorizontalGridLinesBrush" Value="Transparent" />
            <Setter Property="VerticalGridLinesBrush" Value="#FFD3D0" />
            <Setter Property="RowHeaderWidth" Value="0" />
        </Style>
        
        <!-- Enable rows as drop targets -->
        <Style TargetType="{x:Type toolkit:DataGridRow}">
            <Setter Property="AllowDrop" Value="True" />
        </Style>

    </Window.Resources>
    <DockPanel>

        <DockPanel LastChildFill="False" DockPanel.Dock="Top" Margin="5,0,5,0">
        	<TextBlock Text="Grocery List" TextWrapping="Wrap" VerticalAlignment="Bottom" FontFamily="Cambria" FontSize="21.333">
        		<TextBlock.Foreground>
        			<SolidColorBrush Color="#FF437CA6"/>
        		</TextBlock.Foreground>
        	</TextBlock>
        	<Button Command="{Binding DeleteItem}" DockPanel.Dock="Right" ToolTip="Delete selected item from the grocery list">
        		<Image Source="..\Images\delete_item.png" Height="16" Width="16" />
        	</Button>
        </DockPanel>
        <StackPanel DockPanel.Dock="Bottom" Orientation="Horizontal">
        	<TextBlock Text="Number of items: " TextWrapping="NoWrap"/>
        	<TextBlock Text="{Binding ItemCount}" TextWrapping="Wrap"/>
        </StackPanel>

        <toolkit:DataGrid x:Name="MainGrid" 
                          ItemsSource="{Binding GroceryList}" 
                          SelectedItem="{Binding SelectedItem,Converter={StaticResource ignoreNewItemPlaceHolderConverter}}" 
                          AutoGenerateColumns="False" 
                          DockPanel.Dock="Top"
                          MouseMove="OnMainGridMouseMove"
                          DragEnter="OnMainGridCheckDropTarget"
                          DragLeave="OnMainGridCheckDropTarget"
                          DragOver="OnMainGridCheckDropTarget"
                          Drop="OnMainGridDrop" DataContextChanged="OnMainGridDataContextChanged" >
            <toolkit:DataGrid.Columns>
                <toolkit:DataGridTextColumn Header="Sequence" Width="63" Binding="{Binding SequenceNumber}" IsReadOnly="false" />
                <toolkit:DataGridTextColumn Header="Item" Width="*" Binding="{Binding Name}" IsReadOnly="false" />
            </toolkit:DataGrid.Columns>
        </toolkit:DataGrid>

    </DockPanel>
</Window>