silverlight wpf DataTemplate Command binding

<Grid x:Name="LayoutRoot" Background="White">
<CommonControl:NoapDataGrid MinHeight="120" Margin="10,0,10,10" HorizontalAlignment="Stretch" AutoGenerateColumns="False" ItemsSource="{Binding FtpFiles}" >
<CommonControl:NoapDataGrid.Columns>
<c1:DataGridTextColumn Header="文件名称" Binding="{Binding FileName}"  />
<c1:DataGridTextColumn Header="文件类型" Binding="{Binding FileType}"  />
<c1:DataGridTemplateColumn Header="操作">
<c1:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<HyperlinkButton  Content="下载" Command="{Binding DataContext.DownloadFtpFileCommand, RelativeSource={RelativeSource AncestorLevel=2,AncestorType=Grid}}"/>
</DataTemplate>
</c1:DataGridTemplateColumn.CellTemplate>
</c1:DataGridTemplateColumn>
</CommonControl:NoapDataGrid.Columns>
</CommonControl:NoapDataGrid>
</Grid>

<c1:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<HyperlinkButton  Content="下载" Command="{Binding DataContext.DownloadFtpFileCommand, RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=UserControl}}"/>
</DataTemplate>
</c1:DataGridTemplateColumn.CellTemplate>