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>