bind description of sublayer of ArcGISDynamicMapServiceLayer

[MostafaKhoshabi]

Hi

I want to build legend that with tooltip of description of sublayer.

may xaml is :

<esri:Map x:Name="MyMap" WrapAround="True" Extent="44,25,63,39">

            <esri:ArcGISTiledMapServiceLayer ID="Street Map"

                    Url="http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_StreetMap_World_2D/MapServer"/>

            <esri:ArcGISDynamicMapServiceLayer ID="Cancer" Opacity="0.6"

                    Url="http://localhost:6080/arcgis/rest/services/Cancer/MapServer"/>

</esri:Map> 

<!-- Data template for sublayers within a map layer-->

                <esri:Legend.LayerTemplate>

                    <DataTemplate>

                             <CheckBox Content="{Binding Label}"

                                     IsChecked="{Binding IsEnabled, Mode=TwoWay}"

                                     IsEnabled="{Binding IsInScaleRange}" ToolTipService.ToolTip="{Binding Layer.Description}">

                             </CheckBox>

                    </DataTemplate>

               </esri:Legend.LayerTemplate>

with this code bind map layer description as tool tip but i want bindind Sub Layer description. How Can it?

best regards