Here is a simple example...Assume you have the following layers in your UserControl:
<esri:Map x:Name="MyMap">
<esri:ArcGISTiledMapServiceLayer ID="BaseLayer"
Url="http://services.arcgisonline.com/ArcGIS/rest/services/ESRI_StreetMap_World_2D/MapServer" />
<esri:FeatureLayer ID="Fire" OutFields="*" DisableClientCaching="True" AutoSave="False" MouseLeftButtonUp="FeatureLayer_MouseLeftButtonUp"
Url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/Fire/Sheep/FeatureServer/0" />
<esri:FeatureLayer ID="HomelandSecurity" OutFields="*" DisableClientCaching="True" AutoSave="False" MouseLeftButtonUp="FeatureLayer_MouseLeftButtonUp"
Url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/HomelandSecurity/operations/FeatureServer..." />
</esri:Map>
I added a StackPanel containing my ComboBox and a FeatureDataForm for attribute editing. The GraphicSource property of the FeatureDataForm has set to the selected graphic of the feature layers:
<StackPanel HorizontalAlignment="Right" VerticalAlignment="Top" Margin="10">
<StackPanel Orientation="Horizontal">
<TextBlock Text="Select Layer" />
<ComboBox x:Name="cboLayers" Margin="3,0,0,0">
<ComboBoxItem Content="Fire" DataContext="{Binding Path=Layers[1], ElementName=MyMap}" />
<ComboBoxItem Content="HomelandSecurity" DataContext="{Binding Path=Layers[2], ElementName=MyMap}" />
</ComboBox>
</StackPanel>
<esri:FeatureDataForm x:Name="MyFeatureDataForm" Width="400" Height="400"
FeatureLayer="{Binding Path=SelectedItem.DataContext, ElementName=cboLayers}"
GraphicSource="{Binding Path=SelectedItem.DataContext.SelectedGraphics[0], ElementName=cboLayers}" />
</StackPanel>
And here is the MouseLeftButtonUp event handler of both FeatureLayers:
private void FeatureLayer_MouseLeftButtonUp(object sender, ESRI.ArcGIS.Client.GraphicMouseButtonEventArgs e)
{
FeatureLayer featureLayer = sender as FeatureLayer;
for (int i = 0; i < featureLayer.SelectionCount; i++)
featureLayer.SelectedGraphics.ToList().UnSelect();
e.Graphic.Select();
}
Hope this helps.