However, after trying around I still could find out how to use the FeatureSymbol property. It is in Symbol class of ESRI.ArcGIS.Client.Symbols Namespace, I don't know how to put it into the StackPanel and arrange it with other controls such as text blocks and checkboxes.
As Morten said, you can use the ESRI.ArcGIS.Client.Toolkit.Primitives.SymbolDisplay control.For example the following code will give the attached result:
<Grid x:Name="LayoutRoot" >
<Grid.Resources>
<esri:FillSymbol x:Name="FillSymbol" Fill="Yellow" />
<esri:SimpleMarkerSymbol x:Name="MarkerSymbol0" Color="Brown" Style="Circle" />
<esri:SimpleMarkerSymbol x:Name="MarkerSymbol1" Color="BlueViolet" Style="Circle"/>
</Grid.Resources>
<esri:Map x:Name="MyMap" Extent="-120,20,-90,60">
<esri:Map.Layers>
<esri:ArcGISTiledMapServiceLayer ID="StreetMapLayer" Url="http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_StreetMap_World_2D/MapServer" />
<esri:FeatureLayer ID="Warning" FeatureSymbol="{StaticResource FillSymbol}" Url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/PublicSafety/PublicSafetyFeedSample/MapServer/2"/>
<esri:FeatureLayer ID="Wind" FeatureSymbol="{StaticResource MarkerSymbol0}" Url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/PublicSafety/PublicSafetyFeedSample/MapServer/0"/>
<esri:FeatureLayer ID="Stream Gauges" FeatureSymbol="{StaticResource MarkerSymbol1}" Url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/PublicSafety/PublicSafetyFeedSample/MapServer/1"/>
</esri:Map.Layers>
</esri:Map>
<Border Background="#77919191" BorderThickness="1" CornerRadius="5"
HorizontalAlignment="Right" VerticalAlignment="Top"
Margin="20" Padding="5" BorderBrush="Black" >
<ListBox x:Name="MyList" ItemsSource="{Binding ElementName=MyMap, Path=Layers}">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<!--Layer visibility checkbox-->
<CheckBox IsChecked="{Binding Visible, Mode=TwoWay}" />
<!-- Symbol -->
<esriPrimitives:SymbolDisplay Symbol="{Binding FeatureSymbol}" Margin="5,0,0,0" Height="18" Width="25" VerticalAlignment="Center" HorizontalAlignment="Center"/>
<!--Opacity slider-->
<Slider Margin="0,0,0,0" Minimum="0" Maximum="1" Width="30"
Value="{Binding Opacity, Mode=TwoWay}" Height="18" />
<!--Layer name-->
<TextBlock Text="{Binding ID, Mode=OneWay}" Margin="5,0,0,0" >
<!-- Tooltip on hover-->
<ToolTipService.ToolTip>
<StackPanel MaxWidth="400">
<TextBlock FontWeight="Bold" Text="{Binding CopyrightText}" TextWrapping="Wrap" />
<TextBlock Text="{Binding Description}" TextWrapping="Wrap" />
</StackPanel>
</ToolTipService.ToolTip>
</TextBlock>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Border>
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</Grid>
