How to define layer's URL programmatically? (ESRI Silverlight API)

Question

I have defined a simple tiled map service layer:

<esri:ArcGISTiledMapServiceLayer x:Name="BaseMap"
ID="BaseMap" Opacity="50" Initialized="MapServiceLayer_Initialized"
InitializationFailed="Layer_InitializationFailed" />

Then I assign its Url property programmatically:

BaseMap.Url = App._componentDict["BaseMapRestUrl"];

The app bombs with this message, and the entire Silverlight app goes blank:

Layer 'BaseMap' is currently unavailable. Error" Value cannot be null. Parameter name: Url

Is there a particular way to go about this that I'm missing? I suspect I'm setting the Url too late in the lifecycle.

Answer 1

Following Kirk's suggestion in the comments, I just did this:

// add base map layer
ArcGISTiledMapServiceLayer baseMap = new ArcGISTiledMapServiceLayer();
baseMap.Url = App._componentDict["BaseMapRestUrl"];
baseMap.ID = "BaseMap";
baseMap.Opacity = 50;
baseMap.Initialized += MapServiceLayer_Initialized;
baseMap.InitializationFailed += MapServiceLayer_InitializationFailed;
MyMap.Layers.Add(baseMap);

It worked great.

Answer 2

To set the url at runtime you might consider using a trigger action, as illustrated in Esri's Standard Map Application project template (under the Silverlight folder when you create a new project in visual studio).

This is from the Base Map Switcher section of MainPage.xaml:

<RadioButton Content="Streets" IsChecked="True"
    ToolTipService.ToolTip="Worldwide Street Map"
    GroupName="BaseLayer"
    Margin="5,0,0,0" Foreground="White" FontSize="11" >
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="Checked">
            <actions:SetLayerUrlAction TargetName="Map" LayerID="BaseLayer" 
                Url="http://services.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer"/>
        </i:EventTrigger>
    </i:Interaction.Triggers>
</RadioButton>
<RadioButton Content="Topo"
    GroupName="BaseLayer"
    ToolTipService.ToolTip="United States Topographic Map"
    Margin="5,0,0,0" Foreground="White" FontSize="11" >
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="Checked">
            <actions:SetLayerUrlAction TargetName="Map" LayerID="BaseLayer" 
                  Url="http://services.arcgisonline.com/ArcGIS/rest/services/USA_Topo_Maps/MapServer"/>
        </i:EventTrigger>
    </i:Interaction.Triggers>
</RadioButton>