NullReferenceException for Adding the Switch Map component

Question

I tried to customize a template received by adding the Switch Map component sourced from ESRI: http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#SwitchMap. The procedure I used is described below: 1) Create Switch.xaml as well as .cs file in \UserControls\. 2) Copy and paste the related source codes into the 2 files mentioned in 1). E.g., in the xaml file: 3) Add the following code into MainPage.xaml 4) Compile and run debugging. I received NullReferenceException at private void RadioButton_Click(object sender, RoutedEventArgs e) { ArcGISTiledMapServiceLayer arcgisLayer = _Parent.MyMap.Layers["AGOLayer"] as ArcGISTiledMapServiceLayer; I don't know how to fix it. If you can share your experience, I will greatly appreciate.

ShaningYu · Answer

Dominique:
Great!  Using your code, it works well.  Thanks a LOT!!!
Shaning

DominiqueBroux · Answer

If I understand well, your _Parent should be the current main page and not one created with the new keyword. If you are sure that the map is inside the root control you can use _Parent = (Mainpage)System.Windows.Application.Current.RootVisual; Or, more elegant, you can add a 'Map' property to your switch control in order to get the map buddied to your control. In switch.cs add : #region Map ///

/// Gets or sets the map that the switch control is buddied to. ///

/// The map. public ESRI.ArcGIS.Client.Map Map { get { return GetValue(MapProperty) as Map; } set { SetValue(MapProperty, value); } } /// ///

/// Identifies the dependency property. ///

public static readonly DependencyProperty MapProperty = DependencyProperty.Register("Map", typeof(Map), typeof(Switch),null); #endregion In switch.cs, replace all '_Parent.MyMap' by 'Map' In MainPage.xaml, bind the Map property of your Switch control :

ShaningYu · Answer

Jenniffer:
Thanks for your response.  As you pointed out, the _Parebt is null.  I added
_Parent = new MainPage();
then 
arcgisLayer.Url = ((RadioButton)sender).Tag as string;
also shows right URL.  However, neither imagery nor topo map can be loaded.  Please advise me how to fix this problem.  Thanks again.
Shaning