'ConstrainExtentBehavior' cannot be added to a collection or dictionary of type 'BehaviorCollection'

Question

i am currently doing a silverlight c# application , following a sample code in the arcgis website, i added this code in the xaml page to add the constrainextent function. there is no other code in the xaml.cs page.

 <i:Interaction.Behaviors>                
 <esriBehaviors:ConstrainExtentBehavior ConstrainedExtent="-120,30,-60,60" />
  </i:Interaction.Behaviors>

however it gives me this error, A value of type 'ConstrainExtentBehavior' cannot be added to a collection or dictionary of type 'BehaviorCollection'.

could anyone help me to solve this? this is my xaml page if there is any code i need to add in , i am a beginner in this section and have posted questions before but cant get a working solution for this.

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"   
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"             
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"     
 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    xmlns:esriSymbols="clr-namespace:ESRI.ArcGIS.Client.Symbols;assembly=ESRI.ArcGIS.Client"  
  xmlns:esri="clr-namespace:ESRI.ArcGIS.Client;assembly=ESRI.ArcGIS.Client"
 xmlns:esriBehaviors="clr- namespace:ESRI.ArcGIS.Client.Behaviors;assembly=ESRI.ArcGIS.Client.Behaviors"  
 xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"   

  mc:Ignorable="d"     d:DesignHeight="300" d:DesignWidth="400"     
      >
 <Grid x:Name="LayoutRoot" Background="White">   

  <esri:Map x:Name="MyMap" Background="White"    >      

        <esri:Map.Layers>     

   <esri:ArcGISTiledMapServiceLayer ID="StreetMapLayer"    

         Url="http://www.onemap.sg/arcgis/rest/services/Basemap/MapServer"/>      

        <esri:GraphicsLayer ID="MyGraphicsLayer" />            

     </esri:Map.Layers>     

   <i:Interaction.Behaviors>        
        <esriBehaviors:ConstrainExtentBehavior ConstrainedExtent="-120,30,-60,60" />           
  </i:Interaction.Behaviors>    
     </esri:Map>   

  </Grid> 

 </UserControl>