Kindly look at the SDK sample for MaintainExtentBehavior http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#MaintainExtentBehavior
The error message in your other post seem related to this. In both cases you were using "x:Uid". Have you tried compiling your app without this?
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
Maybe the error is unrelated to "x:Uid" then. Have you tried the recreating SDK sample? Was the issue related to xaml namespace definition or referencing the correct assemblies?
I think you just need to update this line.xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
The type mismatch may be caused by this.
Here's more information about the error you got: http://msdn.microsoft.com/en-us/library/bb546975(v=VS.100).aspx
I don't see why you would get this error message. I found this related thread: http://forums.arcgis.com/threads/3323-SL4-Breaks-ESRI-Behaviors Can you try adding the behavior in code-behind? Both methods should work actually (XAML / code-behind).
xmlns:esri="http://schemas.esri.com/arcgis/client/2009"
<esri:MaintainExtentBehavior .../>
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
I think the SDK sample is correct.
Are you using VS 2010 with SL API v2.1? If so, the following is supportedxmlns:esri="http://schemas.esri.com/arcgis/client/2009"
Any public XAML-able properties that belong to esri namespace, except for the Primitives can be accessed through namespace as long as you have reference to their assemblies.
This means you still need to add reference to ESRI.ArcGIS.Client.Behavior.dll in your project in order to do the following:<esri:MaintainExtentBehavior .../>
Now that you mentioned the error message, I think the issue is resolving this namespace and adding reference to the correct version in your project.xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
An older may have been installed and it picks up the older version, you can try to add hint path as described here http://forums.silverlight.net/forums/p/194833/453085.aspx