How to deserialize json data in windows mobile CE 6.0 device

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VS2008

C#3.5

I have created a smart device project in VS 2008 c#.
This project invoke wcf services, which return json data.

I am desrializing json data and added reference as System.Web.Extensions.dll 3.5
Here is a my code...

===========
HttpWebResponse webresponse = (HttpWebResponse)webrequest.GetResponse();
StreamReader loResponseStream = new StreamReader(webresponse.GetResponseStream(), enc);
string strResult = loResponseStream.ReadToEnd();
RootObject ss = new JavaScriptSerializer().Deserialize<rootobject>(strResult);
foreach (var item in ss.IsValidStatus)
{
sRetMessage = item.RtnMessage;
}
===========
However this code working properly, if i execute exe file from \bin\debug\SmartAppl.exe
But, if i deploy and run in device, it throws error as "could not load the assembly from .net framework".
I have set System.Web.Extensions.dll to LocalCopy=True also.

Why this is happening?
Is there any dll to deserialize json data for smart device ?

Please help me.

Thanks