use this method maybe useful
public static T Deserialise<T>(string json)
{
    T obj = Activator.CreateInstance<T>();
    using (MemoryStream ms = new MemoryStream(Encoding.Unicode.GetBytes(json)))
    {
        DataContractJsonSerializer serializer = new DataContractJsonSerializer(obj.GetType());
        obj = (T)serializer.ReadObject(ms); // 
        return obj;
    } 
}
Also, just for reference, here is the Serialize method :
public static string Serialize<T>(T obj)
{
    DataContractJsonSerializer serializer = new DataContractJsonSerializer(obj.GetType());
    using (MemoryStream ms = new MemoryStream())
    {
        serializer.WriteObject(ms, obj);
        return Encoding.Default.GetString(ms.ToArray());
    }
}