找不到System.Runtime.Serialization。Json在MonoTouch

I am a beginner in Monotouch.
I would like to deserialize Json data using DataContractJsonSerializer. But I cannot reference System.Runtime.Serialization.Json(Only .Formatters under System.Runtime.Serialization) in MonoDevelop. I have referenced System.Runtime.Serialization. My config and installation sequences are: 1. iPhone SDK 4.2 2. Mono 2.8.2 (not CSDK version) 3. Monotouch 3.2.4 Eval 4. MonoDevelop 2.4 I would like to d