[C#/VisualStudio2010] Error The type or namespace name 'Json' does not exist in the namespace 'System.Runtime.Serialization' (are you missing an assembly reference?)

Solution:


Add a reference to the System.Servicemodel.Web assembly:
step1. Right-click the References folder in Solution Explorer
step2. select Add Reference
step3. select the .NET tab
step4. select the System.Servicemodel.Web assembly from the list of components and click OK


Note:
The System.Servicemodel.Web assembly contains the DataContractJsonSerializer that the application uses to serialize objects using JavaScript Object Notation (JSON) before persisting them in Isolated Storage.

Source: http://msdn.microsoft.com/en-us/windowsphonetrainingcourse_yourfirstwp7applab_topic4