You need to capture the data type in a class that is serializable.
In this example I use Employee to do so. The first action will be to construct an IEnumerable of Employee:
XElement empXml = GetEmployees();
var empJson = from emp in empXml.Descendants("Employee")
select new Employee
ID = emp.Element("ID").Value,
FirstName = emp.Element("FirstName").Value,
Department = emp.Element("Department").Value,
City = emp.Element("City").Value
The second step is to serialize the enumeration into the memory stream.
var ser = new DataContractJsonSerializer(typeof(IEnumerable
var ms = new MemoryStream();
And at last, we can take the generated stream as a string just before closing the stream.
string json = Encoding.Default.GetString(ms.ToArray());