C# 如何让datacontractjsonserializer使用字符串？

我有以下代码，它只在使用filestream和读取文件时工作，如何使用json格式的字符串使其工作

using (FileStream stream = File.OpenRead("c://test"))
{
    DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(datastruct));
    datastruct struct= (datastruct )ser.ReadObject(stream);

                var ms = new MemoryStream();
                var sw = new StreamWriter(ms);
                sw.Write(json);
                sw.Flush();
                ms.Position = 0;
                DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(Item));
                var type = ser.ReadObject(ms);

---

考虑到json是您的字符串：

                var ms = new MemoryStream();
                var sw = new StreamWriter(ms);
                sw.Write(json);
                sw.Flush();
                ms.Position = 0;
                DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(Item));
                var type = ser.ReadObject(ms);