DataContractSerializer、KnownType和继承

我读过很多关于已知类型的文章,我相信我的示例应该有效。但事实并非如此。我在反序列化方面遇到以下异常,不明白原因:

第1行位置2出错。需要命名空间中的元素“a” http://schemas.datacontract.org/2004/07/ConsoleApplication2 …遇到名为“c”、命名空间为的“element” http://schemas.datacontract.org/2004/07/consoleapplication2 '.

using System;
using System.Runtime.Serialization;
using System.Xml;
using System.IO;

namespace ConsoleApplication2
{
    [DataContract][KnownType(typeof(C))]class A { }
    [DataContract]class C : A { }

    class Program
    {
        static void Main(string[] args)
        {
            A a = new C();
            string data;

            var serializer = new DataContractSerializer(a.GetType());
            using (var sw = new StringWriter())
            {
                using (var xw = new XmlTextWriter(sw))
                    serializer.WriteObject(xw, a);
                data = sw.ToString();
            }

            serializer = new DataContractSerializer(typeof(A));
            using (var sr = new StringReader(data))
            using (var xr = new XmlTextReader(sr))
                a = (A)serializer.ReadObject(xr);
        }
    }
}

我错过了什么?