[爱心链接：拯救一个25岁身患急性白血病的女孩[内有苏州电视台经济频道《天天山海经》为此录制的节目视频（苏州话）]]DataContractSerializer承载着所有数据契约对象的序列化和反序列化操作。在上面一篇文章（《数据契约（Data Contract）和数据契约序列化器（DataContractSerializer）》）中，我们谈到DataContractSerializer基本的序列化规则；如何控制DataContractSerializer序列化或者反序列化对象的数量；以及如何在序列化后的XML中保存被序列化对象的对象引用结构。在这篇文章中，我们会详细讨论WCF序列化中一个重要的话题：已知类型（Known Type）。
WCF下的序列化与反序列化解决的是数据在两种状态之间的相互转化：托管类型对象和XML。由于类型定义了对象的数据结构，所以无论对于序列化还是反序列化，都必须事先确定对象的类型。如果被序列化对象或者被反序列化生成的对象包含不可知的类型，序列化或者反序列化将会失败。为了确保DataContractSerializer的正常序列化和反序列化，我们需要将“未知”类型加入DataContractSerializer“已知”类型列表中。
一、未知类型导致序列化失败
.NET的类型可以分为两种：声明类型和真实类型。我们提倡面向接口的编程，对象的真实类型往往需要在运行时才能确定，在编程的时候往往只需要指明类型的声明类型，比如类型实现的接口或者抽象类。当我们使用基于接口或者抽象类创建的DataContractSerializer去序列化一个实现了该接口或者继承该抽象类的实例的时候，往往会因为对对象的真实类型无法识别造成不能正常地序列化。比如下面的代码中，我们定义了3个类型，一个接口、一个抽象类和一个具体类。
1: namespace Artech.DataContractSerializerDemos
2: {
3: public interface IOrder
4: {
5: Guid ID
6: { get; set; }
7:
8: DateTime Date
9: { get; set; }
10:
11: string Customer
12: { get; set; }
13:
14: string ShipAddress
15: { get; set; }
16: }
17:
18: [DataContract]
19: public abstract class OrderBase : IOrder
20: {
21: [DataMember]
22: public Guid ID
23: { get; set; }
24:
25: [DataMember]
26: public DateTime Date
27: { get; set; }
28:
29: [DataMember]
30: public string Customer
31: { get; set; }
32:
33: [DataMember]
34: public string ShipAddress
35: { get; set; }
36: }
37:
38: [DataContract]
39: public class Order : OrderBase
40: {
41: [DataMember]
42: public double TotalPrice
43: { get; set; }
44: }
45: }
当我们通过下面的方式去序列化一个Order对象（注意泛型类型为IOrder或者OrderBase），将会抛出如图1所示SerializationException异常，提示Order类型无法识别。
注：Serialize
1: Order order = new Order()
2: {
3: ID = Guid.NewGuid(),
4: Customer = "NCS",
5: Date = DateTime.Today,
6: ShipAddress = "#328, Airport Rd, Industrial Park, Suzhou Jiangsu Province",
7: TotalPrice = 8888.88
8: };
9:
10: Serialize(order, @"E:\order.xml");
11: //或者
12: Serialize(order, @"E:\order.xml");
![[原创]WCF技术剖析之十三：序列化过程中的已知类型（Known Type）_第1张图片](http1://img.it610.com/image/product/74003752ac294a32a5af3a9bcb8d39a5.jpg "clip_image002")
图1 “未知”类型导致的序列化异常
二、DataContractSerializer的已知类型集合
解决上面这个问题的唯一途径就是让DataContractSerializer能够识别Order类型，成为DataContractSerializer的已知类型（Known Type）。DataContractSerializer内部具有一个已知类型的列表，我们只需要将Order的类型添加到这个列表中，就能从根本上解决这个问题。通过下面6个重载构造函数中的任意一个，均可以通过knownTypes参数指定DataContractSerializer的已知类型集合，该集合最终反映在DataContractSerializer的制度属性KnownTypes上。
1: public sealed class DataContractSerializer : XmlObjectSerializer
2: {
3: public DataContractSerializer(Type type, IEnumerableknownTypes);
4: public DataContractSerializer(Type type, string rootName, string rootNamespace, IEnumerableknownTypes);
5: public DataContractSerializer(Type type, XmlDictionaryString rootName, XmlDictionaryString rootNamespace, IEnumerableknownTypes);
6: public DataContractSerializer(Type type, IEnumerableknownTypes, int maxItemsInObjectGraph, bool ignoreExtensionDataObject, bool preserveObjectReferences, IDataContractSurrogate dataContractSurrogate);
7: public DataContractSerializer(Type type, string rootName, string rootNamespace, IEnumerableknownTypes, int maxItemsInObjectGraph, bool ignoreExtensionDataObject, bool preserveObjectReferences, IDataContractSurrogate dataContractSurrogate);
8: public DataContractSerializer(Type type, XmlDictionaryString rootName, XmlDictionaryString rootNamespace, IEnumerableknownTypes, int maxItemsInObjectGraph, bool ignoreExtensionDataObject, bool preserveObjectReferences, IDataContractSurrogate dataContractSurrogate);
9:
10: public ReadOnlyCollectionKnownTypes { get; }
11: }
为了方便后面的演示，我们对我们使用的泛型服务方法Serialize
1: public static void Serialize(T instance, string fileName, IList konwnTypes)
2: {
3: DataContractSerializer serializer = new DataContractSerializer(typeof(T), konwnTypes, int.MaxValue, false, false, null);
4: using (XmlWriter writer = new XmlTextWriter(fileName, Encoding.UTF8))
5: {
6: serializer.WriteObject(writer, instance);
7: }
8: Process.Start(fileName);
9: }
三、基于接口的序列化
DataContractSerializer的创建必须基于某个确定的类型，这里的类型既可以是接口，也可以是抽象类或具体类。不过基于接口的DataContractSerializer与基于抽象数据契约类型的DataContractSerializer，在进行序列化时表现出来的行为是不相同的。
在下面的代码中，在调用Serialize
1: Order order = new Order()
2: {
3: ID = Guid.NewGuid(),
4: Customer = "NCS",
5: Date = DateTime.Today,
6: ShipAddress = "#328, Airport Rd, Industrial Park, Suzhou Jiangsu Province",
7: TotalPrice = 8888.88
8: };
9:
10: Serialize(order, @"E:\order.interface.xml", new List {typeof(Order)});
11: Serialize(order, @"E:\order.class.xml", new List { typeof(Order) });
1: <z:anyType xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns:d1p1="http://schemas.datacontract.org/2004/07/Artech.DataContractSerializerDemos" i:type="d1p1:Order" xmlns:z="http://schemas.microsoft.com/2003/10/Serialization/">
2: <d1p1:Customer>NCSd1p1:Customer>
3: <d1p1:Date>2008-12-04T00:00:00+08:00d1p1:Date>
4: <d1p1:ID>04c07e41-6302-48d1-ac06-87ebbff2b75fd1p1:ID>
5: <d1p1:ShipAddress>#328, Airport Rd, Industrial Park, Suzhou Jiangsu Provinced1p1:ShipAddress>
6: <d1p1:TotalPrice>8888.88d1p1:TotalPrice>
7: z:anyType>
1: <OrderBase xmlns:i="http://www.w3.org/2001/XMLSchema-instance" i:type="Order" xmlns="http://schemas.datacontract.org/2004/07/Artech.DataContractSerializerDemos">
2: <Customer>NCSCustomer>
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