原文:WCF技术剖析之十五：数据契约代理（DataContractSurrogate）在序列化中的作用
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比如下面定义了两个类型Contact和Customer，其中Customer是数据契约，Contact的Sex属性相当于Customer的Gender属性，而Contact的FullName可以看成是Customer的FirstName和LastName的组合。现在我们要做的是将一个Contact对象序列化成基于Customer数据契约对应的结构的XML，或者对于一段基于Customer数据契约对应结构的XML，将其反序列化生成Contact对象。
1: public class Contact
2: {
3: public string FullName
4: { get; set; }
5:
6: public string Sex
7: { get; set; }
8:
9: public override bool Equals(object obj)
10: {
11: Contact contact = obj as Contact;
12: if (contact == null)
13: {
14: return false;
15: }
16:
17: return this.FullName == contact.FullName && this.Sex == contact.Sex;
18: }
19:
20: public override int GetHashCode()
21: {
22: return this.FullName.GetHashCode() ^ this.Sex.GetHashCode();
23: }
24: }
1: [DataContract(Namespace = "http://www.artech.com")]
2: public class Customer
3: {
4: [DataMember(Order = 1)]
5: public string FirstName
6: { get; set; }
7:
8: [DataMember(Order = 2)]
9: public string LastName
10: { get; set; }
11:
12: [DataMember(Order = 3)]
13: public string Gender
14: { get; set; }
15: }
为实现上面的要求，要涉及WCF中一个特殊的概念：数据契约代理（DataContract Surrogate）。WCF通过一个接口System.Runtime.Serialization.IDataContractSurrogate来表示数据契约代理。IDataContractSurrogate用于实现在序列化、反序列化、数据契约的导入和导出过程中对对象或者类型的替换。以上面Contact和Customer为例，在正常的情况下，DataContractSerializer针对类型Customer对一个真正的Customer对象进行序列化，现在要求的是通过DataContractSerializer序列化一个Contact对象，并且要生成与Customer等效的XML，就要在序列化的过程中实现类型的替换（由Contact类型替换成Customer类型）和对象的替换（由Contact对象替换成Customer对象）。
我们先来看看IDataContractSurrogate的定义，序列化相关的方法有以下3个，如果想具体了解IDataContractSurrogate在数据契约导入、导出，以及代码生成方面的应用可以参考MSDN相关文档，在这里就不多作介绍了。
- GetDataContractType：获取进行序列化、反序列化或者数据契约导入导出基于的数据契约的类型，实现此方法相当于实现了类型的替换；
- GetObjectToSerialize：在序列化之前获取序列化的对象，实现了此方法相当于为序列化实现了对象替换；
- GetDeserializedObject：当完成反序列化工作后，通过方法获得被反序列化生成的对象，通过此方法可以用新的对象替换掉真正被反序列化生成的原对象。
1: public interface IDataContractSurrogate
2: {
3: Type GetDataContractType(Type type);
4: object GetObjectToSerialize(object obj, Type targetType);
5: object GetDeserializedObject(object obj, Type targetType);
6:
7: object GetCustomDataToExport(MemberInfo memberInfo, Type dataContractType);
8: object GetCustomDataToExport(Type clrType, Type dataContractType);
9: void GetKnownCustomDataTypes(Collection<Type> customDataTypes);
10: Type GetReferencedTypeOnImport(string typeName, string typeNamespace, object customData);
11: CodeTypeDeclaration ProcessImportedType(CodeTypeDeclaration typeDeclaration, CodeCompileUnit compileUnit);
12: }
现在我专门为Contact和Customer之间的转换创建了一个自定义的DataContractSurrogate:ContractSurrogate。在GetDataContractType中，如果发现类型是Contact，则替换成Customer。在GetObjectToSerialize方法中，将用于序列化的Contact对象用Customer对象替换，而在GetDeserializedObject中则用Contact对象替换反序列化生成的Customer对象。
1: public class ContractSurrogate : IDataContractSurrogate
2: {
3:
4: public object GetCustomDataToExport(Type clrType, Type dataContractType)
5: {
6: return null;
7: }
8:
9: public object GetCustomDataToExport(MemberInfo memberInfo, Type dataContractType)
10: {
11: return null;
12: }
13:
14: public Type GetDataContractType(Type type)
15: {
16: if (type == typeof(Contact))
17: {
18: return typeof(Customer);
19: }
20:
21: return type;
22: }
23:
24: public object GetDeserializedObject(object obj, Type targetType)
25: {
26: Customer customer = obj as Customer;
27: if (customer == null)
28: {
29: return obj;
30: }
31:
32: return new Contact
33: {
34: FullName = string.Format("{0} {1}", customer.FirstName, customer.LastName),
35: Sex = customer.Gender
36: };
37: }
38:
39: public void GetKnownCustomDataTypes(Collection<Type> customDataTypes)
40: {
41:
42: }
43:
44: public object GetObjectToSerialize(object obj, Type targetType)
45: {
46: Contact contact = obj as Contact;
47: if (contact == null)
48: {
49: return obj;
50: }
51:
52:
53: return new Customer
54: {
55: FirstName = contact.FullName.Split(" ".ToCharArray())[0],
56: LastName = contact.FullName.Split(" ".ToCharArray())[1],
57: Gender = contact.Sex
58: };
59: }
60:
61: public Type GetReferencedTypeOnImport(string typeName, string typeNamespace, object customData)
62: {
63: return null;
64: }
65:
66: public CodeTypeDeclaration ProcessImportedType(CodeTypeDeclaration typeDeclaration, CodeCompileUnit compileUnit)
67: {
68: return typeDeclaration;
69: }
70: }
为了演示ContractSurrogate在序列化和反序列化中所起的作用，创建了Serialize<T>和Deserialize<T>两个辅助方法，通过创建DataContractSerializer进行序列化和反序列化。方法中的dataContractSurrogate参数被传入DataContractSerializer的构造函数中。
1: public static void Serialize<T>(T instance, string fileName, IDataContractSurrogate dataContractSurrogate)
2: {
3: DataContractSerializer serializer = new DataContractSerializer(typeof(T), null, int.MaxValue, false, false, dataContractSurrogate);
4: using (XmlWriter writer = new XmlTextWriter(fileName, Encoding.UTF8))
5: {
6: serializer.WriteObject(writer, instance);
7: Process.Start(fileName);
8: }
9: }
10:
11: public static T Deserialize<T>( string fileName, IDataContractSurrogate dataContractSurrogate)
12: {
13: DataContractSerializer serializer = new DataContractSerializer(typeof(T), null, int.MaxValue, false, false, dataContractSurrogate);
14: using (XmlReader reader = new XmlTextReader(fileName))
15: {
16: return (T)serializer.ReadObject(reader);
17: }
18: }
借助于上面定义的ContractSurrogate和两个辅助方法，我们通过下面的程序演示IDataContractSurrogate在序列化和反序列化过程中所起的作用。
1: string fileName = @"E:\contact.xml";
2: Contact contactToSerialize = new Contact
3: {
4: FullName = "Bill Gates",
5: Sex = "Male"
6: };
7: IDataContractSurrogate dataContractSurrogate = new ContractSurrogate();
8: Serialize<Contact>(contactToSerialize, fileName, dataContractSurrogate);
9:
10: Contact contactToDeserialize = Deserialize<Contact>(fileName, dataContractSurrogate);
11: Console.WriteLine("contactToSerialize.Equals(contactToDeserialize) = {0}", contactToSerialize.Equals(contactToDeserialize) );
下面的XML表述Contract对象被序列化后的结果，显而易见，这和真正序列化一个Customer对象是完全一样的。不仅如此，基于下面一段XML反序列化生成的Contact对象和用于序列化的对象是相等的，这通过最终的输出结果可以看出来。
1: <Customer xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.artech.com">
2: <FirstName>Bill</FirstName>
3: <LastName>Gates</LastName>
4: <Gender>Male</Gender>
5: </Customer>
输出结果：
1: contactToSerialize.Equals(contactToDeserialize) = True
在进行服务寄宿的时候，可以通过如下代码指定IDataContractSurrogate。
1: using (ServiceHost serviceHost = new ServiceHost(typeof(InventoryCheck)))
2: foreach (ServiceEndpoint ep in serviceHost.Description.Endpoints)
3: {
4: foreach (OperationDescription op in ep.Contract.Operations)
5: {
6: DataContractSerializerOperationBehavior dataContractBehavior =
7: op.Behaviors.Find<DataContractSerializerOperationBehavior>()
8: as DataContractSerializerOperationBehavior;
9: if (op.Behaviors.Find<DataContractSerializerOperationBehavior>()
10: != null)
11: dataContractBehavior.DataContractSurrogate = new ContractSurrogate();
12: op.Behaviors.Add(op.Behaviors.
13: Find<DataContractSerializerOperationBehavior>());
14:
15: dataContractBehavior = new DataContractSerializerOperationBehavior(op);
16: dataContractBehavior.DataContractSurrogate =
17: new ContractSurrogate ();
18: op.Behaviors.Add(dataContractBehavior);
19: }
20: }
注：部分内容节选自《WCF技术剖析（卷1）》第五章:序列化与数据契约（Serialization and Data Contract）
出处：http://artech.cnblogs.com
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