WCF学习（五）数据契约之已知类型

准备技术：

1.C#基础知识

2.了解WCF基础知识

在正常的c#开发中我们是允许用子类去替换基类的，这也是所谓的替换原则。但是我们在WCF中确不能用数据契约的子类来替换父类的，因为这中间存在一个序列化的问题。举个例子：

我们有数据契约：

[DataContract]

class Employee{...}

服务契约中：

[ServiceContract]

interface IEmployeeManager

{

[OperationContract]

void AddEmployee(Employee employee);

}

然后我们在客户端的代理中就算有类：Intern继承于

[DataContract]

class Intern:Employee{...}

然后再客户端调用时：

proxy.AddEmployee(new Intern())是会出错的。因为在服务器端无法识别Intern对象，因为他无法去反序列化Intern成Employee对象(WCF序列化)。

WCF提供给我们了一个解决的办法就是使用KnownTypeAttribute特性，在基类上标识对应的子类就可以了。KnownTypeAttribute特性可以使用在Struct跟Class上。示例：

[DataContract]

[KnownType(typeof(Customer))]

class Employee{...}

[DataContract]

class Intern:Employee{...}

这样我们就在所有的契约跟操作上，能跨越所有的服务和终结点，允许服务接受子类。但是这样会遇到一个问题，我们不能去特定的指定某一个服务操作，所以KnownType的缺陷就是范围过于广泛。WCF提供了另外一个Attribute--ServiceKnownType.

ServiceKnownType 特性

KnownType只能应用在数据契约的基类上，而ServiceKnownType可以在Interface、Method、Class上标识。看一个示例Employee.cs：

Code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ServiceModel;
using System.Runtime.Serialization;

namespace HenllyeeDataContract
{
    [DataContract]
    public class Employee
    {
        #region Fields
        private string _name;
        private int _age;
        #endregion

        #region Properties
        /// <summary>
        /// The employee's name
        /// </summary>
        [DataMember(Order=0)]
        public string Name
        {
            get
            {
                return this._name;
            }
            set
            {
                this._name = value;
            }
        }

        /// <summary>
        /// The employee's age
        /// </summary>
        [DataMember(Order=1)]
        public int Age
        {
            get
            {
                return this._age;
            }
            set
            {
                this._age = value;
            }
        }
        #endregion
    }

    [DataContract]
    public class Intern : Employee
    {
        private int _internship;

        /// <summary>
        /// The intern's working days
        /// </summary>
        [DataMember]
        public int Internship
        {
            get
            {
                return this._internship;
            }
            set
            {
                this._internship = value;
            }
        }
    }
}

在数据契约中我们并没有去指定KnownType，我们在服务契约的操作上去标识ServiceKnownType特性，EmployeeManage.cs:

Code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ServiceModel;
using System.Runtime.Serialization;

namespace HenllyeeServiceContract
{
    [ServiceContract]
    public interface IEmployeeManage
    {
        [OperationContract]
        [ServiceKnownType(typeof(HenllyeeDataContract.Intern))]
        void AddEmployee(HenllyeeDataContract.Employee emlpoyee);

        [OperationContract]
        [ServiceKnownType(typeof(HenllyeeDataContract.Intern))]
        HenllyeeDataContract.Employee GetEmployee();
    }

    [ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
    public class EmployeeManage : IEmployeeManage
    {
        private HenllyeeDataContract.Employee _employee;

        /// <summary>
        /// Set employee
        /// </summary>
        /// <param name="emlpoyee">the employee's object</param>
        public void AddEmployee(HenllyeeDataContract.Employee emlpoyee)
        {
            this._employee = emlpoyee;
        }

        /// <summary>
        /// Get a employee
        /// </summary>
        /// <returns></returns>
        public HenllyeeDataContract.Employee GetEmployee()
        {
            return this._employee;
        }
    }
}

在客户端我们调用：

Code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Client
{
    class Program
    {
        static void Main(string[] args)
        {
            EmployeeService.EmployeeManageClient proxy = new Client.EmployeeService.EmployeeManageClient();
            EmployeeService.Intern intern = new Client.EmployeeService.Intern();
            intern.Age = 22;
            intern.Name = "Henllyee Cui";
            intern.Internship = 120;
            proxy.AddEmployee(intern);

            EmployeeService.Employee internOut = proxy.GetEmployee();
            Console.Write("The Employee Name:{0}\nAge:{1}\n",
                internOut.Name,
                internOut.Age
                );
            Console.Read();
        }
    }
}

运行后：