WCF 實作雙向通訊（三）：Callback 模式完整範例

雙向通訊架構設計

WCF 雙向通訊允許 Service 主動回呼 Client。本範例沿用數學運算情境：Client 傳遞運算元與一個回呼物件，Server 運算完成後透過 Callback 將結果回傳 Client 顯示。專案結構包含五個專案：Contract、Service、Hosting (Self-host)、Client、IIS Host 網站。

定義雙向 Contract

在 Contract 專案中定義兩個介面：

[ServiceContract(CallbackContract = typeof(ICalculatorCallback))]
public interface IDuplexCalculator
{
    [OperationContract]
    void Add(double x, double y);
}

[ServiceContract]
public interface ICalculatorCallback
{
    [OperationContract]
    void ShowResult(double x, double y, double result);
}

關鍵點：

• CallbackContract 參數指定回呼介面，使 Service 知道如何回呼 Client。
• 雙向通訊中 Client 與 Server 角色可互換：Client 呼叫 Service 時為 Client，接收 Callback 時實際扮演 Service 角色。

實作 Service

[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple)]
public class DuplexCalculatorService : IDuplexCalculator
{
    public void Add(double x, double y)
    {
        double result = x + y;
        ICalculatorCallback callback = OperationContext.Current.GetCallbackChannel<ICalculatorCallback>();
        callback.ShowResult(x, y, result);
    }
}

注意事項：

• 必須設定 ConcurrencyMode.Multiple 或 Reentrant，避免死鎖。預設 Single 模式下，InstanceContext 被鎖定，Service 發起外呼叫時無法重入。
• 透過 OperationContext.Current.GetCallbackChannel<T>() 取得 Client 端的 Callback 通道。

Self-Host 設定

App.config 關鍵設定：

<services>
  <service name="Artech.WCFService.Service.DuplexCalculatorService" behaviorConfiguration="calculatorServieBehavior">
    <endpoint binding="wsDualHttpBinding" contract="Artech.WCFService.Contract.IDuplexCalculator" />
    <host>
      <baseAddresses>
        <add baseAddress="https://localhost:7777/DuplexCalculator" />
      </baseAddresses>
    </host>
  </service>
</services>

Program.cs 啟動 Host：

using (ServiceHost host = new ServiceHost(typeof(DuplexCalculatorService)))
{
    host.Opened += delegate { Console.WriteLine("Duplex calculator Service has begun to listen"); };
    host.Open();
    Console.Read();
}

Client 端實作

Client 需實作 Callback 介面並建立雙向 Proxy：

public class CalculatorCallbackHandler : ICalculatorCallback
{
    public void ShowResult(double x, double y, double result)
    {
        Console.WriteLine("x + y = {2} where x = {0} and y = {1}", x, y, result);
    }
}

public class DuplexCalculatorClient : ClientBase<IDuplexCalculator>, IDuplexCalculator
{
    public DuplexCalculatorClient(InstanceContext callbackInstance) : base(callbackInstance) { }
    public void Add(double x, double y) { this.Channel.Add(x, y); }
}

Client 端 App.config 需設定 clientBaseAddress 讓 Service 能回呼：

<wsDualHttpBinding>
  <binding name="wsDualBinding_IDuplexCalculator" clientBaseAddress="https://localhost:6666/myClient/" />
</wsDualHttpBinding>

Program.cs 呼叫：

CalculatorCallbackHandler handler = new CalculatorCallbackHandler();
using (DuplexCalculatorClient client = new DuplexCalculatorClient(new InstanceContext(handler)))
{
    Console.WriteLine("Begin to invocate duplex calculator");
    client.Add(1, 2);
}

部署至 IIS

建立 .svc 檔並複製 Host 端的 system.serviceModel 設定至 Web.config，修改 Client 端 endpoint address 指向 .svc 網址即可在 IIS 中執行相同功能。