WCF 行為擴展深度解析：從參數攔截到訊息檢查器的完整實作指南

WCF 擁有極具彈性的擴展架構，其中行為擴展是最常見且強大的切入點。本文系統梳理伺服器端與客戶端擴展點、四種行為類型、配置與屬性兩種啟用方式，並提供完整程式碼範例。

核心擴展點介面

所有介面定義於 System.ServiceModel.Dispatcher 命名空間。IParameterInspector 可同時用於雙端。

參數攔截器實作

public class CalculatorParameterInspector : IParameterInspector {
    public object BeforeCall(string operationName, object[] inputs) {
        int x = (int)inputs[0], y = (int)inputs[1];
        if (x < 0 || y < 0)
            throw new FaultException("The number can not be less than zero.");
        return null; // 相關狀態傳給 AfterCall
    }

    public void AfterCall(string operationName, object[] outputs, object returnValue, object correlationState) {
        // 可記錄回傳值、效能監控等
    }
}

訊息攔截器實作

public class PrintMessageInterceptor : IDispatchMessageInspector {
    public object AfterReceiveRequest(ref Message request, IClientChannel channel, InstanceContext instanceContext) {
        var buffer = request.CreateBufferedCopy(int.MaxValue);
        request = buffer.CreateMessage();
        Console.WriteLine("After Receive Request:");
        foreach (var header in request.Headers) Console.WriteLine(header);
        Console.WriteLine(new string('*', 20));
        return null;
    }

    public void BeforeSendReply(ref Message reply, object correlationState) {
        var buffer = reply.CreateBufferedCopy(int.MaxValue);
        reply = buffer.CreateMessage();
        Console.WriteLine("Before Send Reply:");
        foreach (var header in reply.Headers) Console.WriteLine(header);
        Console.WriteLine(new string('*', 20));
    }
}

注意：IDispatchMessageInspector（伺服端）方法順序為 AfterReceiveRequest → BeforeSendReply；IClientMessageInspector（客戶端）則相反：BeforeSendRequest → AfterReceiveReply。

四種行為類型與作用範圍

三種啟用方式比較

方式一：自訂屬性啟用

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface)]
public class MyServiceBehavior : Attribute, IServiceBehavior {
    // 實作 IServiceBehavior 三個方法...
}

[MyServiceBehavior]
public interface IService { ... }

方式二：配置檔啟用（需 BehaviorExtensionElement）

public class MyBehaviorExtensionElement : BehaviorExtensionElement {
    public override Type BehaviorType => typeof(MyServiceBehavior);
    protected override object CreateBehavior() => new MyServiceBehavior();

    [ConfigurationProperty("providerName", IsRequired = true)]
    public string ProviderName {
        get => (string)this["providerName"];
        set => this["providerName"] = value;
    }
}

配置檔：

<system.serviceModel>
  <services>
    <service name="Demo.Calculator">
      <endpoint behaviorConfiguration="messageInspectorBehavior" ... />
    </service>
  </services>
  <behaviors>
    <serviceBehaviors>
      <behavior name="messageInspectorBehavior">
        <myBehaviorExtensionElement providerName="Test" />
      </behavior>
    </serviceBehaviors>
  </behaviors>
  <extensions>
    <behaviorExtensions>
      <add name="myBehaviorExtensionElement"
           type="Demo.MyBehaviorExtensionElement, Demo, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
    </behaviorExtensions>
  </extensions>
</system.serviceModel>

關鍵細節：type 必須為完整型別名稱（含命名空間、版本、文化特性、公開金鑰），逗號後必須保留一空格，否則反射載入失敗。

行為中掛載攔截器

參數檢查 → IOperationBehavior

public class CalculatorParameterValidation : Attribute, IOperationBehavior {
    public void ApplyClientBehavior(OperationDescription desc, ClientOperation op) {
        op.ParameterInspectors.Add(new CalculatorParameterInspector());
    }
    public void ApplyDispatchBehavior(OperationDescription desc, DispatchOperation op) {
        op.ParameterInspectors.Add(new CalculatorParameterInspector());
    }
    // AddBindingParameters, Validate 留空
}

進階：合併介面實作

public class CalculatorParameterValidation : Attribute, IParameterInspector, IOperationBehavior {
    // 實作 IParameterInspector 方法...
    public void ApplyClientBehavior(OperationDescription desc, ClientOperation op) {
        op.ParameterInspectors.Add(this); // this 即為參數檢查器
    }
    public void ApplyDispatchBehavior(OperationDescription desc, DispatchOperation op) {
        op.ParameterInspectors.Add(this);
    }
}

操作調用器 → IOperationBehavior + DispatchOperation.Invoker

public class MyOperationInvokerBehavior : Attribute, IOperationBehavior {
    public void ApplyDispatchBehavior(OperationDescription desc, DispatchOperation op) {
        op.Invoker = new MyOperationInvoker(op.Invoker); // 裝飾模式包裝原調用器
    }
}

訊息檢查器 → IEndpointBehavior / IServiceBehavior

public class PrintMessageInspectorBehavior : IDispatchMessageInspector, IEndpointBehavior {
    // 實作 IDispatchMessageInspector...

    public void ApplyClientBehavior(ServiceEndpoint endpoint, ClientRuntime clientRuntime) {
        clientRuntime.MessageInspectors.Add(this);
    }
    public void ApplyDispatchBehavior(ServiceEndpoint endpoint, EndpointDispatcher dispatcher) {
        dispatcher.DispatchRuntime.MessageInspectors.Add(this);
    }
}

若實作 IServiceBehavior，需遍歷 ServiceHostBase.ChannelDispatchers：

public void ApplyDispatchBehavior(ServiceDescription desc, ServiceHostBase host) {
    foreach (var chDisp in host.ChannelDispatchers)
        foreach (var epDisp in chDisp.Endpoints)
            epDisp.DispatchRuntime.MessageInspectors.Add(this);
}

總結

• 行為擴展是 WCF AOP 風格切面的核心實現機制
• 選擇正確的行為類型（服務/端點/合約/操作）對應作用範圍
• 屬性啟用適合編譯期靜態綁定，配置檔啟用適合部署期動態調整
• 攔截器（Inspector/Invoker）與行為類別分離符合單一職責，亦可合併簡化
• 配置檔 type 字串格式嚴格，逗號後空格不可省略