WCF: Building an HTTP User Agent Message Inspector : CSharp Feeds

WCF: Building an HTTP User Agent Message Inspector

by Paulo Morgado via Paulo Morgado : C# on 4/26/2007 11:50:50 PM

Yesterday I had to build a custom message encoder to be able to call a legacy POX service with iso-8859-1 encoding.

It turned out that the service had another surprise to me: it needs an HTTP user-agent header.

It's something quite simple to accomplish with WCF. All it's needed is to add the required header to the HTTP request message property of the request message. Something like this:

HttpRequestMessageProperty httpRequestMessage = new HttpRequestMessageProperty();
httpRequestMessage.Headers.Add(USER_AGENT_HTTP_HEADER, this.m_userAgent);
requestMessage.Properties.Add(HttpRequestMessageProperty.Name, httpRequestMessage);

But I was not willing to pollute my code with this and, besides that, it doesn't look much WCFish having to do this.

Since I don't like to spend too much time trying to figure things out when I know someone who knows it, I asked António Cruz what to do. And the answer was a client message inspector.

With the help of a sample I have built an HTTP user agent client message inspector.

Message Inspector

The message inspector is very simple. We just need to implement the IClientMessageInspector Interface.

public class HttpUserAgentMessageInspector : IClientMessageInspector
{
    private const string USER_AGENT_HTTP_HEADER = "user-agent";
 
    private string m_userAgent;
 
    public HttpUserAgentMessageInspector(string userAgent)
    {
        this.m_userAgent = userAgent;
    }
 
    #region IClientMessageInspector Members
 
    public void AfterReceiveReply(ref System.ServiceModel.Channels.Message reply, object correlationState)
    {
    }
 
    public object BeforeSendRequest(ref System.ServiceModel.Channels.Message request, System.ServiceModel.IClientChannel channel)
    {
        HttpRequestMessageProperty httpRequestMessage;
        object httpRequestMessageObject;
        if (request.Properties.TryGetValue(HttpRequestMessageProperty.Name, out httpRequestMessageObject))
        {
            httpRequestMessage = httpRequestMessageObject as HttpRequestMessageProperty;
            if (string.IsNullOrEmpty(httpRequestMessage.Headers[USER_AGENT_HTTP_HEADER]))
            {
                httpRequestMessage.Headers[USER_AGENT_HTTP_HEADER] = this.m_userAgent;
            }
        }
        else
        {
            httpRequestMessage = new HttpRequestMessageProperty();
            httpRequestMessage.Headers.Add(USER_AGENT_HTTP_HEADER, this.m_userAgent);
            request.Properties.Add(HttpRequestMessageProperty.Name, httpRequestMessage);
        }
        return null;
    }
 
    #endregion

Behavior

But this message inspectors are extensions to the client runtime. Such extensions are configured using behaviors. A behavior is a class that changes the behavior of the service model runtime by changing the default configuration or adding extensions (such as message inspectors) to it.

The implementation of a behavior is, also, very simple. We just need to implement the IEndpointBehavior Interface. In this case, since it's a client message inspector only, all we need to implement is the ApplyClientBehavior method.

public class HttpUserAgentEndpointBehavior : IEndpointBehavior
{
    private string m_userAgent;
 
    public HttpUserAgentEndpointBehavior(string userAgent)
    {
        this.m_userAgent = userAgent;
    }
 
    #region IEndpointBehavior Members
 
    public void AddBindingParameters(ServiceEndpoint endpoint, System.ServiceModel.Channels.BindingParameterCollection bindingParameters)
    {
    }
 
    public void ApplyClientBehavior(ServiceEndpoint endpoint, System.ServiceModel.Dispatcher.ClientRuntime clientRuntime)
    {
        HttpUserAgentMessageInspector inspector = new HttpUserAgentMessageInspector(this.m_userAgent);
        clientRuntime.MessageInspectors.Add(inspector);
    }
 
    public void ApplyDispatchBehavior(ServiceEndpoint endpoint, System.ServiceModel.Dispatcher.EndpointDispatcher endpointDispatcher)
    {
    }
 
    public void Validate(ServiceEndpoint endpoint)
    {
    }
 
    #endregion
}

Adding the Message Inspector through Configuration

For configuring an endpoint in the application configuration file to use our custom behavior, the service model requires us to create a configuration extension element represented by a class derived from BehaviorExtensionElement.

public class HttpUserAgentBehaviorExtensionElement : BehaviorExtensionElement
{
    public override Type BehaviorType
    {
        get
        {
            return typeof(HttpUserAgentEndpointBehavior);
        }
    }
 
    protected override object CreateBehavior()
    {
        return new HttpUserAgentEndpointBehavior(UserAgent);
    }
 
    [ConfigurationProperty("userAgent", IsRequired = true)]
    public string UserAgent
    {
        get { return (string)base["userAgent"]; }
        set { base["userAgent"] = value; }
    }
}

This extension must then be added to the service model's configuration section for extensions:

<system.serviceModel>
...
  <extensions>
    <behaviorExtensions>
      <add name="httpUserAgent" type="Pajocomo.ServiceModel.Dispatcher.HttpUserAgentBehaviorExtensionElement, Esi.ServiceModel, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
    </behaviorExtensions>
  </extensions>
...
</system.serviceModel>

Note: We must use the full assembly qualified name here, otherwise it won't work.

Now we need to apply our behavior extension to a behavior configuration:

<system.serviceModel>
...
  <behaviors>
    <endpointBehaviors>
      <behavior name="LegacyServiceEndpointBehavior">
        <httpUserAgent userAgent="test user agent" />
      </behavior>
    </endpointBehaviors>
  </behaviors>
...
</system.serviceModel>

Adding the configuration to the service configuration

Now we can apply this configuration to our sevice configuration:

<system.serviceModel>
...
  <client>
    <endpoint
      address="..."
      binding="..."
      bindingConfiguration="..."
      behaviorConfiguration="LegacyServiceEndpointBehavior"
      contract="..."
      name="..." />
  </client>
...
</system.serviceModel>