Adding custom headers to every WCF call – a solution

At last, a solution presented itself. While I must admit that at first I was very skeptical of the extensibilty model for WCF which seemed far too involved and complicated, but after implementing a simple extension I must say it’s simple and quite intuitive.

My goal was to have several custom headers added to each call made through a proxy, rather than manually adding them to each call’s OperationContext. With a little help from Ralph Squillace’s blog, I was able to get an extension up and running within minutes, and it Just Works.

The first item to build is the actual extension logic. In this case, I needed an IProxyMessageInspector object that inspects each outgoing message and adds the custom headers:

public class ContextPusher : IProxyMessageInspector
    {
        public void AfterReceiveReply(ref Message reply, object correlationState)
        {
            // Do nothing.
        }

        public object BeforeSendRequest(ref Message request, IClientChannel channel)
        {
            MessageHeader myHeader = new MessageHeader(“HeaderValue”).GetUntypedHeader(“MyHeader”, “ns”);
            request.Headers.Add(
myHeader);
            return null;
        }
    }
}

Now we want to attach that inspector to proxy. We do that wth Behavior objects. This can be done on an operation-by-operation basis or for all operations on a given channel. We can make the same class implement both behaviors, for flexibility:

public class CallContextAttribute : Attribute, IChannelBehavior, IOperationBehavior
    {
        #region IOperationBehavior Members

        public void ApplyBehavior(OperationDescription description, ProxyOperation proxy, BindingParameterCollection parameters)
        {      
            // Add my MessageInspector to each message sent by the proxy.
            proxy.Parent.MessageInspectors.Add(new ContextPusher());
  
        }

        public void ApplyBehavior(OperationDescription description, DispatchOperation dispatch, BindingParameterCollection parameters)
        {
            // No server-side behaviors for now.
        }

        #endregion
               
        #region IChannelBehavior Members

        public void ApplyBehavior(ChannelDescription description, ProxyBehavior behavior, BindingParameterCollection parameters)
        {
            // Add the MessageInspector to every message sent through the channel.
            behavior.MessageInspectors.Add(new ContextPusher());
        }

        #endregion
    }

And then simply put it on a Service or Operation.
I think in this case, putting the attribute on both will result in an error when trying to add duplicate headers. But this allows me flexibility in adding the headers only to certain calls.

[ServiceContract]
[CallContext]
public interface ILocalizationServices
{
   [OperationContract]
   [CallContext]
   string DoSomething(string param);
}

I think this is the first time I got really excited by the WCF framework and the ease of using and extending it. This is FUN.

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