Adding HTTP Headers to WCF Calls

To add HTTP headers, request messages have to be intercepted before they are sent to the server. This can be done by implementing the IClientMessageInspector.BeforeSendRequestmethod:

 /// <summary>
/// Represents a message inspector object that can be added to the <c>MessageInspectors</c> collection to view or modify messages.
/// </summary>
public class ClientMessageInspector : IClientMessageInspector
{
    /// <summary>
    /// Enables inspection or modification of a message before a request message is sent to a service.
    /// </summary>
    /// <param name="request">The message to be sent to the service.</param>
    /// <param name="channel">The WCF client object channel.</param>
    /// <returns>
    /// The object that is returned as the <paramref name="correlationState " /> argument of
    /// the <see cref="M:System.ServiceModel.Dispatcher.IClientMessageInspector.AfterReceiveReply(System.ServiceModel.Channels.Message@,System.Object)" /> method.
    /// This is null if no correlation state is used.The best practice is to make this a <see cref="T:System.Guid" /> to ensure that no two
    /// <paramref name="correlationState" /> objects are the same.
    /// </returns>
    public object BeforeSendRequest(ref Message request, IClientChannel channel)
    {
        HttpRequestMessageProperty property = new HttpRequestMessageProperty();

        property.Headers["User-Agent"] = "value";
        request.Properties.Add(HttpRequestMessageProperty.Name, property);

        return null;
    }

    /// <summary>
    /// Enables inspection or modification of a message after a reply message is received but prior to passing it back to the client application.
    /// </summary>
    /// <param name="reply">The message to be transformed into types and handed back to the client application.</param>
    /// <param name="correlationState">Correlation state data.</param>
    public void AfterReceiveReply(ref System.ServiceModel.Channels.Message reply, object correlationState)
    {
        // Nothing special here
    }
}

This interception is a behavior or an extension of WCF endpoints, so one more method needs to be implemented to apply that behavior to the endpoint, the IEndpointBehavior.ApplyClientBehavior method:

 /// <summary>
/// Represents a run-time behavior extension for a client endpoint.
/// </summary>
public class CustomEndpointBehavior : IEndpointBehavior
{
    /// <summary>
    /// Implements a modification or extension of the client across an endpoint.
    /// </summary>
    /// <param name="endpoint">The endpoint that is to be customized.</param>
    /// <param name="clientRuntime">The client runtime to be customized.</param>
    public void ApplyClientBehavior(ServiceEndpoint endpoint, ClientRuntime clientRuntime)
    {
        clientRuntime.ClientMessageInspectors.Add(new ClientMessageInspector());
    }
            
    /// <summary>
    /// Implement to pass data at runtime to bindings to support custom behavior.
    /// </summary>
    /// <param name="endpoint">The endpoint to modify.</param>
    /// <param name="bindingParameters">The objects that binding elements require to support the behavior.</param>
    public void AddBindingParameters(ServiceEndpoint endpoint, BindingParameterCollection bindingParameters)
    {
        // Nothing special here
    }

    /// <summary>
    /// Implements a modification or extension of the service across an endpoint.
    /// </summary>
    /// <param name="endpoint">The endpoint that exposes the contract.</param>
    /// <param name="endpointDispatcher">The endpoint dispatcher to be modified or extended.</param>
    public void ApplyDispatchBehavior(ServiceEndpoint endpoint, EndpointDispatcher endpointDispatcher)
    {
        // Nothing special here
    }

    /// <summary>
    /// Implement to confirm that the endpoint meets some intended criteria.
    /// </summary>
    /// <param name="endpoint">The endpoint to validate.</param>
    public void Validate(ServiceEndpoint endpoint)
    {
        // Nothing special here
    }
}

The last step is to add the new behavior to the WCF endpoint:

 Endpoint.EndpointBehaviors.Add(new CustomEndpointBehavior());

Voilà!