Custom Global HttpMessageHandlers don't work with the SwashBuckle HttpMessageHandlers

When I add my own HttpMessageHandler to the MessageHandler collection for an IIS hosted application like so:

GlobalConfiguration.Configuration.MessageHandlers.Add(new AuthenticationHandler());

I can then either hit my standard api endpoints:

e.g.

http://somehost.com/api/companies

OR

I can hit the swagger endpoint.

http://somehost.com/api/swagger

I CANNOT hit one and then the other.

If I hit one of those endpoints and then try to hit the other endpoint I get the following:

Server Error in '/api' Application.

The 'DelegatingHandler' list is invalid because the property 'InnerHandler' of 'AuthenticationHandler' is not null.
Parameter name: handlers

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.ArgumentException: The 'DelegatingHandler' list is invalid because the property 'InnerHandler' of 'AuthenticationHandler' is not null.
Parameter name: handlers

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 


[ArgumentException: The 'DelegatingHandler' list is invalid because the property 'InnerHandler' of 'AuthenticationHandler' is not null.
Parameter name: handlers]
   System.Net.Http.HttpClientFactory.CreatePipeline(HttpMessageHandler innerHandler, IEnumerable`1 handlers) +380
   System.Web.Http.HttpServer.Initialize() +30
   System.Web.Http.HttpServer.<EnsureInitialized>b__b() +10
   System.Threading.LazyInitializer.EnsureInitializedCore(T& target, Boolean& initialized, Object& syncLock, Func`1 valueFactory) +86
   System.Web.Http.HttpServer.EnsureInitialized() +95
   System.Web.Http.<SendAsync>d__0.MoveNext() +147
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +93
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +52
   System.Web.Http.WebHost.<ProcessRequestAsyncCore>d__0.MoveNext() +400
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +93
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +52
   System.Web.TaskAsyncHelper.EndTask(IAsyncResult ar) +60
   System.Web.HttpTaskAsyncHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +6
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9651188
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

[johncrim]

Just to contribute a hopefully helpful insight - this error occurs when you have multiple HttpServers that use a single HttpConfiguration, and the HttpConfiguration has one or more delegating handlers implicitly or explicitly configured.

I explain it in more depth here: http://stackoverflow.com/questions/12008686/webapi-batching-and-delegating-handlers/31445355#31445355

[cryo75]

I’m getting the same problem when adding the custom message handler to the pipeline. When I add the message handler to the routing, it works fine.