Pitfall with ASP.NET Web Api, OWIN and FxCop

As I wrote in Self-host ASP.NET Web API and SignalR together, you can easily combine ASP.NET Web Api and OWIN. My Startup class contained at least the Configuration method:

public void Configuration(IAppBuilder app)
{
  HttpConfiguration config = new HttpConfiguration();
  config.Routes.MapHttpRoute("API Default", "api/{controller}/{id}", new { id = RouteParameter.Optional });
  app.UseWebApi(config);
}

With this, everything worked fine.

(Unfortunately) I am a well-behaved man. Therefore I ran the code analysis (aka FxCop). It returned the warning

CA2000: Microsoft.Reliability: In method 'Startup.Configuration(IAppBuilder)', call System.IDisposable.Dispose on object 'config' before all references to it are out of scope.

Understandable for me. And since Microsoft suggests to fix it, I changed my method to:

public void Configuration(IAppBuilder app)
{
  using (HttpConfiguration config = new HttpConfiguration())
  {
    config.Routes.MapHttpRoute("API Default", "api/{controller}/{id}", new { id = RouteParameter.Optional });
    app.UseWebApi(config);
  }
}

Sadly, I didn’t test it immediately. Instead I fixed a lot of other FxCop warnings. And I improved my coding otherwise. Finally I forgot this special fix.

When I finally put all together, I got only HTTP 500 Internal Server Error, without any further information. A lot of people suggest in such a case to debug the server, but also there no exception was thrown. It simply didn’t work. I slimmed down my coding until I removed the using statement. And then it worked again!

So it’s better to keep the original code and to add only a SuppressMessage attribute:

[SuppressMessage("Microsoft.Reliability", "CA2000:Dispose objects before losing scope",
                 Justification = "HttpConfiguration must not be disposed, otherwise web api will not work")]
public void Configuration(IAppBuilder app)
{
  HttpConfiguration config = new HttpConfiguration();
  config.Routes.MapHttpRoute("API Default", "api/{controller}/{id}", new { id = RouteParameter.Optional });
  app.UseWebApi(config);
}