All requests to ASP.NET Web API return 404 error

Thanks to Kiran Challa’s comment and source code from this answer I was able to figure out that an assembly (the ReportViewer 11 assembly for SQL Server Reporting Services) was missing on the production server.

Although no ApiController is in this assembly it seems to cause that controllers in assemblies – in this case my Web project’s assembly – that are referencing the missing assembly are not found.

Apparently this behaviour is related to this piece of code from the Web API’s DefaultHttpControllerTypeResolver source:

List<Type> result = new List<Type>();

// Go through all assemblies referenced by the application
// and search for types matching a predicate
ICollection<Assembly> assemblies = assembliesResolver.GetAssemblies();
foreach (Assembly assembly in assemblies)
{
    Type[] exportedTypes = null;
    if (assembly == null || assembly.IsDynamic)
    {
        // can't call GetExportedTypes on a dynamic assembly
        continue;
    }

    try
    {
        exportedTypes = assembly.GetExportedTypes();
    }
    catch (ReflectionTypeLoadException ex)
    {
        exportedTypes = ex.Types;
    }
    catch
    {
        // We deliberately ignore all exceptions when building the cache. If 
        // a controller type is not found then we will respond later with a 404.
        // However, until then we don't know whether an exception at all will
        // have an impact on finding a controller.
        continue;
    }

    if (exportedTypes != null)
    {
        result.AddRange(exportedTypes.Where(x => IsControllerTypePredicate(x)));
    }
}

I don’t know if it has to be this way and I am not quite convinced by the comment in the code but this catch ... continue block is rather silent about a possible problem and it took me a huge amount of time and frustration to find it. I even knew that the ReportViewer wasn’t installed yet. I tried to install it and dependent assemblies but it was blocked by another running process on the server, so I decided to postpone the installation until I could contact the administrator and focus on MVC and WebAPI testing first – big mistake! Without Kiran’s debugging code snippet I had never had the idea that the existence of a ReportViewer.dll could have anything to do with controller type resolution.

In my opinion there is room for improvement for the average developer like me who doesn’t have a deeper knowledge about the inner workings of Web API.

After installing the missing ReportViewer.dll the problem disappeared.

Here are questions about the same symptom which might have the same reason:

Edit

I have issued a request for improvement on CodePlex:

http://aspnetwebstack.codeplex.com/workitem/1075

Edit 2 (Aug 11 ’13)

The issue has been fixed for WebAPI v5.0 RC. See the link above to the workitem and its comments section for details.

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