MediatR - Handler not found error when the DataContext couldn't be initialized

TL;DR

If you use MediatR package and it suddenly it fails with Handler was not found for request of type <type> inspect the dependencies of the handler it fails to create/invoke. One or more of those dependencies (a DbContext in my case) throws an error when instantiated and the error is making MediatR fail.

Jimmy Bogards' MediatR is a little gem of a package I like using because it enables a good separation of business logic from the boilerplate code and provides a clean and structured enforcement of the Single Responsibility Principle.

I use this package extensively in one of my outside work projects (I'm proud to say that it's not a pet project anymore) to delegate requests/commands to their respective request/command handlers. The project itself consists of two parts - an ASP.NET MVC application for public access and back office management and a WebAPI application used for registering payments. In order to keep both Web Application and Web API URLs consistent (and pretty) I have hosted the Web API application as a virtual directory inside the main Web Application.

Recently, after an update of the application the payment module went down (giving me a tiny heart attack). As expected I dove into the application logs and after some thorough search I found the culprit with the following error message:

An unhandled exception of type 'System.InvalidOperationException' occurred in MediatR.dll Additional information: Handler was not found for request of type GetAuthorizedUserRequest.
Container or service locator not configured properly or handlers not registered with your container.

The exception was popping inside the IsAuthorized method of a custom AuthorizeAttribute

  protected override bool IsAuthorized(HttpActionContext actionContext)
  {
      try
      {
            var authorizationToken = new AuthorizationToken(actionContext.Request);
            if (String.IsNullOrEmpty(authorizationToken.Value))
            {
                  return false;
            }
            var request = new GetAuthorizedUserRequest
            {
                  AuthorizationToken = authorizationToken.Value
            };
            var user = _securityService.GetAuthorizedUser(request);
            return user != null;
      }
      catch (Exception)
      {
            return false;
      }
  }

The first thing to do was to thoroughly inspect what does the IoC container (StructureMap in my case) has registered. After a glimpse through the output of WhatDoIHave() method I saw that the handler GetAuthorizedUserRequestHandler was indeed registered as a IRequestHandler<GetAuthorizedUserRequest, GetAuthorizedUserResponse>.

So, what is the problem then? The InnerException property of the exception that was caught was null and I was stuck.

On the dawn of divine inspiration I decided to comment out the existing constructor of the request handler and create a default one (also return a dummy user). It worked - the exception wasn't thrown and the user got authenticated.

However, the next request (dispatched through MediatR) that had to query something in the database failed which gave me the idea that there must be some issues with the DbContext initialization (I use Entity Framework).
Sure enough - when I put a breakpoint in the constructor of my DataContext class (derived from DbContext) I got an exception mentioning that the key "mssqllocaldb" is missing from <connectionStrings> section.

Then, I remembered that the latest code update also came with an update of Entity Framework NuGet package and it dawned upon me why the MediatR was failing. As I said in the beginning, the Web API application is hosted under the main Web Application. This means that the <entityFramework> configuration element in the child application is inherited from the parent one so the Web.config file of the child application did not contain any section related to Entity Framework.
When I did the upgrade of the NuGet package the installer added the configuration section with default values. Those default and wrong values were read by the DbContext class constructor and since the values were wrong the constructor failed. After deleting the <entityFramework> configuration element the application went back online.