In this blog post I want to show you difference between the ASP.NET Core methods AddMvc() and AddMvcCore() when working with ASP.NET Core.

ASPNETCore-WebAPI-Sample

Startup.cs

When creating an ASP.NET Core WebAPI you often see a Startup.cs file to configure your services and configure your pipeline. Thats what the methods ConfigureServices(IServiceCollection services) and Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory) are for.

ConfigureServices is preparing your services for being used as you configure them.

Here is also the place to add dependency injection, but that is another seperate topic

You can have a lot of configuration in here. But we want to focus on the main point: Adding the mvc framework.

When starting with “File” –> “New Project” in Visual Studio the default setting in the method is AddMvc(). And it works straight away. Lets take a look:

public void ConfigureServices(IServiceCollection services)
{
    // ...
    services.AddMvc();
}

When now an API with Controllers kicks in we can consume it like normal.

dotnet run makes the API work on localhost:5000

GET localhost:5000/api/house

brings

[
  {
    id: 1,
    street: 'Street1',
    city: 'Town1',
    zipCode: 1234,
  },
  {
    id: 2,
    street: 'Street2',
    city: 'Town2',
    zipCode: 1234,
  },
  {
    id: 3,
    street: 'Street3',
    city: 'Town3',
    zipCode: 1234,
  },
  {
    id: 4,
    street: 'Street4',
    city: 'Town4',
    zipCode: 1234,
  },
];

What happens, if we change AddMvc() to AddMvcCore()?

public void ConfigureServices(IServiceCollection services)
{
    // ...
    services.AddMvcCore();
}

Lets run the same command again:

GET localhost:5000/api/house

now brings a 406 Error saying “Not Acceptable”.

If we check the console which hosts the webapi we see more information about this error:

warn: Microsoft.AspNetCore.Mvc.Internal.ObjectResultExecutor[1]No output formatter was found for content type ” to write the response.

So we deactivated something we better should have not deactivated ;-).

The error says that we do not have an output formatter which can parse our output. Even if we add an accept header in the request like Accept: application/json we would get the same error message, because the server does not know how ot handle the respose.

Lets take a closer look on what is the real difference between AddMvc() and AddMvcCore().

Due to the fact that the framework is open source we can take a look at the sources:

public static IMvcBuilder AddMvc(this IServiceCollection services)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    var builder = services.AddMvcCore();

    builder.AddApiExplorer();
    builder.AddAuthorization();

    AddDefaultFrameworkParts(builder.PartManager);

    // Order added affects options setup order

    // Default framework order
    builder.AddFormatterMappings();
    builder.AddViews();
    builder.AddRazorViewEngine();
    builder.AddCacheTagHelper();

    // +1 order
    builder.AddDataAnnotations(); // +1 order

    // +10 order
    builder.AddJsonFormatters();

    builder.AddCors();

    return new MvcBuilder(builder.Services, builder.PartManager);
}

From MvcServiceCollectionExtensions.cs tells us, that we are adding the complete MVC Services you need to get the whole MVC functionality.

It is adding Authorization, the RazorViewEngine and the JsonFormatters we need to get our output going. And most interesting it is also calling the AddMvcCore() method itself.

So if we use the AddMvc() method we got the ability to render view with razor and so on.

Lets have a look at AddMvcCore() on the other hand:

MvcCoreServiceCollectionExtensions.cs

public static IMvcCoreBuilder AddMvcCore(this IServiceCollection services)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    var partManager = GetApplicationPartManager(services);
    services.TryAddSingleton(partManager);

    ConfigureDefaultFeatureProviders(partManager);
    ConfigureDefaultServices(services);
    AddMvcCoreServices(services);

    var builder = new MvcCoreBuilder(services, partManager);

    return builder;
}

This method is a lot shorter and only adding the basic things to get started. But both methods are returning an IMvcCoreBuilder. Interesting is the AddMvcCoreServices(services); method which is adding the ability to return FileContents, RedirectToRouteResults, ActionResolvers, use Controllers, use routing and so on. Really basic functionality.

So when using AddMvcCore() we have to add everything by ourselves. This means, that we only have in our application what we really want and for example do not include the razor functionality which we do not need anyway.

Now that we know the difference between those two methods: How can we get our webapi going again? We still have the error and we can not return any data.

We can fix that by telling ASP.NET that it should use JsonFormatters like

public void ConfigureServices(IServiceCollection services)
{
	// ...

	// Add framework services.
	services.AddMvcCore().AddJsonFormatters();
}

If we now call our

GET localhost:5000/api/house

again we should see the output as json like we expected it to be.

I hope this clarified a bit what is the main difference between AddMvc() and AddMvcCore().

Best regards

Fabian

Links

When I develop ASP.NET Core MVC, which service should I use? AddMvc or AddMvcCore?

MvcCoreServiceCollectionExtensions.cs

MvcServiceCollectionExtensions.cs