.Net Core 3.1 WebApplicationFactory TestServer: cannot find Razor view when override Startup

Question

I am trying to write an integration test for a controller with a view. I do it as part of migration to .Net Core 3.1 from 2.2. There is a lot of configuration in ConfigureServices that we need to mock or disable in the tests, so, we derive from existing Startup class and override the parts needed.

Now, I can make it working in .Net Core 3.1 using WebApplicationFactory and overriding ConfigureWebHost. However, I rather hoped to not rewrite the existing class that derives from Startup.

I tried to use the approach from https://gunnarpeipman.com/aspnet-core-integration-test-startup/ where I specify the derived Startup for WebApplicationFactory and call UseSolutionRelativeContentRoot (which has UseContentRoot inside which I also tried). However, the views cannot be found. Part of the exception returned is:

System.InvalidOperationException: The view 'Index' was not found. The following locations were searched:
Features\Dummy\Index.cshtml
Features\Shared\Index.cshtml
\Features\Index\Dummy.cshtml

How could I fix the tests?

I have a "mock" project where I reproduce the issue.

public class Program
{
    public static void Main(string[] args)
    {
        var host = BuildHost(args);
        host.Run();
    }

    public static IHost BuildHost(string[] args) =>
        Host.CreateDefaultBuilder(args)
        .ConfigureWebHostDefaults(webBuilder => webBuilder
            .UseStartup<Startup>())
        .Build();
}

public class Startup
{
    protected virtual void AddTestService(IServiceCollection services)
    {
        services.TryAddSingleton<IServiceToMock, ServiceToMock>();
    }

    public void ConfigureServices(IServiceCollection services)
    {
        AddTestService(services);

        services.AddMvc()
            .AddFeatureFolders();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        app.UseRouting();

        app.UseEndpoints(endpoints => endpoints.MapControllerRoute("default", "{controller=Home}/{action=Index}"));
    }
}

public interface IServiceToMock
{
    Task DoThing();
}

public class ServiceToMock : IServiceToMock
{
    public async Task DoThing() =>
        throw new Exception(await Task.FromResult("service exception"));
}

[Route("candidates/[controller]/[action]")]
public class DummyController : Controller
{
    private readonly IServiceToMock serviceToMock;

    public DummyController(IServiceToMock serviceToMock)
    {
        this.serviceToMock = serviceToMock;
    }

    [HttpGet]
    public IActionResult Index()
    {
        return View();
    }

    [HttpGet]
    public async Task<bool> IsExternal(string email)
    {
        await serviceToMock.DoThing();
        return await Task.FromResult(!string.IsNullOrWhiteSpace(email));
    }
}

Index.cshtml (goes in the same folder as the controller)

@{
    ViewData["Title"] = "Title";
}

<p>Hello.</p>

csproj:

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="OdeToCode.AddFeatureFolders" Version="2.0.3" />
  </ItemGroup>

</Project>

The test part:

public class TestServerFixture : TestServerFixtureBase<Startup, TestStartup>
{
}

public class TestServerFixtureBase<TSUTStartus, TTestStartup> : WebApplicationFactory<TTestStartup>
    where TTestStartup : class where TSUTStartus : class
{
    private readonly Lazy<HttpClient> m_AuthClient;

    public TestServerFixtureBase()
    {
        m_AuthClient = new Lazy<HttpClient>(() => CreateAuthClient());
    }

    protected override IWebHostBuilder CreateWebHostBuilder()
    {
        return WebHost.CreateDefaultBuilder()
            .UseStartup<TTestStartup>();
    }

    public HttpClient AuthClient => m_AuthClient.Value;
    protected virtual HttpClient CreateAuthClient() => WithWebHostBuilder(builder =>
    {
        builder.UseSolutionRelativeContentRoot("NetCore31IntegrationTests3");

        builder.ConfigureTestServices(services =>
        {
            services.AddMvc().AddApplicationPart(typeof(TSUTStartus).Assembly);
        });

    }).CreateClient();
}

public class TestStartup : Startup
{
    protected override void AddTestService(IServiceCollection services)
    {
        services.AddSingleton<IServiceToMock, TestServiceToMock>();
    }
}

public class TestServiceToMock : IServiceToMock
{
    public async Task DoThing() => await Task.CompletedTask;
}

public class HomeControllerTests : IClassFixture<TestServerFixture>
{
    private readonly TestServerFixture _factory;

    public HomeControllerTests(TestServerFixture factory)
    {
        _factory = factory;
    }

    [Theory]
    [InlineData("/candidates/dummy/IsExternal?email=aaa")]
    [InlineData("/candidates/dummy/index")]
    [InlineData("candidates/dummy/index")]
    public async Task Get_EndpointsReturnSuccessAndCorrectContentType(string url)
    {
        // Arrange
        var client = _factory.AuthClient;

        // Act
        var response = await client.GetAsync(url);

        // Assert
        response.EnsureSuccessStatusCode();
    }
}

The working fix I am trying to avoid:

public class TestServerFixtureBase<TSUTStartus, TTestStartup> : WebApplicationFactory<TSUTStartus>
    where TTestStartup : class where TSUTStartus : class
{
    private readonly Lazy<HttpClient> m_AuthClient;

    public TestServerFixtureBase()
    {
        m_AuthClient = new Lazy<HttpClient>(() => CreateAuthClient());
    }

    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureServices(services =>
        {
            var descriptor = services.SingleOrDefault(
            d => d.ServiceType ==
                typeof(IServiceToMock));

            if (descriptor != null)
                services.Remove(descriptor);

            services.AddSingleton<IServiceToMock, TestServiceToMock>();
        });
    }
    protected override IWebHostBuilder CreateWebHostBuilder()
    {
        return WebHost.CreateDefaultBuilder()
            .UseStartup<TSUTStartus>();
    }

    public HttpClient AuthClient => m_AuthClient.Value;
    protected virtual HttpClient CreateAuthClient() => WithWebHostBuilder(builder => { }).CreateClient();
}

Answer 1

To fix the problem into WithWebHostBuilder I added

services
    .AddMvc()
    .AddRazorRuntimeCompilation()
    .AddApplicationPart(typeof(TTestStartup).Assembly);

However, there were other fixes suggested, for example:

var builder = new WebHostBuilder();
        builder.ConfigureAppConfiguration((context, b) => {
            context.HostingEnvironment.ApplicationName = typeof(HomeController).Assembly.GetName().Name;
        });

from https://github.com/dotnet/aspnetcore/issues/17655#issuecomment-581418168