Asp .Net Core 3.0 adding a custom Startup class in tests causes that the Views are not served anymore

I test an Asp .Net Core 3.0 MVC application. This a basic sample app create with the command dotnet new mvc. In the xunit test project I test returning of the view Index from the HomeController. The test looks like this:

public class HomeControllerTests
{
    [Fact]
    public async Task ReturnsView()
    {
        const string testProjectDir = "ViewsTestingTests";
        var factory = new TestServerFactory();
        var client = factory.WithWebHostBuilder(builder =>
        {
            builder.UseSolutionRelativeContentRoot(testProjectDir);
            builder.ConfigureTestServices(services =>
            {
                services.AddControllersWithViews()
                    .AddApplicationPart(typeof(Startup).Assembly);
            });
        }).CreateClient();

        var resultIndex = await client.GetAsync("/");
        resultIndex.StatusCode.Should().Be(200);
        var content = await resultIndex.Content.ReadAsStringAsync();
        content.Should().Contain("Welcome");
    }
}

public class TestServerFactory : WebApplicationFactory<TestStartup>
{
    protected override IWebHostBuilder CreateWebHostBuilder()
    {
        return WebHost.CreateDefaultBuilder(null)
            .UseStartup<TestStartup>();
    }
}

public class TestStartup : Startup
{
    public TestStartup(IConfiguration configuration) : base(configuration)
    {
    }
}

I use a test startup class because I’ll be overriding some methods. When I use this TestStartup class the test doesn’t pass with the following reason:

The view 'Index' was not found. The following locations were searched:
/Views/Home/Index.cshtml
/Views/Shared/Index.cshtml

But if I use the Startup class from the main project like this:

public class TestServerFactory : WebApplicationFactory<Startup>
{
    protected override IWebHostBuilder CreateWebHostBuilder()
    {
        return WebHost.CreateDefaultBuilder(null)
            .UseStartup<Startup>();
    }
}

And create Client like this:

var client = factory.WithWebHostBuilder(builder =>
            {
                builder.UseSolutionRelativeContentRoot(testProjectDir);
            }).CreateClient();

Then the test pass. Of course I copied the whole Views folder to the test project. What is interesting the same test used to work in Asp. Net Core 2.2.

Using builder.UseContentRoot inside factory.WithWebHostBuilder and setting a path to the test project folder doesn’t help. Please help me how to approach this problem.

[pikoscielniak]

@mykola384 for me even better solution. I must only add the package Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation. Thank you.

[mykola384]

I fixed it by adding

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

But thanks for the suggestion!