WebApplicationFactory: How to use .NET 6.0 WebApplication.CreateBuilder instead of older Host.CreateDefaultBuilder()?

Question

I have successfully converted my web applications to start using the new WebApplication.CreateBuilder(args) method, but I am not managing to replicate this success in my tests using a custom (derived) WebApplicationFactory.

I decided to keep a loose concept of a Startup class in my codebase (due to inheritance and what not), but I now call it manually as per https://mcmap.net/q/590861/-how-to-use-startup-class-inside-program-file-on-net-6.

My code used to work like this:

public class TestWebApiFactory<TStartup> : WebApplicationFactory<TStartup>
    where TStartup : class
{
    public TestWebApiFactory()
    {
    }

    protected override void ConfigureClient(HttpClient client)
    {
        base.ConfigureClient(client);
    }

    protected override IHostBuilder CreateHostBuilder()
    {
        var builder = Host.CreateDefaultBuilder()
                        .ConfigureAppConfiguration((context, config) =>
                        {
                            config.AddJsonFile("appsettings.json");                                    
                        })
                      .ConfigureWebHostDefaults(x =>
                      {
                          x.UseStartup<TStartup>().UseTestServer();
                      });
        return builder;
    }       
}

How do I achieve the same thing using the WebApplication model instead?

Unreflective

Just use the standard integration testing approach for minimal hosting model (do not confuse it with Minimal APIs - it is another concept) - for example as described in this answer. Startup in the new (minimal) hosting model is just an POCO class with no special meaning. – Ascribe

Does this answer your question? Integration test and hosting ASP.NET Core 6.0 without Startup class – Ascribe

Thanks @GuruStron, I did come across that link, but what I'm trying to understand is how can I create tests which call different WebApplicationFactory instances which each take a different TEntryPoint using this approach. I cannot create multiple "Program" classes due to the following error: "CS8802: Only one compilation unit can have top-level statements." This used to be easy before; all you had to do is use a different startup class as an entry point. – Unreflective

Why do you have multiple Startup classes at all (that is a bit pointless in the minimal hosting model)? How Program will select correct startup class to use? Use the same mechanism to make the WebApplicationFactory's (I assume something with configuration). – Ascribe

Also note that there is no need to migrate to the "new" minimal hosting model. You can keep the "old" generic one. – Ascribe

I figured I'd use multiple preset Startup classes instead of using the .WithWebHostBuilder method calls. I've now started using that method instead, and that solved it, so thanks! Now I have a new problem though, I am getting a "BadRequest" response on any request I send to a test controller I have in the test project. I am calling CreateClient() and then sending a request. What am I missing now? – Unreflective

Without full repro it is hard to tell. Can you post a minimal reproducible example somewhere (like github)? – Ascribe

I don't understand why you try to accomplish. I suggest you add in the question a test that use TestWebApiFactory as desired. – Strongroom

I found out that the "BadRequest" response is happening whenever I add the [ApiController] attribute to my controller. I am trying really hard to reproduce this behaviour in a new simple test project, but to no avail. There is something in my production code which is causing this, but can't for the life of me figure it out. (Note: This used to work without any issue before I switched to the minimal hosting model) – Unreflective