WebApplication.Start suggestion

WebApplication.cs

public interface IWebApplication : IDisposable
{
    public Func<IDictionary<string, object>, Task> AppFunc { get; }
    
    public string Url { get; }
}

// Start an app listen = false so that it is pure in-proc for testing.
using(IWebApplication webApp = WebApplication.Start(
    builder => builder.UseThing(_foo),
    listen: false))
{
    var httpClient = new HttpClient(new OwinHttpMessageHandler(webApp.AppFunc));
    httpClient.GetAsync("http://doesntmatter.com/");
}

// Alternatively use the network stack
using(IWebApplication webApp = WebApplication.Start(
    builder => builder.UseThing(_foo),
    url: "http://localhost:1234/"))
{
    var httpClient = new HttpClient();
    httpClient.GetAsync(webApp.Url);
}

In the second sample, I assume you still need to pass new OwinHttpMessageHandler(webApp.AppFunc) to the HttpClient constructor?

Very nice, by the way!

Cheers :)

Yes, you would. This would be in your test setup, not normal client usage though.