[WIP] Decouple WebApplicationFactory from TestServer implementation

[mkArtakMSFT]

Background and Motivation

We have a long-standing popular issue tracking improvement of automated browser testing with real server. Part of the ask is to decouple the WebApplicationFactory from the TestServer implementation, as they're currently [tightly coupled] (

aspnetcore/src/Mvc/Mvc.Testing/src/WebApplicationFactory.cs

Line 29 in 2edcf3b

private TestServer? _server;

).

Unfortunately, the TestServer is also exposed in WebApplicationFactory via a public property Server. Hence, decoupling requires an API change proposed below.

The second change is exposing the initialization logic publicly. Without this change, customers had to call the CreateClient() API which would internally initialize the server but wouldn't use the returned HttpClient instance. This will make it more intuitive and avoid creating unnecessary objects. Here is a screenshot from a blogpost pointing out this odd usage pattern:

With the proposed changes, customers can now simply call server.Initialize() instead which is more intuitive.

Proposed API

New abstraction / interface to depend on instead of the TestServer implementation.

namespace Microsoft.AspNetCore.TestHost;

public interface ITestServer : IServer
{
    IWebHost Host { get; }
    HttpMessageHandler CreateHandler();
    HttpClient CreateClient();
}

- public class TestServer : IServer
+ public class TestServer : ITestServer

Usage / API change in WebApplicationFactory:

namespace Microsoft.AspNetCore.Mvc.Testing;

public partial class WebApplicationFactory<TEntryPoint> : IDisposable, IAsyncDisposable where TEntryPoint : class
{
    [Obsolete("This property is obsolete. Consider utilizing the TestServer property instead.")]
    public TestServer? Server { get; }

   +public ITestServer? TestServer { get; }

   - private void EnsureServer()
   + public void Initialize()

   - protected virtual TestServer CreateServer(IWebHostBuilder builder)
   + protected virtual ITestServer CreateServer(IWebHostBuilder builder)
}

Usage Examples

Alternative Designs

Risks