When upgrading an ASP.NET Core application to .NET 9, I encountered the following error in my integration tests: System.InvalidOperationException: No application configured. Please specify an application via IWebHostBuilder.UseStartup, IWebHostBuilder.Configure, or specifying the startup assembly via StartupAssemblyKey in the web host configuration. I had updated my custom TestHost to switch from using a Startup.cs file to directly using the Program.cs file and the Minimal API approach. Therefore I added a partial Program.cs and updated the WebApplicationFactory class to use the Program.cs instead (more about this change in this post ). Here is the updated code: But this code didn’t work and resulted in the error message above. While giving the code a second look, I noticed that I was still referring to the Startup.cs that I didn't remove yet. I updated the code to use my Program.cs file instead: Doing that resulted in another error: A public method named ...