TestServer & ASP.NET Core 5: Fix “System.InvalidOperationException : Solution root could not be located using application root” with a custom Startup file

I am very happy to write this post today because I have had a lot of trouble over the last few days getting my integration tests with TestServer to work on ASP.NET Core 5. I usually use a dedicated Startup.cs file to my integration tests rather than using the web application’s one.Tthe simple good reason is that I do not like to replace instances of services or other configuration, I prefer a dedicated file for that. With each version of ASP.NET Core comes a change in the behavior of integration tests and in this post I will show you how to fix the external “System.InvalidOperationException : Solution root could not be located using application root” error when we use a Startup.cs file dedicated to our integration tests.