ASP.NET Core Web App/Api

If you want to test a web app, you can utilise the Microsoft.Mvc.Testing packages to wrap your web app within an in-memory test server.

public class WebAppFactory : WebApplicationFactory<Program>, IAsyncInitializer
{
    public Task InitializeAsync()
    {
        // You can also override certain services here to mock things out

        // Grab a reference to the server
        // This forces it to initialize.
        // By doing it within this method, it's thread safe.
        // And avoids multiple initialisations from different tests if parallelisation is switched on
        _ = Server;

        return Task.CompletedTask;
    }
}

This factory can then be injected into your tests, and whether you have one shared instance, or shared per class/assembly, or a new instance each time, is up to you!

The IAsyncInitializer.InitializeAsync method that you can see above will be called before your tests are invoked, so you know all the initialisation has already been done for you.

public class MyTests
{
    [ClassDataSource<WebAppFactory>(Shared = SharedType.PerTestSession)]
    public required WebAppFactory WebAppFactory { get; init; }
    
    [Test]
    public async Task Test1()
    {
        var client = WebAppFactory.CreateClient();

        var response = await client.GetAsync("/my/endpoint");
        
        await Assert.That(response.StatusCode).IsEqualTo(HttpStatusCode.OK);
    }
}

Alternatively, you can use the [NotInParallel] attribute to avoid parallelism and multi-initialisation. But you'll most likely be sacrificing test speeds if tests can't run in parallel.