Full TestServer and InMemory DB Test

ApplicationUsersController.cs

public class ApplicationUsersControllerGetApplicationUser
{
    private readonly ApplicationDbContext _context;
    private readonly HttpClient _client;

    public ApplicationUsersControllerGetApplicationUser()
    {
        var builder = new WebHostBuilder()
            .UseEnvironment("Testing")
            .UseStartup<Startup>();

        var server = new TestServer(builder);
        _context = server.Host.Services.GetService(typeof(ApplicationDbContext)) as ApplicationDbContext;
        _client = server.CreateClient();
    }

    [Fact]
    public async Task DoesReturnNotFound_GivenUserDoesNotExist()
    {
        // Act
        var response = await _client.GetAsync($"/api/ApplicationUsers/abc"); // No users with ID abc

        // Assert
        Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);
    }

    [Fact]
    public async Task DoesReturnOk_GivenUserExists()
    {
        // Arrange
        var user = new ApplicationUser
        {
            Id = "123",
            Email = "test@test.com"
        };

        _context.Users.Add(user);
        _context.SaveChanges();

        // Act
        var response = await _client.GetAsync($"/api/ApplicationUsers/{user.Id}");

        // Assert
        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
        string jsonResult = await response.Content.ReadAsStringAsync();
        ApplicationUser userFromJson = JsonConvert.DeserializeObject<ApplicationUser>(jsonResult);
        Assert.Equal(user.Id, userFromJson.Id);
        Assert.Equal(user.Email, userFromJson.Email);
    }
}