Handling Null Reference handling in ASP.NET Core with Dependency Injection
I'm trying to configure I'm currently developing an ASP.NET Core 6.0 application and I'm running into a `NullReferenceException` when trying to access a service that I injected via the constructor of my controller. The behavior occurs only when I try to access certain endpoints, and I'm not sure if it's related to how I'm configuring my services or if there's something wrong with my dependency injection setup. Here's the relevant part of my controller: ```csharp public class UserController : ControllerBase { private readonly IUserService _userService; public UserController(IUserService userService) { _userService = userService ?? throw new ArgumentNullException(nameof(userService)); } [HttpGet("/{id}")] public async Task<IActionResult> GetUser(int id) { var user = await _userService.GetUserByIdAsync(id); return user == null ? NotFound() : Ok(user); } } ``` My `IUserService` implementation is registered in `Startup.cs` like this: ```csharp public void ConfigureServices(IServiceCollection services) { services.AddControllers(); services.AddScoped<IUserService, UserService>(); } ``` When I test the `GetUser` endpoint with a valid ID, I receive a `NullReferenceException` on the line where I call `_userService.GetUserByIdAsync(id)`. I've checked and the service is correctly registered and should not be null at that point. To debug, I inserted breakpoints and noticed that the `userService` parameter in the constructor is indeed not null, but I still get this exception. I've also looked at the lifetime of the service; since I'm using `AddScoped`, it should be created per request. I suspect there might be a race condition or an scenario with how the DI container is resolving the service in certain scenarios. Has anyone encountered this before or can you suggest how to properly debug this scenario? Any insights on best practices for setting up DI in ASP.NET Core would also be appreciated. This is part of a larger service I'm building. What am I doing wrong?