How to correctly mock a service in unit tests using Moq for an ASP.NET Core application?
I'm trying to unit test a service class in my ASP.NET Core application, but I'm running into issues with mocking dependencies using the Moq library. My service class depends on an external API client, which I've abstracted behind an interface, `IApiClient`. When I write my unit tests, I'm getting unexpected behavior, particularly an `ArgumentNullException` when I try to call a method on the mocked client. Here's a simplified version of my setup: ```csharp public interface IApiClient { Task<string> GetDataAsync(string id); } public class MyService { private readonly IApiClient _apiClient; public MyService(IApiClient apiClient) { _apiClient = apiClient ?? throw new ArgumentNullException(nameof(apiClient)); } public async Task<string> ProcessDataAsync(string id) { var data = await _apiClient.GetDataAsync(id); return data.ToUpper(); } } ``` In my unit test project, I'm using the following test method: ```csharp [TestClass] public class MyServiceTests { private Mock<IApiClient> _apiClientMock; private MyService _myService; [TestInitialize] public void Setup() { _apiClientMock = new Mock<IApiClient>(); _myService = new MyService(_apiClientMock.Object); } [TestMethod] public async Task ProcessDataAsync_ReturnsUpperCaseData() { // Arrange var id = "123"; _apiClientMock.Setup(m => m.GetDataAsync(id)).ReturnsAsync("some data"); // Act var result = await _myService.ProcessDataAsync(id); // Assert Assert.AreEqual("SOME DATA", result); } } ``` When I run this test, it fails with the error `ArgumentNullException: Value cannot be null. (Parameter 'apiClient')`. I've double-checked that I'm passing the mocked object correctly in the service constructor. I even tried initializing `_apiClientMock` in the test method itself instead of `Setup()`, but it didn't resolve the issue. Could you help me understand why I'm getting this error? Are there any best practices for mocking dependencies in unit tests that I might be overlooking? I'm using .NET Core 3.1 and Moq version 4.14.7. Any assistance would be greatly appreciated!