How to correctly mock a service in unit tests using Moq for an ASP.NET Core application?
I'm wondering if anyone has experience with After trying multiple solutions online, I still can't figure this out. I'm building a feature where I'm attempting to set up Does anyone know how to 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 behavior `ArgumentNullException: Value want to 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 scenario. Could you guide to understand why I'm getting this behavior? 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! Is there a simpler solution I'm overlooking? My development environment is CentOS. For context: I'm using Csharp on Ubuntu 20.04. Any suggestions would be helpful. I recently upgraded to Csharp LTS. Thanks for your help in advance!