How to properly mock dependencies in unit tests using Moq for a .NET Core service?
I've been researching this but I'm performance testing and I've searched everywhere and can't find a clear answer. This might be a silly question, but I'm currently working on a .NET Core 3.1 web API and I've run into some issues when trying to write unit tests for my service layer... Specifically, I'm using Moq to mock dependencies, but I'm working with unexpected behavior when the mocks are involved. My service class depends on an `IRepository<T>` interface for data access. I created a test like this: ```csharp public class MyServiceTests { private readonly Mock<IRepository<MyEntity>> _mockRepository; private readonly MyService _service; public MyServiceTests() { _mockRepository = new Mock<IRepository<MyEntity>>(); _service = new MyService(_mockRepository.Object); } [Fact] public async Task GetEntityById_ReturnsCorrectEntity() { var testEntity = new MyEntity { Id = 1, Name = "Test" }; _mockRepository.Setup(repo => repo.GetByIdAsync(1)).ReturnsAsync(testEntity); var result = await _service.GetEntityById(1); Assert.NotNull(result); Assert.Equal(testEntity.Name, result.Name); } } ``` However, when I run this test, I receive a `System.NullReferenceException` at the line where I'm calling `GetEntityById` on my service. I suspect that the setup for mocking might not be working as expected. I even tried adding `Verify()` calls to ensure the repository method was hit, but that didn't help clarify things. I checked that the method in the service is correctly implemented: ```csharp public async Task<MyEntity> GetEntityById(int id) { return await _repository.GetByIdAsync(id); } ``` Could it be an scenario with how I'm setting up the mock, or is there something wrong with the way the service is designed? Any help with this would be greatly appreciated! I'm just looking for best practices when mocking dependencies in unit tests, especially in the context of .NET Core and Moq. For context: I'm using C# on Ubuntu. Any help would be greatly appreciated! What am I doing wrong? I'm coming from a different tech stack and learning C#. Could this be a known issue? What would be the recommended way to handle this?