C#中使用Moq模拟MediatR进行集成测试返回null问题排查

你的测试返回null且不符合预期的核心原因有两个：

1. Moq未正确匹配MediatR的泛型Send方法 #

MediatR的IMediator.Send是泛型方法，定义为：

Task<TResponse> Send<TResponse>(IRequest<TResponse> request, CancellationToken cancellationToken = default);

你当前的Setup未指定泛型参数，Moq无法识别你要模拟的是返回Result<Unit>的Send调用，实际调用时未触发模拟逻辑，因此返回null。

2. 测试预期对象未正确初始化 #

你用new Result<Unit>()创建的预期对象未设置Error属性值，即便模拟成功，断言也会失败。应使用Result<Unit>.Failure()静态方法创建符合预期的失败结果。

修复后的测试代码 #

[TestFixture]
public class UserControllerTests
{
    private Mock<IMediator> _mockMediator;
    private UserController _userController;

    [SetUp] // 改用SetUp初始化，符合NUnit规范
    public void Setup()
    {
        _mockMediator = new Mock<IMediator>();
        _userController = new UserController(_mockMediator.Object);
    }

    [Test]
    public async Task DeleteUser_NotSuccessful_NoIdDeleted()
    {
        // 用Failure方法创建预期的失败结果
        var expected = Result<Unit>.Failure("User Id doesn't exsist");
        
        // 指定泛型参数，精准匹配目标Send调用
        _mockMediator.Setup(x => x.Send<Result<Unit>>(
                It.Is<DeleteUserCommand>(cmd => cmd.UserId == 6),
                It.IsAny<CancellationToken>()))
            .ReturnsAsync(expected); // 替代Task.FromResult，代码更简洁

        var result = await _userController.DeleteUser(6);

        Assert.AreEqual("User Id doesn't exsist", result.Value.Error);
        Assert.IsFalse(result.Value.IsSuccess);
    }

    [Test]
    public async Task DeleteUser_Successful_UserIdDeleted()
    {
        var expected = Result<Unit>.Success(Unit.Value);
        
        _mockMediator.Setup(x => x.Send<Result<Unit>>(
                It.Is<DeleteUserCommand>(cmd => cmd.UserId == 7),
                It.IsAny<CancellationToken>()))
            .ReturnsAsync(expected);

        var result = await _userController.DeleteUser(7);

        Assert.IsTrue(result.Value.IsSuccess);
        Assert.AreEqual(Unit.Value, result.Value.Value);
    }
}

额外优化建议 #

• 用[SetUp]替代构造函数初始化测试依赖，更符合测试框架规范且灵活
• 对DeleteUserCommand的UserId做精准匹配，避免测试逻辑被其他ID的调用干扰
• 使用ReturnsAsync简化异步返回值的模拟

内容的提问来源于stack exchange，提问作者Sean Elias