如何在NUnit中使用Moq正确填充Mock<DbSet>以测试仓储删除方法

我是NUnit测试新手，尝试测试仓储中的DeleteQuestion方法，但填充Mock<DbSet>测试实体时不断抛出异常。

待测试的DeleteQuestion方法代码 #

public async Task DeleteQuestion(Question questionToRemove)
{
    if (questionToRemove is not null)
    {
        var questionsAnswers = await _context.Answers.Where(a => a.Question == questionToRemove).ToListAsync();
        foreach (Answer a in questionsAnswers)
        {
            _context.Answers.Remove(a);
            await _context.SaveChangesAsync();
        }

        _context.Questions.Remove(questionToRemove);
        await _context.SaveChangesAsync();                
        var questions = await _context.Questions.Where(q => q.Quiz == questionToRemove.Quiz).ToListAsync();
        int order = 1;
        foreach (Question question in questions)
        {
            question.QuestionOrderId = order;
            await _context.SaveChangesAsync();
            order++;
        }
    }
}

我的测试代码 #

public async Task DeleteQuestionFunction()
{
    //Arrange
    Question testQuestion = new Question { QuestionId = 1, QuestionText = "Test" };
    Answer testAnswer = new Answer { Question = testQuestion, AnswerId = 1, AnswerText = "TestAnswer" };
    _appDBContextMock.SetupAdd(x => x.Questions.Add(testQuestion));
    _appDBContextMock.SetupAdd(x => x.Answers.Add(testAnswer));

    //Act
    var questionListcomponent = new QuestionListComponent();
    questionListcomponent.DeleteQuestion(testQuestion);

    var testresult = await _questionRepositoryMock.Object.GetQuestionById(testQuestion.QuestionId);

    //Assert
    Assert.That(testresult, Is.Null);
}

遇到的异常 #

1. 使用Setup方法时抛出：

System.NotSupportedException : Unsupported expression: x =>
x.Questions Non-overridable members (here: AppDbContext.get_Questions)
may not be used in setup / verification expressions.

2. 使用SetupAdd方法时抛出：

System.ArgumentException : Can not instantiate proxy of class:
TietoQuiz.Models.DbContexts.AppDbContext. Could not find a
parameterless constructor. (Parameter 'constructorArguments')

我认为问题在于不知道如何正确设置Mock<DbSet>，搜索到的方案杂乱且褒贬不一，作为新手十分困惑，希望获得帮助。

核心问题分析 #

1. 非可重写成员报错：直接MockAppDbContext的Questions/Answers属性时，EF Core的DbSet<T>默认是类属性，并非虚成员，Moq无法拦截非虚成员的调用，因此抛出异常。
2. 无参构造函数报错：AppDbContext没有无参构造函数，Moq无法直接创建它的代理对象，导致SetupAdd调用失败。

正确设置Mock的步骤 #

1. 准备测试数据并封装成可查询的Mock DbSet

先将测试实体转为IQueryable，再用Moq创建Mock<DbSet<T>>，并配置它支持LINQ查询和异步操作：

// 创建测试数据列表
var testQuestions = new List<Question>
{
    new Question { QuestionId = 1, QuestionText = "Test", Quiz = new Quiz() } // 确保Quiz不为null，方法中会用到
};

var testAnswers = new List<Answer>
{
    new Answer { Question = testQuestions[0], AnswerId = 1, AnswerText = "TestAnswer" }
};

// 转为IQueryable
var questionsQueryable = testQuestions.AsQueryable();
var answersQueryable = testAnswers.AsQueryable();

// 创建Mock DbSet<Question>并配置IQueryable支持
var mockQuestionsDbSet = new Mock<DbSet<Question>>();
mockQuestionsDbSet.As<IQueryable<Question>>().Setup(m => m.Provider).Returns(questionsQueryable.Provider);
mockQuestionsDbSet.As<IQueryable<Question>>().Setup(m => m.Expression).Returns(questionsQueryable.Expression);
mockQuestionsDbSet.As<IQueryable<Question>>().Setup(m => m.ElementType).Returns(questionsQueryable.ElementType);
mockQuestionsDbSet.As<IQueryable<Question>>().Setup(m => m.GetEnumerator()).Returns(questionsQueryable.GetEnumerator());

// 配置异步ToListAsync支持
mockQuestionsDbSet.As<IAsyncEnumerable<Question>>()
    .Setup(m => m.GetAsyncEnumerator(It.IsAny<CancellationToken>()))
    .Returns(new AsyncEnumerator<Question>(questionsQueryable.GetEnumerator()));

// 同理创建并配置Mock DbSet<Answer>
var mockAnswersDbSet = new Mock<DbSet<Answer>>();
mockAnswersDbSet.As<IQueryable<Answer>>().Setup(m => m.Provider).Returns(answersQueryable.Provider);
mockAnswersDbSet.As<IQueryable<Answer>>().Setup(m => m.Expression).Returns(answersQueryable.Expression);
mockAnswersDbSet.As<IQueryable<Answer>>().Setup(m => m.ElementType).Returns(answersQueryable.ElementType);
mockAnswersDbSet.As<IQueryable<Answer>>().Setup(m => m.GetEnumerator()).Returns(answersQueryable.GetEnumerator());

mockAnswersDbSet.As<IAsyncEnumerable<Answer>>()
    .Setup(m => m.GetAsyncEnumerator(It.IsAny<CancellationToken>()))
    .Returns(new AsyncEnumerator<Answer>(answersQueryable.GetEnumerator()));

需要添加以下辅助类支持异步枚举：

public class AsyncEnumerator<T> : IAsyncEnumerator<T>
{
    private readonly IEnumerator<T> _enumerator;

    public AsyncEnumerator(IEnumerator<T> enumerator)
    {
        _enumerator = enumerator ?? throw new ArgumentNullException(nameof(enumerator));
    }

    public ValueTask DisposeAsync() => ValueTask.CompletedTask;

    public ValueTask<bool> MoveNextAsync() => new ValueTask<bool>(_enumerator.MoveNext());

    public T Current => _enumerator.Current;
}

2. 正确Mock DbContext

由于AppDbContext没有无参构造函数，Mock时需要传入它所需的构造参数（比如DbContextOptions<AppDbContext>）：

// 创建空的DbContextOptions
var options = new DbContextOptionsBuilder<AppDbContext>().Options;
var _appDBContextMock = new Mock<AppDbContext>(options);

// 配置Context的属性返回Mock的DbSet
_appDBContextMock.Setup(c => c.Questions).Returns(mockQuestionsDbSet.Object);
_appDBContextMock.Setup(c => c.Answers).Returns(mockAnswersDbSet.Object);

// Mock SaveChangesAsync，方法中多次调用该方法
_appDBContextMock.Setup(c => c.SaveChangesAsync(It.IsAny<CancellationToken>()))
    .ReturnsAsync(1); // 返回1表示保存成功

3. 修正测试代码

调整测试方法，确保依赖注入正确，异步方法调用加await：

public async Task DeleteQuestionFunction()
{
    //Arrange
    // 将上述测试数据、Mock DbSet、DbContext的代码放置此处

    // 注入Mock的DbContext到组件中（替换为实际构造注入方式）
    var questionListcomponent = new QuestionListComponent(_appDBContextMock.Object);

    //Act
    await questionListcomponent.DeleteQuestion(testQuestions[0]); // 异步方法必须加await

    //Assert
    // 验证Questions的Remove方法被调用一次
    mockQuestionsDbSet.Verify(m => m.Remove(testQuestions[0]), Times.Once);
    // 验证Answers的Remove方法被调用一次
    mockAnswersDbSet.Verify(m => m.Remove(testAnswers[0]), Times.Once);
    // 验证SaveChangesAsync的调用次数（删除Answer1次+删除Question1次+更新Question1次，共3次）
    _appDBContextMock.Verify(c => c.SaveChangesAsync(It.IsAny<CancellationToken>()), Times.Exactly(3));
    // 验证QuestionOrderId被正确更新
    Assert.That(testQuestions[0].QuestionOrderId, Is.EqualTo(1));
}

额外建议 #

1. 将DbContext的DbSet属性改为虚属性，简化Moq拦截：

public class AppDbContext : DbContext
{
    public virtual DbSet<Question> Questions { get; set; }
    public virtual DbSet<Answer> Answers { get; set; }
    // ...其他代码
}

2. 可以使用EntityFrameworkCore.Testing.Moq第三方库，无需手动编写异步枚举辅助类，快速完成DbSet的Mock。

内容的提问来源于stack exchange，提问作者Yumaisch