单元测试中使用Moq对EF的DbSet进行mock

刚用上Moq，就用它解决了一个IUnitOfWork的mock问题，在这篇博文中记录一下。

开发场景

Application服务层BlogCategoryService的实现代码如下：

public class BlogCategoryService : IBlogCategoryService
{
    private IBlogCategoryRepository _blogCategoryRepository;

    public BlogCategoryServiceImp(IBlogCategoryRepository blogCategoryRepository)
    {
        _blogCategoryRepository = blogCategoryRepository;
    }

    public async Task<IList<BlogCategory>> GetCategoriesAsync(int blogId)
    {
        return await _blogCategoryRepository.GetCategories(blogId).ToListAsync();
    }
}

这里用到了Entity Framework中System.Data.Entity命名空间下的ToListAsync()扩展方法。

Repository层BlogCategoryRepository的实现代码如下：

public class BlogCategoryRepository : IBlogCategoryRepository
{
    private IQueryable<BlogCategory> _categories;

    public BlogCategoryRepository(IUnitOfWork unitOfWork)
    {
        _categories = unitOfWork.Set<BlogCategory>();
    }

    public IQueryable<BlogCategory> GetCategories(int blogId)
    {
        return _categories.Where(c => c.BlogId == blogId);
    }
}

这里在BlogCategoryRepository的构造函数中通过IUnitOfWork接口获取BlogCategory的数据集。

在单元测试中一开始是这样用Moq对IUnitOfWork接口进行mock的——让IUnitOfWork.Set()方法直接返回IQueryable类型的BlogCategory集合，代码如下：

[Fact]
public async Task GetCategoriesTest()
{
    var blogCategories = new List<BlogCategory>()
    {
        new BlogCategory {  BlogId = 1, Active = true, CategoryId = 1, Title = "C#" },
        new BlogCategory {  BlogId = 1, Active = false, CategoryId = 2, Title = "ASP.NET Core" }
    }.AsQueryable();

    var mockUnitOfWork = new Mock<IUnitOfWork>();
    mockUnitOfWork.Setup(u => u.Set<BlogCategory>()).Returns(blogCategories);

    _categoryService = new BlogCategoryServiceImp(new BlogCategoryRepository(mockUnitOfWork.Object));

    var actual = await _categoryService.GetCategoriesAsync(1);
    Assert.Equal(2, actual.Count());
    actual.ToList().ForEach(c => Assert.Equal(1, c.BlogId));
}

遇到问题

运行单元测试时，却出现下面的错误：

The source IQueryable doesn't implement IDbAsyncEnumerable<BlogCategory>. 
Only sources that implement IDbAsyncEnumerable can be used for Entity Framework asynchronous operations.

出现这个错误是由于在BlogCategoryService中用到了EF的ToListAsync()扩展方法，使用这个扩展方法需要实现IDbAsyncEnumerable相关接口，而通过List转换过来的IQueryable并没有实现这个接口。要解决这个问题，我们需要使用实现IDbAsyncEnumerable相关接口的集合类型，而EF中已经天然内置了这样的集合类型，它就是DbSet。只要能mock出DbSet，问题就迎刃而解。

解决问题

那如何mock呢？比想象中复杂得多，幸好在msdn网站上发现了现成的mock实现代码（详见 Testing with a mocking framework ），照此就可以轻松mock。
mock之前需要实现这三个接口：IDbAsyncEnumerator，IDbAsyncEnumerable，IDbAsyncQueryProvider 。
1）TestDbAsyncEnumerator 实现 IDbAsyncEnumerator

public class TestDbAsyncEnumerator<T> : IDbAsyncEnumerator<T>
{
    private readonly IEnumerator<T> _inner;

    public TestDbAsyncEnumerator(IEnumerator<T> inner)
    {
        _inner = inner;
    }

    public void Dispose()
    {
        _inner.Dispose();
    }

    public Task<bool> MoveNextAsync(CancellationToken cancellationToken)
    {
        return Task.FromResult(_inner.MoveNext());
    }

    public T Current
    {
        get { return _inner.Current; }
    }

    object IDbAsyncEnumerator.Current
    {
        get { return Current; }
    }
}

2）TestDbAsyncEnumerable 实现 IDbAsyncEnumerable

public class TestDbAsyncEnumerable<T> : EnumerableQuery<T>, IDbAsyncEnumerable<T>, IQueryable<T>
{
    public TestDbAsyncEnumerable(IEnumerable<T> enumerable)
        : base(enumerable)
    { }

    public TestDbAsyncEnumerable(Expression expression)
        : base(expression)
    { }

    public IDbAsyncEnumerator<T> GetAsyncEnumerator()
    {
        return new TestDbAsyncEnumerator<T>(this.AsEnumerable().GetEnumerator());
    }

    IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator()
    {
        return GetAsyncEnumerator();
    }

    IQueryProvider IQueryable.Provider
    {
        get { return new TestDbAsyncQueryProvider<T>(this); }
    }
}

3）TestDbAsyncQueryProvider 实现 IDbAsyncQueryProvider

public class TestDbAsyncQueryProvider<TEntity> : IDbAsyncQueryProvider
{
    private readonly IQueryProvider _inner;

    public TestDbAsyncQueryProvider(IQueryProvider inner)
    {
        _inner = inner;
    }

    public IQueryable CreateQuery(Expression expression)
    {
        return new TestDbAsyncEnumerable<TEntity>(expression);
    }

    public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
    {
        return new TestDbAsyncEnumerable<TElement>(expression);
    }

    public object Execute(Expression expression)
    {
        return _inner.Execute(expression);
    }

    public TResult Execute<TResult>(Expression expression)
    {
        return _inner.Execute<TResult>(expression);
    }

    public Task<object> ExecuteAsync(Expression expression, CancellationToken cancellationToken)
    {
        return Task.FromResult(Execute(expression));
    }

    public Task<TResult> ExecuteAsync<TResult>(Expression expression, CancellationToken cancellationToken)
    {
        return Task.FromResult(Execute<TResult>(expression));
    }
}

然后将之前的mock代码：

var mockUnitOfWork = new Mock<IUnitOfWork>();
mockUnitOfWork.Setup(u => u.Set<BlogCategory>()).Returns(blogCategories);

改为下面的代码：

#region mockSet
var mockSet = new Mock<DbSet<BlogCategory>>();
mockSet.As<IDbAsyncEnumerable<BlogCategory>>()
    .Setup(m => m.GetAsyncEnumerator())
    .Returns(new TestDbAsyncEnumerator<BlogCategory>(blogCategories.GetEnumerator()));

mockSet.As<IQueryable<BlogCategory>>()
    .Setup(m => m.Provider)
    .Returns(new TestDbAsyncQueryProvider<BlogCategory>(blogCategories.Provider));

mockSet.As<IQueryable<BlogCategory>>().Setup(m => m.Expression).Returns(blogCategories.Expression);
mockSet.As<IQueryable<BlogCategory>>().Setup(m => m.ElementType).Returns(blogCategories.ElementType);
mockSet.As<IQueryable<BlogCategory>>().Setup(m => m.GetEnumerator()).Returns(blogCategories.GetEnumerator());
#endregion

var mockUnitOfWork = new Mock<IUnitOfWork>();
mockUnitOfWork.Setup(u => u.Set<BlogCategory>()).Returns(mockSet.Object);

这样成功mock出DbSet之后，单元测试成功通过，问题就解决了。

1 passed, 0 failed, 0 skipped, took 2.75 seconds (xUnit.net 1.9.2 build 1705).