Testing EF async methods with sync methods with MOQ

Question

I have this method:

    public async Task DeleteUserAsync(Guid userId)
    {
        using (var context = this.contextFactory.Create())
        {
            var user = await context.Users.FirstOrDefaultAsync(x => x.Id.Equals(userId));

            if (user == null)
            {
                throw new Exception("User doesn't exist");
            }

            context.Users.Remove(user);

            await context.SaveChangesAsync();
        }
    }

I want to test it out. So I create the test:

    [TestMethod]
    public async Task DeleteUsersSuccessfulCallTest()
    {
        // Arrange
        var id = Guid.NewGuid();
        var user = new User() { Id = id };

        var context = new Mock<IDashboardContext>();
        var usersDbSet = DbSetQueryMocking.GenericSetupAsyncQueryableMockInterfaceSet(new List<User> { user }.AsQueryable());
        context.Setup(x => x.Users).Returns(usersDbSet.Object);

        context.Setup(x => x.Users.Remove(user)).Returns(user).Verifiable();
        context.Setup(x => x.SaveChangesAsync()).ReturnsAsync(1).Verifiable();

        this.contextFactory.Setup(x => x.Create()).Returns(context.Object);

        // Act
        await this.userService.DeleteUserAsync(id);

        context.VerifyAll();
    }
}

I have got this method to create me a mock set:

    public static Mock<DbSet<T>> GenericSetupAsyncQueryableMockSet<T>(IQueryable<T> data) where T : class
    {
        var mockSet = new Mock<DbSet<T>>();
        mockSet.As<IDbAsyncEnumerable<T>>().Setup(m => m.GetAsyncEnumerator()).Returns(new TestDbAsyncEnumerator<T>(data.GetEnumerator()));
        mockSet.As<IQueryable<T>>().Setup(m => m.Provider).Returns(new TestDbAsyncQueryProvider<T>(data.Provider));
        mockSet.As<IQueryable<T>>().Setup(m => m.Expression).Returns(data.Expression);
        mockSet.As<IQueryable<T>>().Setup(m => m.ElementType).Returns(data.ElementType);
        mockSet.As<IQueryable<T>>().Setup(m => m.GetEnumerator()).Returns(data.GetEnumerator());

        return mockSet;
    }

However, because my DeleteUserAsync contains async extension methods and standard sync methods I get this error message:

System.InvalidOperationException: The provider for the source IQueryable doesn't implement IDbAsyncQueryProvider. Only providers that implement IDbAsyncQueryProvider can be used for Entity Framework asynchronous operations. For more details see http://go.microsoft.com/fwlink/?LinkId=287068.

Obviously if I just set up the DbSet<T> with Queryable mocked out then it will throw the same exception.

FYI: the offending line is:

context.Setup(x => x.Users.Remove(user)).Returns(user).Verifiable();

With this line: errors

Without it: a successful test.

How do I fix this?

Answer 1

The EnumerableQuery<T> class which is produced by .AsQueryable() does not implement IDbAsyncQueryProvider but it's easy to extend EnumerableQuery<T> with the implementation. Create one of these instead of calling .AsQueryable() to wrap your collection. I have an implementation below that extends it further into a IDbSet<T> but you may not need to go that far.

class StubSet<T> : EnumerableQuery<T>, IDbSet<T>, IDbAsyncQueryProvider
    where T : class
{
    public StubSet(IEnumerable<T> collection) : base(collection)
    {
        Local = new ObservableCollection<T>(collection);
    }

    public ObservableCollection<T> Local { get; private set; }

    public T Find(params object[] keyValues)
    {
        throw new NotImplementedException();
    }

    public T Add(T entity)
    {
        Local.Add(entity);
        return entity;
    }

    public T Remove(T entity)
    {
        Local.Remove(entity);
        return entity;
    }

    public T Attach(T entity)
    {
        return Add(entity);
    }

    public T Create()
    {
        throw new NotImplementedException();
    }

    public TDerivedEntity Create<TDerivedEntity>() where TDerivedEntity : class, T
    {
        throw new NotImplementedException();
    }

    public void DeleteObject(T entity)
    {
        throw new NotImplementedException();
    }

    public void Detach(T entity)
    {
        throw new NotImplementedException();
    }        

    async Task<object> IDbAsyncQueryProvider.ExecuteAsync(Expression expression, CancellationToken cancellationToken)
    {
        return ((IQueryProvider)this).Execute(expression);
    }

    async Task<TResult> IDbAsyncQueryProvider.ExecuteAsync<TResult>(Expression expression, CancellationToken cancellationToken)
    {
        return ((IQueryProvider)this).Execute<TResult>(expression);
    }
}