How to mock an async repository with Entity Framework Core

I'm trying to create a unit test for a class that calls into an async repository. I'm using ASP.NET Core and Entity Framework Core. My generic repository looks like this.

public class EntityRepository<TEntity> : IEntityRepository<TEntity> where TEntity : class
{
    private readonly SaasDispatcherDbContext _dbContext;
    private readonly DbSet<TEntity> _dbSet;

    public EntityRepository(SaasDispatcherDbContext dbContext)
    {
        _dbContext = dbContext;
        _dbSet = dbContext.Set<TEntity>();
    }

    public virtual IQueryable<TEntity> GetAll()
    {
        return _dbSet;
    }

    public virtual async Task<TEntity> FindByIdAsync(int id)
    {
        return await _dbSet.FindAsync(id);
    }

    public virtual IQueryable<TEntity> FindBy(Expression<Func<TEntity, bool>> predicate)
    {
        return _dbSet.Where(predicate);
    }

    public virtual void Add(TEntity entity)
    {
        _dbSet.Add(entity);
    }
    public virtual void Delete(TEntity entity)
    {
        _dbSet.Remove(entity);
    }

    public virtual void Update(TEntity entity)
    {
        _dbContext.Entry(entity).State = EntityState.Modified;
    }

    public virtual async Task SaveChangesAsync()
    {
        await _dbContext.SaveChangesAsync();
    }
}

Then I have a service class that calls FindBy and FirstOrDefaultAsync on an instance of the repository:

    public async Task<Uri> GetCompanyProductURLAsync(Guid externalCompanyID, string productCode, Guid loginToken)
    {            
        CompanyProductUrl companyProductUrl = await _Repository.FindBy(u => u.Company.ExternalCompanyID == externalCompanyID && u.Product.Code == productCode.Trim()).FirstOrDefaultAsync();

        if (companyProductUrl == null)
        {
            return null;
        }

        var builder = new UriBuilder(companyProductUrl.Url);
        builder.Query = $"-s{loginToken.ToString()}";

        return builder.Uri;
    }

I'm trying to mock the repository call in my test below:

    [Fact]
    public async Task GetCompanyProductURLAsync_ReturnsNullForInvalidCompanyProduct()
    {
        var companyProducts = Enumerable.Empty<CompanyProductUrl>().AsQueryable();
        
        var mockRepository = new Mock<IEntityRepository<CompanyProductUrl>>();
        mockRepository.Setup(r => r.FindBy(It.IsAny<Expression<Func<CompanyProductUrl, bool>>>())).Returns(companyProducts);
        
        var service = new CompanyProductService(mockRepository.Object);

        var result = await service.GetCompanyProductURLAsync(Guid.NewGuid(), "wot", Guid.NewGuid());

        Assert.Null(result);
    }

However, when the test executes the call to the repository, I get the following error:

The provider for the source IQueryable doesn't implement IAsyncQueryProvider. Only providers that implement IEntityQueryProvider can be used for Entity Framework asynchronous operations.

How can I properly mock the repository to get this to work?

Thanks to @Nkosi for pointing me to a link with an example of doing the same thing in EF 6: https://msdn.microsoft.com/en-us/library/dn314429.aspx. This didn't work exactly as-is with EF Core, but I was able to start with it and make modifications to get it working. Below are the test classes that I created to "mock" IAsyncQueryProvider:

internal class TestAsyncQueryProvider<TEntity> : IAsyncQueryProvider
{
    private readonly IQueryProvider _inner;

    internal TestAsyncQueryProvider(IQueryProvider inner)
    {
        _inner = inner;
    }

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

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

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

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

    public IAsyncEnumerable<TResult> ExecuteAsync<TResult>(Expression expression)
    {
        return new TestAsyncEnumerable<TResult>(expression);
    }

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

internal class TestAsyncEnumerable<T> : EnumerableQuery<T>, IAsyncEnumerable<T>, IQueryable<T>
{
    public TestAsyncEnumerable(IEnumerable<T> enumerable)
        : base(enumerable)
    { }

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

    public IAsyncEnumerator<T> GetEnumerator()
    {
        return new TestAsyncEnumerator<T>(this.AsEnumerable().GetEnumerator());
    }

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

internal class TestAsyncEnumerator<T> : IAsyncEnumerator<T>
{
    private readonly IEnumerator<T> _inner;

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

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

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

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

And here is my updated test case that uses these classes:

[Fact]
public async Task GetCompanyProductURLAsync_ReturnsNullForInvalidCompanyProduct()
{
    var companyProducts = Enumerable.Empty<CompanyProductUrl>().AsQueryable();

    var mockSet = new Mock<DbSet<CompanyProductUrl>>();

    mockSet.As<IAsyncEnumerable<CompanyProductUrl>>()
        .Setup(m => m.GetEnumerator())
        .Returns(new TestAsyncEnumerator<CompanyProductUrl>(companyProducts.GetEnumerator()));

    mockSet.As<IQueryable<CompanyProductUrl>>()
        .Setup(m => m.Provider)
        .Returns(new TestAsyncQueryProvider<CompanyProductUrl>(companyProducts.Provider));

    mockSet.As<IQueryable<CompanyProductUrl>>().Setup(m => m.Expression).Returns(companyProducts.Expression);
    mockSet.As<IQueryable<CompanyProductUrl>>().Setup(m => m.ElementType).Returns(companyProducts.ElementType);
    mockSet.As<IQueryable<CompanyProductUrl>>().Setup(m => m.GetEnumerator()).Returns(() => companyProducts.GetEnumerator());

    var contextOptions = new DbContextOptions<SaasDispatcherDbContext>();
    var mockContext = new Mock<SaasDispatcherDbContext>(contextOptions);
    mockContext.Setup(c => c.Set<CompanyProductUrl>()).Returns(mockSet.Object);

    var entityRepository = new EntityRepository<CompanyProductUrl>(mockContext.Object);
        
    var service = new CompanyProductService(entityRepository);

    var result = await service.GetCompanyProductURLAsync(Guid.NewGuid(), "wot", Guid.NewGuid());

    Assert.Null(result);
}

Try to use my Moq/NSubstitute/FakeItEasy extension MockQueryable: supported all Sync/Async operations (see more examples here)

//1 - create a List<T> with test items
var users = new List<UserEntity>()
{
 new UserEntity,
 ...
};

//2 - build mock by extension
var mock = users.AsQueryable().BuildMock();

//3 - setup the mock as Queryable for Moq
_userRepository.Setup(x => x.GetQueryable()).Returns(mock.Object);

//3 - setup the mock as Queryable for NSubstitute
_userRepository.GetQueryable().Returns(mock);

DbSet also supported

//2 - build mock by extension
var mock = users.AsQueryable().BuildMockDbSet();

//3 - setup DbSet for Moq
var userRepository = new TestDbSetRepository(mock.Object);

//3 - setup DbSet for NSubstitute
var userRepository = new TestDbSetRepository(mock);

Notes:

• AutoMapper is also supported from 1.0.4 ver
• DbQuery supported from 1.1.0 ver
• EF Core 3.0 supported from 3.0.0 ver
• .Net 5 supported from 5.0.0 ver

Much less code solution. Use the in-memory db context which should take care of bootstrapping all the sets for you. You no longer need to mock out the DbSet on your context but if you want to return data from a service for example, you can simply return the actual set data of the in-memory context.

DbContextOptions< SaasDispatcherDbContext > options = new DbContextOptionsBuilder< SaasDispatcherDbContext >()
  .UseInMemoryDatabase(Guid.NewGuid().ToString())
  .Options;
        
  _db = new SaasDispatcherDbContext(optionsBuilder: options);

I'm maintaining two open-source projects that do the heavy lifting of setting up the mocks and actually emulates SaveChanges(Async).

For EF Core: https://github.com/huysentruitw/entity-framework-core-mock

For EF6: https://github.com/huysentruitw/entity-framework-mock

Both projects have Nuget packages with integration for Moq or NSubstitute.

Here is a port of the accepted answer to F#, I just did it for myself and thought it may save someone the time. I have also updated the example to match the updated C#8 IAsyncEnumarable API and tweaked the Mock setup to be generic.

    type TestAsyncEnumerator<'T> (inner : IEnumerator<'T> ) =     
        
        let inner : IEnumerator<'T> = inner
        
        interface IAsyncEnumerator<'T> with
            member this.Current with get() = inner.Current
            member this.MoveNextAsync () = ValueTask<bool>(Task.FromResult(inner.MoveNext()))
            member this.DisposeAsync () = ValueTask(Task.FromResult(inner.Dispose))

    type TestAsyncEnumerable<'T> =       
        inherit EnumerableQuery<'T>

        new (enumerable : IEnumerable<'T>) = 
            { inherit EnumerableQuery<'T> (enumerable) }
        new (expression : Expression) = 
            { inherit EnumerableQuery<'T> (expression) }
        
        interface IAsyncEnumerable<'T> with
            member this.GetAsyncEnumerator cancellationToken : IAsyncEnumerator<'T> =
                 new TestAsyncEnumerator<'T>(this.AsEnumerable().GetEnumerator())
                 :> IAsyncEnumerator<'T>

        interface IQueryable<'T> with
            member this.Provider with get() = new TestAsyncQueryProvider<'T>(this) :> IQueryProvider

    and 
        TestAsyncQueryProvider<'TEntity> 
        (inner : IQueryProvider) =       
        
        let inner : IQueryProvider = inner

        interface IAsyncQueryProvider with
                
            member this.Execute (expression : Expression) =
                inner.Execute expression

            member this.Execute<'TResult> (expression : Expression) =
                inner.Execute<'TResult> expression

            member this.ExecuteAsync<'TResult> ((expression : Expression), cancellationToken) =
                inner.Execute<'TResult> expression

            member this.CreateQuery (expression : Expression) =
                new TestAsyncEnumerable<'TEntity>(expression) :> IQueryable

            member this.CreateQuery<'TElement> (expression : Expression) =
                new TestAsyncEnumerable<'TElement>(expression) :> IQueryable<'TElement>


    let getQueryableMockDbSet<'T when 'T : not struct>
        (sourceList : 'T seq) : Mock<DbSet<'T>> =

        let queryable = sourceList.AsQueryable();

        let dbSet = new Mock<DbSet<'T>>()

        dbSet.As<IAsyncEnumerable<'T>>()
            .Setup(fun m -> m.GetAsyncEnumerator())
            .Returns(TestAsyncEnumerator<'T>(queryable.GetEnumerator())) |> ignore

        dbSet.As<IQueryable<'T>>()
            .SetupGet(fun m -> m.Provider)
            .Returns(TestAsyncQueryProvider<'T>(queryable.Provider)) |> ignore

        dbSet.As<IQueryable<'T>>().Setup(fun m -> m.Expression).Returns(queryable.Expression) |> ignore
        dbSet.As<IQueryable<'T>>().Setup(fun m -> m.ElementType).Returns(queryable.ElementType) |> ignore
        dbSet.As<IQueryable<'T>>().Setup(fun m -> m.GetEnumerator ()).Returns(queryable.GetEnumerator ()) |> ignore
        dbSet

A way simpler approach is to write your own ToListAsync in one of the core layers. You dont need any concrete class implementation. Something like:

    public static async Task<List<T>> ToListAsync<T>(this IQueryable<T> queryable)
    {
        if (queryable is EnumerableQuery)
        {
            return queryable.ToList();
        }

        return await QueryableExtensions.ToListAsync(queryable);
    }

This also has the added benefit that you could use ToListAsync from anywhere in your app without needing to drag EF references all along.

Leveraging @Jed Veatch's accepted answer, as well as the comments provided by @Mandelbrotter, the following solution works for .NET Core 3.1 and .NET 5. This will resolve the "Argument expression is not valid" exception that arises from working with the above code in later .NET versions.

TL;DR - Complete EnumerableExtensions.cs code is here.

Usage:

public static DbSet<T> GetQueryableAsyncMockDbSet<T>(List<T> sourceList) where T : class
{
    var mockAsyncDbSet = sourceList.ToAsyncDbSetMock<T>();
    var queryable = sourceList.AsQueryable();
    mockAsyncDbSet.As<IQueryable<T>>().Setup(m => m.GetEnumerator()).Returns(() => queryable.GetEnumerator());
    mockAsyncDbSet.Setup(d => d.Add(It.IsAny<T>())).Callback<T>((s) => sourceList.Add(s));
    return mockAsyncDbSet.Object;
}

Then, using Moq and Autofixture, you can do:

var myMockData = Fixture.CreateMany<MyMockEntity>();
MyDatabaseContext.SetupGet(x => x.MyDBSet).Returns(GetQueryableAsyncMockDbSet(myMockData));

For everyone who stuck at mocking DbContext with async queries, IAsyncQueryProvider and other things. Heres example usage of copy-paste types for netcore3.1 and higher. Based on generic DbContextCreation and generic DbSet seed.

    public class MyDbContext : DbContext
    {
        public DbSet<MyEntity> MyEntities { get; set; }
    }

    public class MyEntity
    {
        public Guid Id { get; set; }
    }

    internal class MockDbContextAsynced<TDbContext>
    {
        private readonly TDbContext _mock;
        public TDbContext Object => _mock;

        public MockDbContextAsynced()
        {
            _mock = Activator.CreateInstance<TDbContext>();
        }
          // suppressed. see full code in source below
    }

    [Fact]
    public void Test()
    {
        var testData = new List<MyEntity>
        {
            new MyEntity() { Id = Guid.NewGuid() },
            new MyEntity() { Id = Guid.NewGuid() },
            new MyEntity() { Id = Guid.NewGuid() },
        };

        var mockDbContext = new MockDbContextAsynced<MyDbContext>();
        mockDbContext.AddDbSetData<MyEntity>(testData.AsQueryable());

        mockDbContext.MyEntities.ToArrayAsync();
        // or
        mockDbContext.MyEntities.SingleAsync();
        // or etc.
        
        // To inject MyDbContext as type parameter with mocked data
        var mockService = new SomeService(mockDbContext.Object);
    }

For full implemented types see this source: https://gist.github.com/Zefirrat/a04658c827ba3ebffe03fda48d53ea11