Unit-testing .ToListAsync() using an in-memory

Hats off to Bradford Dillon for providing the correct answer:

https://msdn.microsoft.com/en-us/library/dn314429(v=vs.113).aspx

The proper way to create unit tests for async methods of repositories is to first create these utility-mockup classes:

using System.Collections.Generic; 
using System.Data.Entity.Infrastructure; 
using System.Linq; 
using System.Linq.Expressions; 
using System.Threading; 
using System.Threading.Tasks; 
 
namespace TestingDemo 
{ 
    internal class TestDbAsyncQueryProvider<TEntity> : IDbAsyncQueryProvider 
    { 
        private readonly IQueryProvider _inner; 
 
        internal 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)); 
        } 
    } 
 
    internal 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); } 
        } 
    } 
 
    internal 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; } 
        } 
    } 
}

And then use them like so:

using Microsoft.VisualStudio.TestTools.UnitTesting; 
using Moq; 
using System.Collections.Generic; 
using System.Data.Entity; 
using System.Data.Entity.Infrastructure; 
using System.Linq; 
using System.Threading.Tasks; 
 
namespace TestingDemo 
{ 
    [TestClass] 
    public class AsyncQueryTests 
    { 
        [TestMethod] 
        public async Task GetAllBlogsAsync_orders_by_name() 
        { 
 
            var data = new List<Blog> 
            { 
                new Blog { Name = "BBB" }, 
                new Blog { Name = "ZZZ" }, 
                new Blog { Name = "AAA" }, 
            }.AsQueryable(); 
 
            var mockSet = new Mock<DbSet<Blog>>(); 
            mockSet.As<IDbAsyncEnumerable<Blog>>() 
                .Setup(m => m.GetAsyncEnumerator()) 
                .Returns(new TestDbAsyncEnumerator<Blog>(data.GetEnumerator())); 
 
            mockSet.As<IQueryable<Blog>>() 
                .Setup(m => m.Provider) 
                .Returns(new TestDbAsyncQueryProvider<Blog>(data.Provider)); 
 
            mockSet.As<IQueryable<Blog>>().Setup(m => m.Expression).Returns(data.Expression); 
            mockSet.As<IQueryable<Blog>>().Setup(m => m.ElementType).Returns(data.ElementType); 
            mockSet.As<IQueryable<Blog>>().Setup(m => m.GetEnumerator()).Returns(data.GetEnumerator()); 
 
            var mockContext = new Mock<BloggingContext>(); 
            mockContext.Setup(c => c.Blogs).Returns(mockSet.Object); 
 
            var service = new BlogService(mockContext.Object); 
            var blogs = await service.GetAllBlogsAsync(); 
 
            Assert.AreEqual(3, blogs.Count); 
            Assert.AreEqual("AAA", blogs[0].Name); 
            Assert.AreEqual("BBB", blogs[1].Name); 
            Assert.AreEqual("ZZZ", blogs[2].Name); 
        } 
    } 
}