Unit Testing FindAsync
asp.net, c#, entity-framework, moq, unit-testing
Solution
`NullReferenceException` inside a `MoveNext` for an `async` method is almost always due to returning `null` from another async method.
In this case, it looks like `FindAsync` is returning `null`, which makes sense since I don't see where you're mocking it. You're currently mocking the `IQueryable` and `GetAsyncEnumerator` aspects, but not `FindAsync`. The example article you posted does not provide a complete `DbSet` mocking solution.
Problem
I've set up a test project using the async query provider found in this excellent MSDN article: http://msdn.microsoft.com/en-US/data/dn314429#async which works great. However when I add a method which calls `FindAsync`: ``` public async Task<Blog> GetBlog(int blogId) { return await _context.Blogs.FindAsync(blogId); } ``` And add the following unit test in the format: ``` [TestMethod] public async Task GetAllBlogsAsync_gets_blog() { var data = new List<Blog> { new Blog { BlogId = 1, Name = "BBB" }, new Blog { BlogId = 2, Name = "ZZZ" }, new Blog { BlogId = 3, 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 blog = await service.GetBlog(2); Assert.AreEqual("ZZZ", blog.Name); } ``` However when `GetBlog` is called from my test method, `await _context.Blogs.FindAsync(blogId);` throws a `NullReferenceException` at `TestingDemo.BlogService.<GetBlog>d__5.MoveNext()` Any suggestions how I can implement unit tests on methods calling `FindAsync` using the testing methodology found in the MSDN article: http://msdn.microsoft.com/en-US/data/dn314429#async ?