NSubstitute DbSet / IQueryable<T>

Question

So EntityFramework 6 is a lot better testable then previous versions. And there are some nice examples on the internet for frameworks like Moq, but the case is, I prefer using NSubstitute. I've got the "non-query" examples translated to work with the use of NSubstitute, but I can't get my head around the 'query-test'.

How does Moq's items.As<IQueryable<T>>().Setup(m => m.Provider).Returns(data.Provider); translate to NSubstitute? I thought something like ((IQueryable<T>) items).Provider.Returns(data.Provider); but that didn't work. I've also tried items.AsQueryable().Provider.Returns(data.Provider); but that didn't work either.

The exeption I'm getting is:

"System.NotImplementedException : The member 'IQueryable.Provider' has not been implemented on type 'DbSet1Proxy' which inherits from 'DbSet1'. Test doubles for 'DbSet`1' must provide implementations of methods and properties that are used."

So let me quote the code example from the link above. This code sample uses Moq to mock the DbContext and DbSet.

public void GetAllBlogs_orders_by_name()
{
  // Arrange
  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<IQueryable<Blog>>().Setup(m => m.Provider).Returns(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);

  // ...
}

And this is how far I come with NSubstitute

public void GetAllBlogs_orders_by_name()
{
  // Arrange
  var data = new List<Blog>
  {
     new Blog { Name = "BBB" },
     new Blog { Name = "ZZZ" },
     new Blog { Name = "AAA" },
  }.AsQueryable();

  var mockSet = Substitute.For<DbSet<Blog>>();
  // it's the next four lines I don't get to work
  ((IQueryable<Blog>) mockSet).Provider.Returns(data.Provider);
  ((IQueryable<Blog>) mockSet).Expression.Returns(data.Expression);
  ((IQueryable<Blog>) mockSet).ElementType.Returns(data.ElementType);
  ((IQueryable<Blog>) mockSet).GetEnumerator().Returns(data.GetEnumerator());

  var mockContext = Substitute.For<BloggingContext>();
  mockContext.Blogs.Returns(mockSet);

  // ...
}

So the question is; How does one Substitute a property of IQueryable (like Provider)?

Answer 1

This happens because of NSubstitute syntax specific. For example in:

((IQueryable<Blog>) mockSet).Provider.Returns(data.Provider);

NSubstitute calls the Provider's getter, then it specifies the return value. This getter call isn't intercepted by the substitute and you get an exception. It happens because of explicit implementation of IQueryable.Provider property in DbQuery class.

You can explicitly create substitutes for multiple interfaces with NSub, and it creates a proxy which covers all specified interfaces. Then calls to the interfaces will be intercepted by the substitute. Please use the following syntax:

// Create a substitute for DbSet and IQueryable types:
var mockSet = Substitute.For<DbSet<Blog>, IQueryable<Blog>>();
    
// And then as you do:
((IQueryable<Blog>) mockSet).Provider.Returns(data.Provider);
((IQueryable<Blog>) mockSet).Expression.Returns(data.Expression);
((IQueryable<Blog>) mockSet).ElementType.Returns(data.ElementType);
((IQueryable<Blog>) mockSet).GetEnumerator().Returns(data.GetEnumerator());

Answer 2

Thanks to Kevin, I've found the problem in my code translation.

The unittest code samples are mocking DbSet, but NSubstitute requires the interface implementation. So the equivalent of Moqs new Mock<DbSet<Blog>>() for NSubstitute is Substitute.For<IDbSet<Blog>>(). You're not always required to provide the Interface, so that's why I was confused. But in this specific case, it turned out to be crucial.

It also turned out that we don't have to cast to Queryable when using the interface IDbSet.

So the working test code:

public void GetAllBlogs_orders_by_name()
{
  // Arrange
  var data = new List<Blog>
  {
    new Blog { Name = "BBB" },
    new Blog { Name = "ZZZ" },
    new Blog { Name = "AAA" },
  }.AsQueryable();

  var mockSet = Substitute.For<IDbSet<Blog>>();
  mockSet.Provider.Returns(data.Provider);
  mockSet.Expression.Returns(data.Expression);
  mockSet.ElementType.Returns(data.ElementType);
  mockSet.GetEnumerator().Returns(data.GetEnumerator());

  var mockContext = Substitute.For<BloggingContext>();
  mockContext.Blogs.Returns(mockSet);

  // Act and Assert ...
}

I've written a small extention method to cleanup the Arrange section of the unit tests.

public static class ExtentionMethods
{
    public static IDbSet<T> Initialize<T>(this IDbSet<T> dbSet, IQueryable<T> data) where T : class
    {
        dbSet.Provider.Returns(data.Provider);
        dbSet.Expression.Returns(data.Expression);
        dbSet.ElementType.Returns(data.ElementType);
        dbSet.GetEnumerator().Returns(data.GetEnumerator());
        return dbSet;
    }
}

// usage like:
var mockSet = Substitute.For<IDbSet<Blog>>().Initialize(data);

Not the question, but in case you also need to be able to support async operations:

public static IDbSet<T> Initialize<T>(this IDbSet<T> dbSet, IQueryable<T> data) where T : class
{
  dbSet.Provider.Returns(data.Provider);
  dbSet.Expression.Returns(data.Expression);
  dbSet.ElementType.Returns(data.ElementType);
  dbSet.GetEnumerator().Returns(data.GetEnumerator());

  if (dbSet is IDbAsyncEnumerable)
  {
    ((IDbAsyncEnumerable<T>) dbSet).GetAsyncEnumerator()
      .Returns(new TestDbAsyncEnumerator<T>(data.GetEnumerator()));
    dbSet.Provider.Returns(new TestDbAsyncQueryProvider<T>(data.Provider));
  }

  return dbSet;
}

// create substitution with async
var mockSet = Substitute.For<IDbSet<Blog>, IDbAsyncEnumerable<Blog>>().Initialize(data);
// create substitution without async
var mockSet = Substitute.For<IDbSet<Blog>>().Initialize(data);

Answer 3

This is my static generic static method to generate fake DbSet. It may by useful.

 public static class CustomTestUtils
{
    public static DbSet<T> FakeDbSet<T>(List<T> data) where T : class
    {
        var _data = data.AsQueryable();
        var fakeDbSet = Substitute.For<DbSet<T>, IQueryable<T>>();
        ((IQueryable<T>)fakeDbSet).Provider.Returns(_data.Provider);
        ((IQueryable<T>)fakeDbSet).Expression.Returns(_data.Expression);
        ((IQueryable<T>)fakeDbSet).ElementType.Returns(_data.ElementType);
        ((IQueryable<T>)fakeDbSet).GetEnumerator().Returns(_data.GetEnumerator());

        fakeDbSet.AsNoTracking().Returns(fakeDbSet);

        return fakeDbSet;
    }

}