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)?

Solution 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());

Solution 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);

Solution 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;
    }

}

Solution 4

I wrote a wrapper about a year ago around the same code you are referencing from Testing with Your Own Test Doubles (EF6 onwards). This wrapper can be found on GitHub DbContextMockForUnitTests. The purpose of this wrapper is to reduce the amount of repetitive/duplicate code needed to setup unit tests that make use of EF where you want to mock that DbContext and DbSets. Most of the mock EF code you have in the OP can reduced down to a 2 lines of code (and only 1 if you are using DbContext.Set<T> instead of DbSet properties) and the mock code is then called in the wrapper.

To use it copy and include the files in folder MockHelpers to your Test project.

Here is an example test using what you had above, notice that there is now only 2 Lines of code are needed to setup the mock DbSet<T> on the mocked DbContext.

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

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

  // Create and assign the substituted DbSet
  var mockSet = data.GenerateMockDbSet();
  mockContext.Blogs.Returns(mockSet);

  // act
}

It is just as easy to make this a test that invokes something that uses the async/await pattern like .ToListAsync() on the DbSet<T>.

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

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

  // Create and assign the substituted DbSet
  var mockSet = data.GenerateMockDbSetForAsync(); // only change is the ForAsync version of the method
  mockContext.Blogs.Returns(mockSet);

  // act
}

View more solutions

Comments

[s.meijer]

In EF 6 DbSet properties should be made virtual for testability. I dont want to create a fake DbSet wrapper. Substituting these 4 properties should be easier and better than creating the wrapper you're proposing. So your suggestion isn't really the answer I'm looking for.

[Kevin]

Judging by the error message I'd say that DbSet proxy created by NSubstitute does not support an explicitly implemented interface, only the specific contract or object you're substituting. This is kind of born out in (github.com/nsubstitute/NSubstitute/issues/95) and may be a limitation of the framework.

[s.meijer]

Thanks! This is really helpful information. I'm not sure yet if this is the best way to go for substituting DbSet (as it turned out using IDbSet solves the problems), but it sure is helpful in other situations.

[s.meijer]

I'm marking you as the answer, because your suggestion is the solution for supporting Async (Substitute.For<IDbSet<Blog>, IDbAsyncEnumerable<Blog>>()). That is not really the problem of the question asked, but it lies in the extension of it. Thanks again for this insight!

[vborutenko]

It's ok.But what is TestDbAsyncEnumerator and TestDbAsyncQueryProvider.

[s.meijer]

It's an inmemory db query provider, as explained by the link I'm providing in my question, as well as this answer. Take a look at the msdn page: msdn.microsoft.com/nl-nl/data/dn314429.aspx#async

[Michael Haren]

Tip: make sure your DbSets in your context are virtual

[Shevek]

I have tried this but get an exception: NSubstitute.Exceptions.CouldNotSetReturnDueToTypeMismatchExc‌​eption: Can not return value of type IDbSet1Proxy for BloggingContext.get_Blogs (expected type DbSet1). The Context was generated by the EF TT template

[John Lieb-Bauman]

Is there an elegant solution to handle .Include() issues? I keep getting source is null errors.

[klyd]

@Shevek Had the same problem, you can make it work by adjusting the interfaces/types in the for call: Substitute.For<IDbSet<DataModel>, DbSet<DataModel>>();

[Stuart Clement]

@Shevek you can also modify your BloggingContext class: public virtual IDbSet<Blog> Blogs { get; set; } public virtual IDbSet<Post> Posts { get; set; }

[Panagiotis Pnevma]

I think that missing the keyword "this" from the definition of the argument "data"

[Ashley Kilgour]

Used this in several projects. I like that it removes the plumbing from my tests