.net core mocking dbcontext not working (static issue?)

Question

I am trying to test .net core 2.2 api's. I am having difficulty mocking (using moq) the dbcontext. What is the syntax for adapting my mockDbContext to be usable. I receive a NullReferenceException. Since the Changetracker is never instantiated I believe. Do I need a different approach? I saw mentioned .UseInMemoryDatabase() but, with very little documentation or good examples.

Below is the code I am trying to use in my test [Fact].

     var mockDbContext = new Mock<dbContext>(optionsBuilder.Options);
     var controller = new HomeController(mockDbContext.object);

Then use controller to test... removed for brevity

var datafromdbcontext = controller.GetData();

Below is an example of my dbcontext.

    public class dbContext:DbContext
    {
        public dbContext(DbContextOptions<dbContext> options)
        : base(options)
        {
            //MAKE IT READONLY
            ChangeTracker.QueryTrackingBehavior = 
       QueryTrackingBehavior.NoTracking;
        }

Robert Perry · Accepted Answer · 2019-02-07 21:36:54Z

2

There are lots of examples of using the InMemory database (which is what you normally use for unit tests) ... Here is an example: https://learn.microsoft.com/en-us/ef/core/miscellaneous/testing/in-memory

You shouldn't try to Mock the actual context, you use the InMemory option instead. So - your context, but with the InMemory "option"... Like so:

var myFakeContext = new DbContextOptionsBuilder<MYDBCONTEXT>().UseInMemoryDatabase("SO-MADE-UP-NAME");

edited Feb 7, 2019 at 21:36

answered Feb 7, 2019 at 21:28

Robert Perry

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13 Comments

Alex Zimmerman Over a year ago

So I should be using .InMemoryDatabase() if I am having moq Issues?

Sangman Over a year ago

Yes, in fact in EF Core you shouldn't mock your DbContext at all but opt for InMemoryDatabase

Robert Perry Over a year ago

@Sangman beat me to it.. Yes, so, you shouldnt try to Mock the actual context, you use the InMemory option instead. So - your context, but with the InMemory "option"... I've added the example to the answer

Eric Farr Over a year ago

Yes, use InMemoryDatabase and see how far it gets you. There are a few things in-memory doesn't support. Should you need to mock a FromSql (eg, calling a stored proc), check out my example here: github.com/efarr/mock-ef-core-fromsql

Robert Perry Over a year ago

@Fabio - It depends totally on what you're trying to test and what you're trying to achieve. Theres two different types of tests designed for different reasons.The one you're referring to is called an integration test and its fundamentally different to a unit test. Mocking typically (but not necessarily always) comes into play in "Unit Tests".

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huysentruitw · Accepted Answer · 2019-02-14 18:09:27Z

I have created a library, available on NuGet and GitHub called EntityFrameworkCoreMock that does the heavy lifting of setting everything up. There's also an implementation for Moq.

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

Example usage

public class User
{
    [Key, Column(Order = 0)]
    public Guid Id { get; set; }

    public string FullName { get; set; }
}

public class TestDbContext : DbContext
{
    public TestDbContext(DbContextOptions<TestDbContext> options)
        : base(options)
    {
    }

    public virtual DbSet<User> Users { get; set; }
}

public class MyTests
{
    [Fact]
    public void Test()
    {
        var initialEntities = new[]
        {
            new User { Id = Guid.NewGuid(), FullName = "Eric Cartoon" },
            new User { Id = Guid.NewGuid(), FullName = "Billy Jewel" },
        };

        var dbContextMock = new DbContextMock<TestDbContext>(DummyOptions);
        var usersDbSetMock = dbContextMock.CreateDbSetMock(x => x.Users, initialEntities);

        // Pass dbContextMock.Object to the class/method you want to test

        // Query dbContextMock.Object.Users to see if certain users were added or removed
        // or use Mock Verify functionality to verify if certain methods were called: usersDbSetMock.Verify(x => x.Add(...), Times.Once);
   }

    public DbContextOptions<TestDbContext> DummyOptions { get; } = new DbContextOptionsBuilder<TestDbContext>().Options;
}

EcchiBi · Accepted Answer · 2022-11-03 11:00:05Z

Actually you can Mock DbContext like this :

source (Synchronous/Asynchronous): https://learn.microsoft.com/fr-fr/ef/ef6/fundamentals/testing/mocking?redirectedfrom=MSDN

            var mockSet = new Mock<DbSet<User>>();
            mockSet.As<IDbAsyncEnumerable<User>>()
              .Setup(m => m.GetAsyncEnumerator())
              .Returns(new TestDbAsyncEnumerator<User>(data.GetEnumerator()));

            mockSet.As<IQueryable<User>>()
               .Setup(m => m.Provider)
               .Returns(new TestDbAsyncQueryProvider<User>(data.Provider));

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

            var mockContext = new Mock<IWebApiDbContext>();
            mockContext.Setup(m => m.User).ReturnsDbSet(mockSet.Object);

But you will cannot alterate(Add/Delete/Update) the data. Only Get them.

so the solution can be Use in Memory :

var options = new DbContextOptionsBuilder<WebApiDbContext>()
                .UseInMemoryDatabase(databaseName: "dbContext")
                .Options;

            var context = new WebApiDbContext(options);

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