Mocking EF core dbcontext and dbset

I see you are using EF core DbContext in your MovieRepository. So instead of using mock, Using EF Core InMemory database will be a great option for you. This will also reduce the complexity.

Write your GetAllTest() method as follows:

[Fact]
public void GetAllTest()
{
        var options = new DbContextOptionsBuilder<MovieDbContext>()
            .UseInMemoryDatabase(databaseName: "MovieListDatabase")
            .Options;

        // Insert seed data into the database using one instance of the context
        using (var context = new MovieDbContext(options))
        {
            context.Movies.Add(new Movie {Id = 1, Title = "Movie 1", YearOfRelease = 2018, Genre = "Action"});
            context.Movies.Add(new Movie {Id = 2, Title = "Movie 2", YearOfRelease = 2018, Genre = "Action"});
            context.Movies.Add(nnew Movie {Id = 3, Title = "Movie 3", YearOfRelease = 2019, Genre = "Action"});
            context.SaveChanges();
        }

        // Use a clean instance of the context to run the test
        using (var context = new MovieDbContext(options))
        {
            MovieRepository movieRepository = new MovieRepository(context);
            List<Movies> movies == movieRepository.GetAll()

            Assert.Equal(3, movies.Count);
        }
}

Note: Don't forget to install Microsoft.EntityFrameworkCore.InMemory nuget package as follows:

Install-Package Microsoft.EntityFrameworkCore.InMemory

For more details: Testing with InMemory

To save your time, try to use my Moq/NSubstitute extension MockQueryable: https://github.com/romantitov/MockQueryable supported all Sync/Async operations

//1 - create a List<T> with test items
var users = new List<UserEntity>()
{
 new UserEntity,
 ...
};

//2 - build mock by extension
var mock = users.AsQueryable().BuildMock();

//3 - setup the mock as Queryable for Moq
_userRepository.Setup(x => x.GetQueryable()).Returns(mock.Object);

//3 - setup the mock as Queryable for NSubstitute
_userRepository.GetQueryable().Returns(mock);

DbSet also supported

//2 - build mock by extension
var mock = users.AsQueryable().BuildMockDbSet();

//3 - setup DbSet for Moq
var userRepository = new TestDbSetRepository(mock.Object);

//3 - setup DbSet for NSubstitute
var userRepository = new TestDbSetRepository(mock);

Note:

• AutoMapper supported from 1.0.4 ver
• DbQuery supported from 1.1.0 ver
• EF Core 3.0 supported from 3.0.0 ver

Use the Moq.EntityFrameworkCore package.

It is as easy as:

var myDbContextMock = new Mock<MyDbContext>();
IList<Entity> entities = new List<Entity>() { new Entity(), new Entity() };
myDbContextMock.Setup(x => x.Entities).ReturnsDbSet(entities);

This is a development of R.Titovs answer done in ASP.NET Core 3.1:

Constructing the Moq (generic method)

The data is cloned to allow for tests to run in parallel and prevent a test to access data changed by another.

public static Mock<DbSet<TEnt>> SetDbSetData<TEnt>(this Mock<IApplicationDbContext> dbMock,
        IList<TEnt> list, bool clone = true) 
    where TEnt : class
{
    var clonedList = clone ? list.DeepClone().ToList() : list.ToList();
    var mockDbSet = clonedList.AsQueryable().BuildMockDbSet();

    dbMock.Setup(m => m.Set<TEnt>()).Returns(mockDbSet.Object);
    dbMock.Setup(m => m.ReadSet<TEnt>()).Returns(mockDbSet.Object.AsQueryable());

    return mockDbSet;
}

Using some test data

_appUserDbSetMock = _dbMock.SetDbSetData(ApplicationUserTestData.ApplicationUserData);

Example test

[Fact]
private async Task Handle_ShouldAddANewUser()
{
    var command = new CreateApplicationUserCommand
    {
        // ...
    };

    await _handler.Handle(command, default);

    _appUserDbSetMock.Verify(m => m.AddAsync(It.IsAny<ApplicationUser>(), default), Times.Once);
}

One advantage of using MoqQueryable is that there is no need for a generic repository since DbSet acts like one and the mocking is very easy.