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(new 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