Fixes #17826.cs

public void UpdateInstructorCourses(int[] selectedCourses, Instructor instructorToUpdate)
{
    if (selectedCourses == null)
    {
        instructorToUpdate.CourseAssignments = new List<CourseAssignment>();
        return;
    }

    var selectedCoursesHS = new HashSet<int>(selectedCourses);

    var instructorCourses = new HashSet<int>
        (instructorToUpdate.CourseAssignments.Select(c => c.Course.CourseID));

    var targetCoursesId = new HashSet<int>(selectedCourses);
    targetCoursesId.UnionWith(instructorCourses);

    foreach (var courseId in targetCoursesId)
    {
        if (selectedCoursesHS.Contains(courseId))
        {
            if (!instructorCourses.Contains(courseId))
            {
                instructorToUpdate.CourseAssignments.Add(
                    new CourseAssignment
                    {
                        InstructorID = instructorToUpdate.ID,
                        CourseID = courseId
                    });
            }
        }
        else
        {
            CourseAssignment courseToRemove
                = instructorToUpdate
                .CourseAssignments
                .SingleOrDefault(i => i.CourseID == courseId);
            _context.Remove(courseToRemove);
        }
    }
}