I built this sample to help example my problem related to navigation properties to insert related entities. I am getting the error Error System.ArgumentNullException: 'Value cannot be null. Parameter name: source' because of student2 not having data for course.

Is there a way to insert this data and not get this error?

    {
        static void Main(string[] args)
        {
            var Course = new Course();
            Course.Id = 1;
            Course.studentID = 1;
            Course.Name = "Math";

            var student1 = new student();
            student1.Id = 1;
            student1.Name = "Sam";
            student1.Courses.Add(Course);

            var student2 = new student();
            student2.Id = 2;
            student2.Name = "Bill";


        }
    }

    public class student
    {
        public int Id {get; set;}
        public string Name { get; set; }
        public List<Course> Courses { get; set; }
    }

    public class Course
    {
        public int Id { get; set; }
        public int studentID { get; set; }
        public string Name {get; set;}
    }
}```

Saving data


```var newStudent = db.Student
                          .Include(x => x.Course);

            var newInsertStudent = new EFModel.Student
                        {
                            Id = s.id,
                            Name = s.Name,
                            Courses = s.Courses.Select(p => new EFModel.Course
                            {
                                Id = p.Id,
                                studentID = s.id,
                                Name = s.name
                            }).ToList()
                        };   
             newStudent.Student.Add(newInsertStudent);```

CodePudding user response：

Try this: add a ? after s.Courses... this will make it not attempt to call into Select if Courses is null

var newInsertStudent = new EFModel.Student
                        {
                            Id = s.id,
                            Name = s.Name,
                            Courses = s.Courses?.Select(p => new EFModel.Course
                            {
                                Id = p.Id,
                                studentID = s.id,
                                Name = s.name
                            }).ToList()
                        };