Object reference not set to an instance of an object error though I am instantiationg the object [closed]

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  public partial class College
  {
    public CollegeDetails CollegeDetails;
    public List<Students> Students;
    public StaffDetails StaffDetails;
  }

 public partial class Students 
 {
    public StudentDetails StudentDetails ;
    public List<Marks> Marks;
 }

in my aspx.cs file

        College college = new College ();
    Students students= new Students (); 
    //List<Students> students = new List<Students>; 
     if (IsValidPost())
            {
                if (Session["xml"] == null)
                {
                    college.students.Add(new students{ });
                    Addtogrid();
                }
                else
                {
                    college = (College)(Session["xml"]);

                    if (college.students.Count == 0)----getting object refernce erro here
                    {
          开发者_如何学JAVA              college.students.Add(new students{ });
                    }

                    Addtogrid();
                 }
            }
            else
            {
                if (Session["xml"] != null)
                {
                 }

Please help.

---

You need to add creation of Students and Marks to constructors.

public College()
{
   Students = new List<Students>();
}

public Students()
{
  Marks = new List<Marks>();
}

Or if constructors are unavailable then create those Lists before usage

---

You never instantiate the Student List inside the College class. Same goes for other objects in the College and Student classes.

public partial class College   
{ 
  public CollegeDetails collegeDetails;
  //Never instantiate
  //public List<Students> students;      
  //Should be:
  public List<Students> students = new List<Students>();

  public StaffDetails staffDetails;   
} 

---

Just because you've declared an instance of College and Student does not mean you've created new instances of the containing objects. You need to create instances of them, too:

public partial class College {
  public College() {
    CollegeDetails = new CollegeDetails();
    Students = new List<Students>();
    StaffDetails = new StaffDetails();
  }
  public CollegeDetails CollegeDetails;
  public List<Students> Students;
  public StaffDetails StaffDetails;
}

public partial class Students {
  public Students() {
    StudentDetails = new StudentDetails();
    Marks = new List<Marks>();
  }
  public StudentDetails StudentDetails;
  public List<Marks> Marks;
}

BTW: Not a great choice of variable names.

---

You're going to need to do something similar to the following so that your types which are exposed by your composite types are not null:

public partial class College
{
    public College()
    {
        CollegeDetails = new CollegeDetails();
        Students = new List<Student>();
        StaffDetails = new StaffDetails();
    }
    public CollegeDetails CollegeDetails;
    public List<Students> Students;
    public StaffDetails StaffDetails;
}

public partial class Student
{
    public Student()
    {
        StudentDetails = new StudentDetails();
        Marks = new List<Mark>();
    }
    public StudentDetails StudentDetails ;
    public List<Marks> Marks;
}

A few notes on the above:

The constructor of a type is called just once, when the instance is created, which allows you to make ready for use, so to speak; in this case that entails instantiating your types that are expected to be used by an outside source (anything public).

I renamed Students to Student (non-plural), as a List<Student> achieves plurality by being accessible through a property named Students; I'd suspect you could do the same for Mark/s.

---

Looks to me like the constructor of the College object does not instantiate the Students list.