Question

I have summarized my problem in following code snippet.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication13
{
    public class Student
    {
        public int Marks { get; set; }
        public Student(int marks)
        {
            this.Marks = marks;

        }
        public void AssignMarks(Student st)
        {
            st = null;
        }
        public void AssignMarks(ref Student st)
        {
            st = null;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Student st = new Student(50);
            st.AssignMarks(st);
            Console.WriteLine(st.Marks);
            Student st1 = new Student(50);
            st.AssignMarks(ref st1); // NullReferenceException
            Console.WriteLine(st1.Marks);
        }
    }
}

Why am I getting NullReferenceException exception on the line marked with the **

Answer 1

You’re actually getting the exception on the WriteLine call., (I tried it)

Because st1 is passed ref, the variable that you pass becomes null.

A ref parameter is a reference to a variable, not a reference to an object.
Therefore, if a method changes its ref parameter to point to a different object, the variable whose reference was passed will be changed.