Is this the correct behaviour (List<class> question)

[Paul Johnson]

Hi,

I enjoy trying new ways (to me) of doing getting my code to be more efficient. I'm currently working on a database system (front end to an access file with added bits) and have hit upon a problem.

I have a number of tutors containing 3 pieces of information (name, email, mobile number). There can be loads of these and rather than muck around, I've come up with the following solution.

My solution looks like this

using System;
using System.Collections.Generic;
using System.Text;
 
namespace class_list_test
{
    class Program
    {
        static void Main(string[] args)
        {
            tester m = new tester();
            List<tester> t = new List<tester>();
            m.a = 1;
            m.c = "wibble";
            m.d.Add(3.14);
            m.d.Add(3.28);
            t.Add(m);
            m.a = 2;
            m.c = "wobble";
            m.d.Add(6.02);
            m.d.Add(6.16);
            t.Add(m);
            m.a = 3;
            m.c = "snurfle";
            m.d.Add(9.44);
            m.d.Add(9.58);
            t.Add(m);
            m.a = 4;
            m.c = "snarfle";
            m.d.Add(7.33);
            m.d.Add(7.47);
            t.Add(m);
            printstuff(t);
        }
 
        private static void printstuff(List<tester> r)
        {
            for (int n = 0; n < r.Count; ++n)
                Console.WriteLine("{0}, {1}, {2}, {3}", r[n].a, r[n].c, r[n].d[0], r[n].d[1]);
            Console.ReadLine();
        }
    }
 
    public class tester
    {
        public int a;
        public string c;
        public List<double> d = new List<double>();
    }
}
 

The code compiles and runs, but not as expected. The output of the code is four copies of the last things m is set to which doesn't seem to be the correct behaviour to me. I can get around it by inserting

m = new tester();
 

after each add - but this seems wasteful to me.

Am I better off adding a get/set in the tester class will I still need a new instance of tester() after each add?

Paul

[Rabbit]

Define correct behavior. It's incorrect behavior based on your requirements. But it's correct behavior from the code that is written.

You've misunderstood how a list works. When you add an object to a list, it doesn't create a copy of it, it only stores a pointer to the object. Because you don't create a new object each time, you are adding the same pointer to the list.

[Paul Johnson]

Thanks for the clarification - much appreciated :)