Data Grid Can't show the saved data about an object

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C#

DataGridView

inheritance

OOP

First of all, I'd like to thank all persons who read and try to help me.
I've been working on a simple -obsolete- Mobile Company to add a line and save the information of a client in a Data Grid, so I've written the following:

I've defined an line and defined the fields, constructors, and properties, with useful methods:


line.cs

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

namespace MobileCompany
{
  public  abstract class line
    {
      protected   int Id;
       protected string mobilenumber;
       protected string customername;
       static  int tmp = 0 ;

        public int Identity
        {
            get { return Id; }
        }

        public string MobileNumber 
        {
            set
            {
                if (value != null)
                { mobilenumber = value; }
            }

            get
            {
                return mobilenumber;
            }

        }

        public string CustomerName
    {
        set 
        {
            if (value != null)
            {
                customername = value;
            }
        }

        get 
        {
            return customername;
        }
    }


        public line()
        {
            tmp++;
            Id = tmp;
 
        }

        public line (string customername,string mobilenumber)
        {
            tmp++;
            Id = tmp;
            this.customername = customername;
            this.mobilenumber = mobilenumber;

        }

        //Methods Area

        public override string ToString()
        {
            return "Identity :\t"+Id+"\nCustomer Name :\t"+customername+"\nMoblie Number :\t"+mobilenumber;
        }

        public abstract decimal BillAmount(); //this is abstract, can't be known till we create a known (concrete) object
    }
}

Then, I've added -as requested- a subscribed line that inherits from line.cs :

SubscriberLine.cs

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

namespace MobileCompany
{
    class SubcriberLine : line 
    {
        int mm;
        int mf;

        //Propreties Area

        public int MoblietoMobile
        {
            set 
            {
                if (value > 0)
                    mm = value;
            }
            get
            { return mm; }

        }

        public int Mobiletofixed
        {
            set
            {
                if (value > 0)
                    mf = value;
            }

            get
            { return mf; }
        }

        //Constructors Area

    public SubcriberLine(string name , string number,int mm, int mf): base( name, number )
    {
        this.mm = mm;
        this.mf = mf;
    }

    public SubcriberLine()
    {
        // TODO: Complete member initialization
    }

        //Methods Area
    public override string ToString()
    {
        return "Identity :\t" + base.Id + "\nCustomer Name :\t" + base.customername + "\nMoblie Number :\t" + base.mobilenumber +
            "Totall Calling minutes :\nPhone Calls to Mobile:\t" + mm + "\tMobile Calls Charge:\t" + mobilecalls_cost()
            + "\nPhone Calls to Landlines:\t" + mf + "\tland Calls Charge:\t" + landcalls_cost() + "\nTotal Tax :\t" + BillAmount();

    }

    public decimal mobilecalls_cost()
    {
        return mm * 4;
    }
    public decimal landcalls_cost()
    {
        return mm * 6;
    }
    public override decimal BillAmount()
    {
        return mobilecalls_cost() + landcalls_cost();
    }

    }
}

Last, but not least, I've encoded the Company:

Company.cs

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


namespace MobileCompany
{
 public   class Company
    {
         ArrayList data = new ArrayList();

        //Method: add line Object

        public void Addline(line l)
        {
            data.Add(l);

        }

        public Company()
        {

        }


        //Method: getline by index

        public line getLine(int index)
        {
            return data[index] as line;
        }




        public line this[int index]
        {
            get 
            {
                return data[index] as line;
            }
        }


        public int Count()
        {
            return data.Count;
        }



    }

  }

Here I've finished writing the important coding for my application.
I've headed next to the windows form application; it consists of two forms only.
The first form is simple, asks for client's name, number, mobile calls, and land calls; by Pressing buttom1 "Add line", a new line object is created.
It's simplified as follows:

Form1.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Collections;
namespace MobileCompany
{
    public partial class Form1 : Form
    {
        //At the start of the Application, we Create a New Company

        public  static  Company MTN = new Company();
       
        
        public Form1()
        {
            InitializeComponent();



        }
        

        private void button1_Click(object sender, EventArgs e)
        {
           //After Entering data into the fields, we Create an Cellphone Line Object 

                SubcriberLine s = new SubcriberLine(txtName.Text,txtNumber.Text,Convert.ToInt32(txtMobile.Text),Convert.ToInt32(txtFixed.Text));

            //We Add the Cellphone Line Object to the Company's DataBase, Our Arraylist in our company MTN

                MTN.Addline(s);

            //confront the MTN Worker that the Adding process has been finished !

                MessageBox.Show("new CellPhone Line has been created !");
            
            



        }


        private void button2_Click(object sender, EventArgs e)
        {
            //In order to show all lines in Our company, we need to SHOW new table, new Page , or a new FORM

            Form2 f = new Form2();
            f.Show();
        }

     }
    }

Now, if you noticed, the functionality of buttom2 "Show All Lines" is Making another instance of Form called Form2, and simply showing it.
Here, I've Placed Only a DataGridView in Form2, wrote some code to ensure the DataGridView shows what is written, by retrieving data from the object :

Form2.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace MobileCompany
{
    public partial class Form2 :Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        private void Form2_Load(object sender, EventArgs e)
        {
            for (int i = 0; i < Form1.MTN.Count(); i++)
            {
                dataGridView1.Rows.Add(Form1.MTN.getLine(i).Identity,Form1.MTN.getLine(i).CustomerName,Form1.MTN.getLine(i).MobileNumber,Form1.MTN.getLine(i).BillAmount());
            }
        }

        private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {

        }
    }
}

Now, I've finished a pile of coding, unfortunately, the data about an added client won't be shown on the DataGridView.
Can you please Help me find a solution?
I'd be pleased to hear comments about how my code is too.
Appreciate any contributor's help, as long as we're under the same Coding roof.