using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
 
namespace tmp
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void Form1_Load(object sender, EventArgs e)
        {
            // 1.
            Klasse1.SetList(ref listBox1);
 
            // 2.
            Klasse2 cKlasse2 = new Klasse2();
            foreach (string sItem in cKlasse2.sItems())
            {
                listBox1.Items.Add(sItem);
            }
        }
    }
}

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
 
namespace tmp
{
    class Klasse1
    {
        public static void SetList(ref ListBox lstBox)
        {
            for (int i = 0; i < 100; i++)
            {
                lstBox.Items.Add("Möpidöpi" + i.ToString());
            }
        }
    }
 
    class Klasse2
    {
        public Klasse2()
        {
 
        }
 
        public string[] sItems()
        {
            return new string[] { "MöpiDöpi1", "MöpiDöpi2" };
        }
    }
 
}

Mit freundlichen Gr??en,
Andy G.