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.IO;
using System.Threading;
using System.Net.Sockets;
using System.Net;
namespace Messenger_Yazilimcik_Com
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            CheckForIllegalCrossThreadCalls = false;
        }
        public static string deger;
        private void Form1_Load(object sender, EventArgs e)
        {
            lblIP.Text = Dns.GetHostByName(Dns.GetHostName()).AddressList[0].ToString();
            lblNick.Text = "Yazilimcik";
            deger = lblNick.Text;
            lstOnlineUyeler.Items.Add(lblNick.Text);
        }
        private void nickDegistir_Click(object sender, EventArgs e)
        {
            try
            {
                Nick n = new Nick();
                if (n.ShowDialog() == DialogResult.OK)
                {
                    lblNick.Text = n.aktifKullanici.isim;
                    deger = lblNick.Text;
                    lstOnlineUyeler.Items.Clear();
                    lstOnlineUyeler.Items.Add(lblNick.Text);
                }
                else
                { }
            }
            catch (Exception)
            { }
        }
        private void oturumuKapat_Click(object sender, EventArgs e)
        {
            this.Close();
        }
        private void btnSohbet_Click(object sender, EventArgs e)
        {
            Pencere frm = new Pencere();
            frm.Show();
        }
        private void arkaPlan_Click(object sender, EventArgs e)
        {
            colorDialog1.ShowDialog();
            this.BackColor = colorDialog1.Color;
        }
        private void yardimToolStripMenuItem_Click(object sender, EventArgs e)
        {
            MessageBox.Show("Yardim için: www.yazilimcik.com", "Yardim", MessageBoxButtons.OK, MessageBoxIcon.Warning);
        }
        private void hakkindaToolStripMenuItem_Click(object sender, EventArgs e)
        {
            MessageBox.Show("Copyright © 2012\nwww.yazilimcik.com", "Messencer | www.yazilimcik.com", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }
        private void lblNick_DoubleClick(object sender, EventArgs e)
        {
            try
            {
                Nick n = new Nick();
                if (n.ShowDialog() == DialogResult.OK)
                {
                    lblNick.Text = n.aktifKullanici.isim;
                    deger = lblNick.Text;
                    lstOnlineUyeler.Items.Clear();
                }
                else
                { }
            }
            catch (Exception)
            { }
            lstOnlineUyeler.Items.Add(lblNick.Text);
        }
        private void cikis_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }
        private void varsayilanArkaplanToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.BackColor = SystemColors.Control;
        }
    }
}
Nick.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 Messenger_Yazilimcik_Com
{
    public partial class Nick : Form
    { 
        public Nick()
        {
            InitializeComponent();
        }
        public Kullanici aktifKullanici = new Kullanici();
        private void Nick_Load(object sender, EventArgs e)
        {
            // www.yazilimcik.com
            this.Opacity = 0.95;
        }
        
        private void btnNickDegistir_Click(object sender, EventArgs e)
        {
            try
            {
                if (txtNick.Text != string.Empty)
            {
                aktifKullanici.isim = txtNick.Text; 
                this.Close();
            }
            else
            {
                MessageBox.Show("Nick Belirlemediniz..!", "Bilgi:");
                aktifKullanici.isim = "Yazilimcik"; 
            }
            }
            catch (Exception)
            { }
        }
    }
}
Kullanici.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Messenger_Yazilimcik_Com
{
    public class Kullanici
    {
        public string isim { get; set; }
    }
}
Pencere.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.IO;
using System.Net;
namespace Messenger_Yazilimcik_Com
{
    public partial class Pencere : Form
    {
        public Pencere()
        {
            InitializeComponent();
        }
        private void Pencere_Load(object sender, EventArgs e)
        {
            // www.yazilimcik.com
            lblIP.Text = Dns.GetHostByName(Dns.GetHostName()).AddressList[0].ToString();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                if (richYazilan.Text != string.Empty)
                {
                    iletiyiGonder();
                }
                else
                { richYazilan.Text = string.Empty; richYazilan.Focus(); }
            }
            catch (Exception)
            { }
        }
        public Kullanici aktifKullanici = new Kullanici();
        private void iletiyiGonder()
        {
            richKayit.Text += Form1.deger + "> " + richYazilan.Text + " \n";
            richYazilan.Clear();
            richYazilan.Text = string.Empty;
            richYazilan.Focus();
        }
        private void btnSil_Click(object sender, EventArgs e)
        {
            richYazilan.Clear();
            richYazilan.Text = string.Empty;
            richKayit.Text = string.Empty;
            richYazilan.Focus();
        }
        private void konusmayiKapat_Click(object sender, EventArgs e)
        {
            saveFileDialog1.FileName = "Sohbet_" + DateTime.Now.Day.ToString() + "_" + DateTime.Now.Month.ToString() + "_" + DateTime.Now.Year.ToString();
            saveFileDialog1.DefaultExt = ".text";
            saveFileDialog1.Filter = "Text documents (.txt)|*.txt";
            Nullable result = saveFileDialog1.ShowDialog();
            if (result == DialogResult.OK)
            {
                string dosyaAdi = saveFileDialog1.FileName.ToString();
                StreamWriter yaz = new StreamWriter(dosyaAdi);
                string yazilanlar = richKayit.Text;
                yaz.Write(yazilanlar);
                yaz.Close();
            } this.Close();
        }
        private void profilResmi_Click(object sender, EventArgs e)
        {
            try
            {
                resimDegistir();
            }
            catch (Exception)
            { }
        }
        private void resimDegistir()
        {
            openFileDialog1.ShowDialog();
            try
            {
                lblResim.Image = Image.FromFile(openFileDialog1.FileName.ToString());
            }
            catch (Exception)
            { }
        }
        private void yaziTipi_Click(object sender, EventArgs e)
        {
            fontDialog1.ShowDialog();
            richYazilan.Font = fontDialog1.Font;
            richKayit.Font = fontDialog1.Font;
        }
        private void sayfaArkaplan_Click(object sender, EventArgs e)
        {
            openFileDialog1.ShowDialog();
            try
            {
                this.BackgroundImage = Image.FromFile(openFileDialog1.FileName.ToString());
            }
            catch (Exception)
            { }
        }
        private void yardim_Click(object sender, EventArgs e)
        {
            MessageBox.Show("Yardim için: www.yazilimcik.com", "Yardim", MessageBoxButtons.OK, MessageBoxIcon.Warning);
        }
        private void hakkinda_Click(object sender, EventArgs e)
        {
            MessageBox.Show("Copyright © 2012\nwww.yazilimcik.com", "Messencer | www.yazilimcik.com", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }
        private void richYazilan_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                Graphics g = richYazilan.CreateGraphics();
                g.DrawEllipse(new Pen(Color.Black), e.X, e.Y, 5, 5);
            }
        }
        private void lblResim_DoubleClick(object sender, EventArgs e)
        {
            try
            {
                resimDegistir();
            }
            catch (Exception)
            { }
        }
        private void richYazilan_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Enter)
            {
                try
                {
                    if (richYazilan.Text != string.Empty)
                    {
                        iletiyiGonder();
                    }
                    else
                    { richYazilan.Text = string.Empty; richYazilan.Focus(); }
                }
                catch (Exception)
                { }
            }
        }
        private void lblResim_MouseHover(object sender, EventArgs e)
        {
            lblResim.Text = "Degistir";
        }
        private void lblResim_MouseLeave(object sender, EventArgs e)
        {
            lblResim.Text = string.Empty;
        }
        private void kaydet_Click(object sender, EventArgs e)
        {
            saveFileDialog1.FileName = "Sohbet_" + DateTime.Now.Day.ToString() + "_" + DateTime.Now.Month.ToString() + "_" + DateTime.Now.Year.ToString();
            saveFileDialog1.DefaultExt = ".text";
            saveFileDialog1.Filter = "Text documents (.txt)|*.txt";
            Nullable result = saveFileDialog1.ShowDialog();
            if (result == DialogResult.OK)
            {
                string dosyaAdi = saveFileDialog1.FileName.ToString();
                StreamWriter yaz = new StreamWriter(dosyaAdi);
                string yazilanlar = richKayit.Text;
                yaz.Write(yazilanlar);
                yaz.Close();
            }
        }
        private void cikisYap_Click(object sender, EventArgs e)
        {
            saveFileDialog1.FileName = "Sohbet_" + DateTime.Now.Day.ToString() + "_" + DateTime.Now.Month.ToString() + "_" + DateTime.Now.Year.ToString();
            saveFileDialog1.DefaultExt = ".text";
            saveFileDialog1.Filter = "Text documents (.txt)|*.txt";
            Nullable result = saveFileDialog1.ShowDialog();
            if (result == DialogResult.OK)
            {
                string dosyaAdi = saveFileDialog1.FileName.ToString();
                StreamWriter yaz = new StreamWriter(dosyaAdi);
                string yazilanlar = richKayit.Text;
                yaz.Write(yazilanlar);
                yaz.Close();
            } this.Close();
        }
        private void arkaPlanRengi_Click(object sender, EventArgs e)
        {
            colorDialog1.ShowDialog();
            this.BackColor = colorDialog1.Color;
        }
        private void varsayilanArkaPlan_Click(object sender, EventArgs e)
        {
            this.BackColor = SystemColors.Control;
        }
    }
}