How do I fill a drawn rectangle on a panel in C#?

using System;
using System.Windows.Forms;
using System.Text;
using System.Net.Sockets;
using System.Threading;
using System.IO;
using System.Collections;
using System.Collections.Generic;
using System.Drawing;
using System.ComponentModel;
using System.Linq;

using System.Drawing;
using System.Drawing.Drawing2D;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
//using System.Windows.Forms;

namespace Client
{
    public partial class Form1 : Form
    {

        int xStart = 20, yStart = 20;
        
        int lungCasuta = 20;
        int nrCasute = 10;
        System.Net.Sockets.TcpClient clientSocket = new System.Net.Sockets.TcpClient();
        NetworkStream serverStream = default(NetworkStream);
        string readData = null;

        public Form1()
        {
            InitializeComponent();
        
        }

        /*DRAW method---------------------*/

       private delegate void LetsDraw();
        private Graphics m_FormGraphs;
        private const int m_iSquare = 8;
        private int iCol = 0, iRow = 0, iX = 30, iY = 80;
        private const int m_iWidth = 50, m_iHeight = 50;
      //  private Holder[,] holders = new Holder[10, 10];

        private void Draw()
        {
            string[] abc = new string[20] { "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "a", "b", "c", "d", "e", "f", "g", "h", "i", "j" };

            // string abc1 = "abcdefghij";
            string[,] sCoordinates = new string[20, 20];
            //    string[] s = new string[10];
            for (int i = 0; i <= 19; i++)
            {
                //populate the array
                if (i == 20)
                {
                    iCol = 0;
                    iRow += 1;
                }
                sCoordinates[iRow, iCol] = abc[i];
                // s[iCol] = abc1.Substring(1);
                //abc = abc.Remove(0, 1);
                iCol += 1;
            }
            float fLocal = (float)60;
            float fX = (float)iX;
            float fY = (float)iY;
            Font font = new Font("Arial", 10);
            SolidBrush sBlack = new SolidBrush(Color.Black);
            Point pLocation = new Point(iX, iY);
            PointF pfLocation = new PointF(fX, fY);
            Pen pen = new Pen(new SolidBrush(Color.Black), 3);
            //Setting the row and column to origin
            iRow = 0;
            iCol = 0;
            pfLocation.X += 25;
            pfLocation.Y = fLocal;

            for (int iCount = 0; iCount < 10; iCount++)
            {
                //put numbers
                m_FormGraphs.DrawString("" + (iCount + 1), font, sBlack, pfLocation);
                pfLocation.X += 50;
            }
            pfLocation = new PointF(10, fY + 10);
            for (int iCount = 0; iCount < 10; iCount++)
            {
                //put letters
                m_FormGraphs.DrawString("" + (char)(iCount + 'A'), font, sBlack, pfLocation);
                pfLocation.Y += 50;
            }
        }

        /*DRAW method------------------------END*/

        private void button1_Click(object sender, EventArgs e)//send the message
        {
            byte[] outStream = System.Text.Encoding.ASCII.GetBytes(textBox2.Text + Environment.NewLine);
          //  byte[] outStream = System.Text.Encoding.ASCII.GetBytes(textBox2.Text + "$");
            serverStream.Write(outStream, 0, outStream.Length);
            serverStream.Flush();
        }

        private void button2_Click(object sender, EventArgs e)//connect to server
        {
            readData = "Conected to Chat Server ...";
            msg();
            clientSocket.Connect("127.0.0.1", 8888);
            serverStream = clientSocket.GetStream();

            byte[] outStream = System.Text.Encoding.ASCII.GetBytes(textBox3.Text + Environment.NewLine);
           // byte[] outStream = System.Text.Encoding.ASCII.GetBytes(textBox3.Text + "$");
            serverStream.Write(outStream, 0, outStream.Length);
            serverStream.Flush();

            Thread ctThread = new Thread(getMessage);
            ctThread.Start();
        }

        private void getMessage()
        {
            while (true)
            {
                     string bytesFrom = "";
                     string dataClient = null;



                NetworkStream networkStream = clientSocket.GetStream();
                StreamReader reader = new StreamReader(networkStream);
                serverStream = clientSocket.GetStream();
                int buffSize = 1024;
                // string inStream = "";
               byte[] inStream = new byte[10025];

                if (serverStream.CanRead)
                {
                  //  inStream = reader.ReadLine();
                  //  buffSize = clientSocket.ReceiveBufferSize;
                    serverStream.Read(inStream, 0, buffSize);
                   
                 //   dataClient = inStream;
                  string returndata = System.Text.Encoding.ASCII.GetString(inStream);
                    readData = "" + returndata;
                    msg();
                }
            }
        }

        private void msg()
        {
            if (this.InvokeRequired)
                this.Invoke(new MethodInvoker(msg));
            else
                textBox1.Text = textBox1.Text + Environment.NewLine + " >> " + readData;
        }

        private void panel2_Paint(object sender, PaintEventArgs e)
        {
            Pen blackpen = new Pen(Color.Black, 1);
            SolidBrush sBlack = new SolidBrush(Color.Black);
            Font font = new Font("Arial", 10);
            Graphics g = e.Graphics;
            
            
            
            for (int i = 0; i <= 10; i++)
            {
                g.DrawLine(blackpen, xStart, yStart + i * lungCasuta, xStart + nrCasute * lungCasuta, yStart + i * lungCasuta);
                g.DrawLine(blackpen, xStart + i * lungCasuta, yStart, xStart + i * lungCasuta, yStart + nrCasute * lungCasuta);
                
            }
            for (int i = 0; i < 10; i++)
            {
                PointF pfLocation = new PointF(xStart - 18, yStart + lungCasuta / 4 + i * lungCasuta);
                g.DrawString("" + (i + 1), font, sBlack, pfLocation);
                pfLocation = new PointF(xStart + lungCasuta / 4 + i * lungCasuta, yStart - 18);
                g.DrawString("" + (char)(i + 'A'), font, sBlack, pfLocation);
            }

            g.Dispose();

        }

        private void panel2_MouseClick(object sender, MouseEventArgs e)
        {

            textBox2.Text = (char)('A'+Math.Abs((e.X - xStart) / this.lungCasuta) )+ "," + Math.Abs(1+(e.Y - yStart) / this.lungCasuta);

            DrawRectangle(xStart, yStart, (Math.Abs((e.X - xStart) / this.lungCasuta)), (Math.Abs(1 + (e.Y - yStart) / this.lungCasuta)));
            txtXStart.Text =Convert.ToString(xStart);
            txtYStart.Text = Convert.ToString(yStart);

            txtX.Text = Convert.ToString(Math.Abs((e.X - xStart) / this.lungCasuta));
            txtY.Text = Convert.ToString(Math.Abs(1 + (e.Y - yStart) / this.lungCasuta));



            button1.PerformClick();
        }

     
        private void DrawRectangle(int XStart, int YStart, int XFinal, int YFinal)
        {
            System.Drawing.SolidBrush myBrush = new System.Drawing.SolidBrush(System.Drawing.Color.Red);
            System.Drawing.Graphics formGraphics = this.CreateGraphics();
            formGraphics.FillRectangle(myBrush, new Rectangle(XStart, YStart, XFinal, YFinal));
            myBrush.Dispose();
            formGraphics.Dispose();
        }
       private void btnDraw_Click(object sender, EventArgs e)
        {
            //DrawRectangle(300,200,30,30);
            DrawRectangle(xStart, yStart, Convert.ToInt16(txtX.Text), Convert.ToInt16(txtY.Text));

        }
      }
}