Problem med at tegne i en tråd

Jeg har leget lidt videre med det, det ser ud til at det er tråden der laver en fejl og ikke koden deri. Når jeg instantierer klassen gør jeg det således:

Ball myBall=new Ball(20,20,e.Graphics)
myBall.startBall();


Dette er nødvendigvis ikke den korrekte kode, men således gør jeg det med to linier....

(¯`·._.·[Brian Hvarregaard]·._.·´¯)

Kode 

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Threading;

namespace WindowsApplication1
{
  /// <summary>
	/// Summary description for Form1.
	/// </summary>
	public class Form1 : System.Windows.Forms.Form
	{
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private System.ComponentModel.Container components = null;
    private System.Drawing.Graphics graph;
    private System.Windows.Forms.Button button1;
    private WindowsApplication1.Ball myBall;

		public Form1()
		{
			//
			// Required for Windows Form Designer support
			//
			InitializeComponent();

			//
			// TODO: Add any constructor code after InitializeComponent call
			//
      myBall = null;
      graph = null;
      this.Paint +=new PaintEventHandler(Form1_Paint);
//      myBall = new Ball(10, 10, g);
//      myBall.startBall();
//      myBall = null;
    }

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if (components != null) 
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}

		#region Windows Form Designer generated code
		/// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
      this.button1 = new System.Windows.Forms.Button();
      this.SuspendLayout();
      // 
      // button1
      // 
      this.button1.Location = new System.Drawing.Point(376, 24);
      this.button1.Name = "button1";
      this.button1.Size = new System.Drawing.Size(64, 24);
      this.button1.TabIndex = 0;
      this.button1.Text = "Test";
      this.button1.MouseUp += new System.Windows.Forms.MouseEventHandler(this.button1_MouseUp);
      // 
      // Form1
      // 
      this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
      this.ClientSize = new System.Drawing.Size(448, 273);
      this.Controls.Add(this.button1);
      this.Name = "Form1";
      this.Text = "Form1";
      this.ResumeLayout(false);

    }
		#endregion

		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main() 
		{
			Application.Run(new Form1());
    }

    private void Form1_Paint(object sender, PaintEventArgs e)
    {
      graph = e.Graphics;
      if (myBall == null)
      {
        Console.WriteLine("Starting ball.");
        myBall = new Ball(10, 10, graph);
        myBall.startBall();
        myBall = null;
      }
    }

    private void button1_MouseUp(object sender, System.Windows.Forms.MouseEventArgs e)
    {
      if (graph != null)
      {
        Console.WriteLine("Graphics found");
        if (myBall == null)
        {
          Console.WriteLine("Starting ball.");
          myBall = new Ball(10, 10, graph);
          myBall.startBall();
          myBall = null;
        }
      }
    }
  }

  public class Ball 
  { 
    #region Instance Variables 
    private float x_pos; 
    private float y_pos; 
    private float x_speed; 
    private float y_speed; 
    private static Graphics mainGraphics; 

    #endregion 

    public Ball(float startX, float startY, Graphics g) 
    { 
      x_pos=startX; 
      y_pos=startY; 
      mainGraphics=g; 
    } 

    #region Other methods 

    public void DrawBall() 
    { 
      int times = 500;
      Pen myPen=new Pen(Brushes.Black, 2); 
      while(times-- > 0) 
      { 
        Console.WriteLine("x_pos: "+x_pos.ToString()+" y_pos: "+y_pos.ToString()); 
        mainGraphics.DrawEllipse(myPen, x_pos, y_pos, 20, 20);
        if ((x_pos += x_speed) > 200)
          x_speed = -1;
        else
          if (x_pos < 10)
          x_speed = 1;
        if ((y_pos += y_speed) > 50)
          y_speed = -1;
        else
          if (y_pos < 10)
          y_speed = 1;
      } 
    } 

    public void startBall() 
    { 
      Thread ballThread = new Thread(new ThreadStart(this.DrawBall)); 
      Console.WriteLine("inde i tråd"); 
      x_speed=1; 
      y_speed=1; 
      ballThread.Start(); 
      while (ballThread.IsAlive)
        Thread.Sleep(0);
      ballThread.Join();
    } 

    #endregion 

    #region Properties 

    public float XPosition 
    { 
      get { return x_pos; } 
      set { x_pos=value; } 
    } 

    public float YPosition 
    { 
      get { return y_pos; } 
      set { y_pos=value; } 
    } 
    #endregion 
  } 
}

Jeg er ved at lave en klasse der styrer en bold (i starten en ellipse) denne klasse har en metode der starter en tråd som så styrer bolden, problemet er at efter ca. 50 gennemløb fejler den med en illegal argument type, nogle, der kan hjælpe ??