Code
private void Page_Load(object sender, System.EventArgs e)
{
    string checkCode = CreateRandomCode(4);
    Session["CheckCode"] = checkCode;
    CreateImage(checkCode);
}

Code
private string CreateRandomCode(int codeCount)
{
    string allChar = "0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,W,X,Y,Z" ;
    string[] allCharArray = allChar.Split(',');
    string randomCode = "";
    int temp = -1;

    Random rand = new Random();
    for(int i = 0; i < codeCount; i++)
    {
        if(temp != -1)
        {
            rand = new Random(i*temp*((int)DateTime.Now.Ticks));
        }
        int t = rand.Next(35);
        if(temp == t)
        {
            return CreateRandomCode(codeCount);
        }
        temp = t;
        randomCode += allCharArray[t];
    }
    return randomCode;
}

Code
private void CreateImage(string checkCode)
{
    int iwidth = (int)(checkCode.Length * 11.5);
    System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 20);
    Graphics g = Graphics.FromImage(image);
    Font f = new System.Drawing.Font("Arial", 10, System.Drawing.FontStyle.Bold);
    Brush b = new System.Drawing.SolidBrush(Color.White);
    //g.FillRectangle(new System.Drawing.SolidBrush(Color.Blue),0,0,image.Width, image.Height);
    g.Clear(Color.Blue);
    g.DrawString(checkCode, f, b, 3, 3);

    Pen blackPen = new Pen(Color.Black, 0);
    Random rand = new Random();
    for (int i=0;i<5;i++)
    {
        int y = rand.Next(image.Height);
        g.DrawLine(blackPen,0,y,image.Width,y);
    }
    
    System.IO.MemoryStream ms = new System.IO.MemoryStream();
    image.Save(ms,System.Drawing.Imaging.ImageFormat.Jpeg);
    Response.ClearContent();
    Response.ContentType = "image/Jpeg";
    Response.BinaryWrite(ms.ToArray());
    g.Dispose();
    image.Dispose();
}

Code
<asp:Image Runat="server" ID="ImageCheck" ImageUrl="ValidateCode.aspx"></asp:Image>