（c#流的概念）写了一段验证码的程序。流的概念至今不理解。请大家帮忙讨论一下！该如何解决

（c#流的概念）写了一段验证码的程序。流的概念至今不理解。请大家帮忙讨论一下！
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Drawing;

public partial class yanzhengma1 : System.Web.UI.Page
{
  private string getvali(int numcount)
  {
  string strsvali = "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,V,W,X,Y,Z";
  string[] valiarray = strsvali.Split(',');
  string returnnum = "";
  int nums = -1;
  Random vrand = new Random();
  for (int n = 1; n < numcount; n++)
  {
  if (nums != -1)
  {
  vrand = new Random(n * nums * unchecked((int)DateTime.Now.Ticks));
  }
  int t = vrand.Next(35);
  nums = t;
  returnnum += valiarray[t];
  }
  Session["Valid"] = returnnum;
  return returnnum;
  }
  private void createimage(string validatenum)
  {
  if (validatenum == null || validatenum.Trim() == string.Empty)
  return;
  {

  System.Drawing.Bitmap image = new System.Drawing.Bitmap(validatenum.Length * 12 + 10, 22);//生成图像
  Graphics g = Graphics.FromImage(image);//定义一个绘图画面
  try
  {
  Random random = new Random();
  g.Clear(Color.White);//清除画面并以背景色填充
  for (int i = 0; i < 25; i++)
  {
  int x1 = random.Next(image.Width);
  int x2 = random.Next(image.Width);
  int y1 = random.Next(image.Height);
  int y2 = random.Next(image.Height);
  g.DrawLine(new Pen(Color.Coral), x1, y1, x2, y2);//绘制连接坐标的线条
  }
  Font font = new System.Drawing.Font("Arial", 12, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));
  System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.Green, 1.2f, true);
  g.DrawString(validatenum, font, brush, 2, 2);//font：定义文字的格式。brush:渐变工具。2，2：是文字到图片边框的距离
  for (int i = 0; i < 100; i++)
  {
  int x = random.Next(image.Width);
  int y = random.Next(image.Height);
  image.SetPixel(x, y, Color.FromArgb(random.Next()));//表示分配给指定像素指定的颜色
  }
  g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);//绘制一个矩形
  System.IO.MemoryStream ms = new System.IO.MemoryStream();
  image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
  Response.ClearContent();
  Response.ContentType = "image/gif";
  Response.BinaryWrite(ms.ToArray());
  }
  finally
  {
  g.Dispose();
  image.Dispose();
  }
  }
  }
  protected void Page_Load(object sender, EventArgs e)
  {
  if (!Page.IsPostBack)