求这个程序出现错误怎么改
public class ValidCode : IHttpHandler {
//添加生成验证码的随机函数
private string GetValidCode(int num)
{
string strRandomCode = "ABCD1EF2GH3IJ4KL5MN6P7QR8ST9UVWXYZ"; //定义要随机抽取的字符串
char[] chastr = strRandomCode.ToCharArray(); //将定义的字符串转成字符数组
StringBuilder sbValidCode = new StringBuilder(); //定义StringBuilder对象用于存放验证码
Random rd = new Random(); //随机函数，随机抽取字符
for (int i = 0; i < num; i++)
{
//以strRandomCode的长度产生随机位置并截取该位置的字符添加到StringBuilder对象中
sbValidCode.Append(strRandomCode.Substring(rd.Next(0, strRandomCode.Length), 1));
}
return sbValidCode.ToString();
}
//生成验证码
public void ProcessRequest (HttpContext context)
{
string strValidCode = GetValidCode(5); // 产生5位随机字符
context.Session["ValidCode"] = strValidCode; //将字符串保存到Session中，以便需要时进行验证
Bitmap image = new Bitmap(120, 30); //定义宽120像素，高30像素的数据定义的图像对象
Graphics g = Graphics.FromImage(image); //绘制图片
try
{
Random random = new Random(); //生成随机对象
g.Clear(Color.White); //清除图片背景色
for (int i = 0; i < 25; i++) // 随机产生图片的背景噪线
{
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.Silver), x1, y1, x2, y2);
}
Font font = new System.Drawing.Font("新宋体", 20, (System.Drawing.FontStyle.Bold)); //设置图片字体风格
LinearGradientBrush brush = new LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 3, true);//设置画笔类型
g.DrawString(strValidCode, font, brush, 5, 2); //绘制随机字符
g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1); //绘制图片的前景噪点
ms = new (); //建立存储区为内存的流
image.Save(ms, ImageFormat.Gif); //将图像对象储存为内存流
context.Response.ClearContent(); //清除当前缓冲区流中的所有内容
context.Response.ContentType = "image/Gif"; //设置输出流的MIME类型
context.Response.BinaryWrite(ms.ToArray()); //将内存流写入到输出流
}
finally
{
g.Dispose();
image.Dispose();
}
}
public bool IsReusable
{
get {
return false;
}
}
}
我写的这段代码运行时 context.Session["ValidCode"] = strValidCode;位置提示System.NullReferenceException错误，怎么改
