怎么在C#类中添加一些函数

using System;
using System.Collections.Generic;
using System.Text;
using System.Drawing;
using System.IO;
using System.Drawing.Imaging;
namespace Chen
{
public class warterfont
{
public void addtexttoimg(string filename, string text)
{
if (!File.Exists(System.Web.HttpContext.Current.Server.MapPath(filename)))
{
throw new FileNotFoundException("the file don't exist!");
}
if (text == string.Empty)
{
return;
}
//还需要判断文件类型是否为图像类型，这里就不赘述了
Image image = Image.FromFile(System.Web.HttpContext.Current.Server.MapPath(filename));
Bitmap bitmap = new Bitmap(image, image.Width, image.Height);
Graphics g = Graphics.FromImage(bitmap);
float fontsize = 12.0f; //字体大小
float textwidth = text.Length * fontsize; //文本的长度
//下面定义一个矩形区域，以后在这个矩形里画上白底黑字
float rectx = 0;
float recty = 0;
float rectwidth = text.Length * (fontsize + 8);
float rectheight = fontsize + 8; //声明矩形域
RectangleF textarea = new RectangleF(rectx, recty, rectwidth, rectheight);
Font font = new Font("宋体", fontsize); //定义字体
Brush whitebrush = new SolidBrush(Color.White); //白笔刷，画文字用
Brush blackbrush = new SolidBrush(Color.Black); //黑笔刷，画背景用
g.FillRectangle(blackbrush, rectx, recty, rectwidth, rectheight);
g.DrawString(text, font, whitebrush, textarea);
MemoryStream ms = new MemoryStream(); //保存为jpg类型
bitmap.Save(ms, ImageFormat.Jpeg); //输出处理后的图像，这里为了演示方便，我将图片显示在页面中了
bitmap.Save(System.Web.HttpContext.Current.Server.MapPath("/" + "aa.jpg"), ImageFormat.Jpeg); //保存到磁盘上
System.Web.HttpContext.Current.Response.Clear();
System.Web.HttpContext.Current.Response.ContentType = "image/jpeg";
System.Web.HttpContext.Current.Response.BinaryWrite(ms.ToArray());
g.Dispose();
bitmap.Dispose();
image.Dispose();
}
}
}