Asp.net 中将文字写入图片中

/**//// <summary>
       /// 功能：将文字写入图片
      /// </summary>
        /// <param name="strfileName">文件名字</param>
        /// <param name="strtext">需要写入的文字</param>
        private void TextToImage(string strfileName,string strtext)
        ...{
            if(!File.Exists(MapPath(strfileName)))
            ...{
                throw new FileNotFoundException("文件不存在!");
            }
          
            if( strtext == string.Empty )
            ...{
                return;
            }
            //还需要判断文件类型是否为图像类型
            System.Drawing.Image image = System.Drawing.Image.FromFile(MapPath(strfileName));
            Bitmap bitmap = new Bitmap(image,image.Width,image.Height);
            Graphics grap = Graphics.FromImage(bitmap);
            float fontSize = 24.0f;             //字体大小
            float textWidth = strtext.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 = strtext.Length*(fontSize+12);
            float rectHeight = fontSize+8;
            string strFont;//定义字体类型
           //声明矩形域
            RectangleF textArea = new RectangleF(rectX,rectY,rectWidth,rectHeight);
            strFont = ddlfont.SelectedItem.Text.ToString().Trim();
            Font font = new Font(strFont,fontSize);   //定义字体
            Brush whiteBrush = new SolidBrush(Color.White);   //白笔刷，画文字用
            Brush blackBrush = new SolidBrush(Color.Transparent);   //黑笔刷，画背景用

            grap.FillRectangle(blackBrush,rectX,rectY,rectWidth,rectHeight);          

            grap.DrawString(strtext,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.//在这里可以将生成后的图片放大Oracle数据库Power中
            
            Response.Clear();
            Response.ContentType = "image/jpeg";
            Response.BinaryWrite(ms.ToArray());
            grap.Dispose();
            bitmap.Dispose();
            image.Dispose();
        }
事件:
private void btnWrite_Click(object sender, System.EventArgs e)
        ...{
            HttpPostedFile httpPostFile;    //客户端上载文件对象
            httpPostFile = DocUpload.PostedFile;
            string strTemp;
            string strFileName;//完成文件名
            strTemp = httpPostFile.FileName.ToString().Trim();
            strFileName = GetRealFileName(strTemp);
            string strImg = "images/";
            TextToImage(strImg +strFileName,txtDocName.Text.ToString());

        }

/**//// <summary>
        ///  功能：获取文件名
        /// </summary>
        /// <param name="strPathName">文件路径</param>
        /// <returns></returns>
        public string GetRealFileName(string strPathName)
        ...{
            string [] strFileName = strPathName.Split('\');
            int iPos = strFileName.GetUpperBound(0);
            return strFileName[iPos];
        }