asp.net 直接 从数据库 下载 文件

当我们想要从网上下载文件时，通常的做法是在服务器上的某个目录下生成一个文件。
protected void DownloadFile(string filename)
{
string saveFileName = "test.xls";
int intStart = filename.LastIndexOf("\\") + 1;
saveFileName = filename.Substring(intStart, filename.Length - intStart);
Response.Clear();
Response.Charset = "utf-8";
Response.Buffer = true;
this.EnableViewState = false;
Response.ContentEncoding = System.Text.Encoding.UTF8;
Response.AppendHeader("Content-Disposition", "attachment;filename=" + saveFileName);
Response.WriteFile(filename);
Response.Flush();
Response.Close();
Response.End();
} 
但会存在这样的情况，直接从数据库中下载图片。
其实原理很简单。就是用MemoryStream代替FileStream。有一点需要注意就是http头的格式要设置正确。 byte[] pic = //从数据库返回的图片
MemoryStream ms = new MemoryStream(pic);
System.Drawing.Image img = null;
Graphics g =null;
try
{
img = new Bitmap(new MemoryStream(pic));
g= Graphics.fromImage(img);
Font objfont = new Font("Verdana", 12, FontStyle.Bold);
Brush objbrush = Brushes.Red;
Rectangle rect = new Rectangle(150, 550, 200, 50);
g.DrawString("添加文字", objfont, objbrush, rect);
img.Save(ms, ImageFormat.Bmp);
}
catch { }
finally
{
if (img != null) img.Dispose();
if (g!= null) g.Dispose();
}
byte[] newbmp = ms.ToArray();
ms.Close();
ms = new MemoryStream(newbmp);
Response.Clear();
Response.AddHeader("Content-type", "binary/octet-stream");
Response.AddHeader("Content-Length", pic.Length.ToString());
Response.AddHeader("Content-Disposition", "attachment; filename="+pic.Length.ToString()+".bmp; size=" + pic.Length.ToString());