下载完后，给用户个提示
string name = @"c:\123.rar";
  FileInfo aFile = new FileInfo(name);

  Response.Clear();
  Response.ClearHeaders();
  Response.BufferOutput = false;
  Response.ContentType = "application/octet-stream";
  Response.AppendHeader("Content-disposition", "attachment;filename=" + HttpUtility.UrlEncode("123.rar", Encoding.UTF8));
  Response.AddHeader("Content-Length", aFile.Length.ToString());
  Response.WriteFile(name);
  Response.Flush();
   
  Response.End();

下载完后，想给用户一个提示框，提示下载成功

------解决方案--------------------
try{
string name = @"c:\123.rar";
FileInfo aFile = new FileInfo(name);

Response.Clear();
Response.ClearHeaders();
Response.BufferOutput = false;
Response.ContentType = "application/octet-stream";
Response.AppendHeader("Content-disposition", "attachment;filename=" + HttpUtility.UrlEncode("123.rar", Encoding.UTF8));
Response.AddHeader("Content-Length", aFile.Length.ToString());
Response.WriteFile(name);
Response.Flush();

Response.End();

Response.Write("<script>alert('下载成功!');</script>");
 
}
 catch{
Response.Write("<script>alert('下载失败!');</script>");
}
------解决方案--------------------
在最后加上
Response.Write(" <script> alert('哈哈'); </script> ");
------解决方案--------------------
RegisterClientScriptBlock("01","<script>alert('成功')</script>");
------解决方案--------------------
Response.Write("<script>alert('下载完成！');</script>");
------解决方案--------------------

C# code

Page.RegisterStartupScript("ALERT", <script language='javascript'>alert('DONE');</script>)