从SQL Server读取Blob并下载

Question

I am updating an asp.net application and saving and retrieving documents from a mssql database. Saving was very simple but is there a way to download a file from an anchor that directly downloads from the database? Lets just say that that blob is bound within a gridview or some other data control, I would like to do this, example: <a href="<%#Eval("blobcolumn")%">Download Doc</a> . I save the file name as well.

Answer 1

First, you wouldn't assign the image to the href element, you would create an img tag to display the image, then the href tag to download it or embed the image tag directly in the href:

<a href="http://somelink.com"><img src="<%#Eval("blobcolumn")%></img></a>

Second, displaying the image inline without Url to a file requires a Data URI , which means that a base-64 version of the image is embedded in the web page:

<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA
AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO
9TXL0Y4OHwAAAABJRU5ErkJggg==" alt="Red dot">

This approach means a substantial amount of extra traffic on your web server and your end user's machines since the image will not be cached as it would be if you provided an actual URL to the image in your web server (ie /images/reddot.png).

I think that you would be much better off saving copies of the image to the web server and providing urls to those images in your page.

If there is concern about the size or number of files, you could create batch jobs to purge files older than a certain number of days or hours from the temporary image cache.

Answer 2

You can create a Handler to download data.

sample.ashx

---

public void ProcessRequest (HttpContext context) 
{
  string id = context.Request["id"];
  if (!string.IsNullOrEmpty(id))
     {
       //read byte[] and other data from database based upon given ID
       byte []ar=//result from database
       string fileName=//filename 
       context.Response.ContentType = "Application/octet-stream";
       context.Response.AddHeader("Content-Length", ar.Length.ToString());
       context.Response.AddHeader("Content-Disposition", "attachment; filename=" +  fileName );
       context.Response.BinaryWrite(ar);
       context.Response.Flush();
       context.Response.End();
      }
}

and request via hyperlinks

<a href="sample.ashx?id=101">Download .zip</a>
<a href='sample.ashx?id=<%#Eval("ID")%>'><%#Eval("Desc")%></a>