问题
On my site, users can upload images that are displayed publicly. When the user uploads a new image, it overwrites one of their old images.
Because the images can be large, I want them to be cached. However, when the user uploads a new image, the old cached images should not be used.
The source of the dynamic images looks like this:
userImage.ashx?id=f488b864-0a0b-46d9-af4a-a43cd0dcf751&type=micrositePhoto
So I can inject headers into the response in the .ashx file if I need to.
I've been told to use etags for this purpose, but I don't know how to do this in asp.net. The .js and .css files on my site already have etags, but I don't know what is generating them. Also, any in-built systems to generate etags will probably not understand how my .ashx file works.
How do I generate etags for my site, and make it so that they work as they are supposed to?
回答1:
Looks to me that you should be able to do, for example:
Response.AddHeader("ETag","\"" + Guid.NewGuid().ToString() + "\"");
ETags must be enclosed inside "
NOTE Obviously, once an ETag is returned with an image it needs to be the same on subsequent requests. My code above was just a proof of concept. :)
Edit: Yep, just tested and confirmed using Firebug that the Response headers do contain the Guid that I created.
来源：https://stackoverflow.com/questions/7900986/generating-etags-for-images-in-asp-net