Question

MVC 3 RTM. I have an action that returns a file (image/jpeg). I am trying to set the ETag for a file with no success (the etag doesn’t come through on header). I have tried both Response.Cache.SetETag and Response.AppenderHeader. If I add my own custom header tag, it works as expected, it just seems to be ETag that I can’t set.

Here is the source.

        [HttpGet, OutputCache(Location= OutputCacheLocation.Client, VaryByParam="userId", Duration=3600, NoStore=true)]
        public ActionResult GetImage(string userId)
        {
            byte[] result;

            using (var client = new WebClient())
            {
                client.Credentials = CredentialCache.DefaultCredentials; 
                result = client.DownloadData(string.Format(IntranetUrl, userId));
            }

            Response.Cache.SetETag("00amyWGct0y_ze4lIsj2Mw");
            //or Response.AppendHeader("ETag", "00amyWGct0y_ze4lIsj2Mw");
            Response.AppendHeader("MyHeader", "HelloWorld");

            return File(result, "image/jpeg");
        }

And here is the resource request/response:

> Request
> URL:http://localhost/MyApp/Employee.mvc/GetImage?userId=myUserId
> Request Method:GET Status Code:200 OK
> Request Headers Accept:*/*
> Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
> Accept-Encoding:gzip,deflate,sdch
> Accept-Language:en-US,en;q=0.8
> Cache-Control:max-age=0
> Connection:keep-alive
> Cookie:ASP.NET_SessionId=mySessionId
> Host:localhost
> Referer:http://localhost/MyApp/Employee/Review/24/Index
> User-Agent:Mozilla/5.0 (Windows; U;
> Windows NT 5.1; en-US)
> AppleWebKit/534.13 (KHTML, like Gecko)
> Chrome/9.0.597.98 Safari/534.13 Query
> String Parameters userId:myUserId
> Response Headers
> Cache-Control:private, no-store,
> max-age=3600 Content-Length:1428
> Content-Type:image/jpeg Date:Thu, 17
> Feb 2011 15:45:30 GMT Expires:Thu, 17
> Feb 2011 16:45:29 GMT
> Last-Modified:Thu, 17 Feb 2011
> 15:45:29 GMT MyHeader:HelloWorld
> Server:Microsoft-IIS/5.1
> X-AspNet-Version:4.0.30319
> X-AspNetMvc-Version:3.0
> X-Powered-By:ASP.NET

Update

I’ve stripped all the code down to this and still no go…

Action:

public FileETagActionResult GetImage()
{
    return new FileETagActionResult();
}

ActionResult:

    public class FileETagActionResult : ActionResult
    {

        public override void ExecuteResult(ControllerContext context)
        {

            byte[] result;

            using (var client = new WebClient())
            {
                result = client.DownloadData("http://myintranet/images/logo.png");
            }

            var hash = MD5.Create().ComputeHash(result);
            string etag = String.Format("\"{0}\"", Convert.ToBase64String(hash));

            TimeSpan expireTs = TimeSpan.FromDays(5);

            context.HttpContext.Response.Cache.SetCacheability(HttpCacheability.Private);
            context.HttpContext.Response.Cache.SetETag(etag);
            context.HttpContext.Response.Cache.SetExpires(DateTime.Now.AddDays(5));
            context.HttpContext.Response.Cache.SetMaxAge(expireTs);

            context.HttpContext.Response.ContentType = "image/png";
            context.HttpContext.Response.BinaryWrite(result);


        }
    }

Answer 1

The ETag will be suppressed if you use HttpCacheability.Private.

You can find more information on this question

If you change it to HttpCacheability.ServerAndPrivate it should work