caching - Create ETag filter in ASP.NET MVC

Question

I would like to create an ETag filter in MVC. The problem is that I can't control the Response.OutputStream, if I was able to do that I would simply calculate the ETag according to the result stream. I did this thing before in WCF but couldn't find any simple idea to do that in MVC.

I want to be able to write something like that

[ETag]
public ActionResult MyAction()
{
    var myModel = Factory.CreateModel();
    return View(myModel);
}

Any idea?

Answer 1

This is the best I could come up with, I didn't really understand what you meant by you can't control the Response.OutputStream.

using System;
using System.IO;
using System.Security.Cryptography;
using System.Web.Mvc;

public class ETagAttribute : ActionFilterAttribute
{
    private string GetToken(Stream stream) {
        MD5 md5 = MD5.Create();
        byte [] checksum = md5.ComputeHash(stream);
        return Convert.ToBase64String(checksum, 0, checksum.Length);
    }

    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        filterContext.HttpContext.Response.AppendHeader("ETag", GetToken(filterContext.HttpContext.Response.OutputStream));
        base.OnResultExecuted(filterContext);
    }
}

This should work, but is doesn't.

Apparently Microsoft overrode System.Web.HttpResponseStream.Read(Byte[] buffer, Int32 offset, Int32 count) so that it returns "Specified method is not supported.", not sure why they would do that, since it inherits for the System.IO.Stream base class...

Which is mix up of the following resources, the Response.OutputStream is a write only stream, so we have to use a Response.Filter class to read the output stream, kind of quirky that you have to use a filter on a filter, but it works =)

http://bytes.com/topic/c-sharp/answers/494721-md5-encryption-question-communication-java
http://www.codeproject.com/KB/files/Calculating_MD5_Checksum.aspx
http://blog.gregbrant.com/post/Adding-Custom-HTTP-Headers-to-an-ASPNET-MVC-Response.aspx
http://www.infoq.com/articles/etags
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

Update

After much fighting I was finally able to get this to work:

using System;
using System.IO;
using System.Security.Cryptography;
using System.Web;
using System.Web.Mvc;

public class ETagAttribute : ActionFilterAttribute {
    public override void OnActionExecuting(ActionExecutingContext filterContext) {
        try {
            filterContext.HttpContext.Response.Filter = new ETagFilter(filterContext.HttpContext.Response);
        } catch (System.Exception) {
            // Do Nothing
        };
    }
}

public class ETagFilter : MemoryStream {
    private HttpResponseBase o = null;
    private Stream filter = null;

    public ETagFilter (HttpResponseBase response) {
        o = response;
        filter = response.Filter;
    }

    private string GetToken(Stream stream) {
        byte[] checksum = new byte[0];
        checksum = MD5.Create().ComputeHash(stream);
        return Convert.ToBase64String(checksum, 0, checksum.Length);
    }

    public override void Write(byte[] buffer, int offset, int count) {
        byte[] data = new byte[count];
        Buffer.BlockCopy(buffer, offset, data, 0, count);
        filter.Write(data, 0, count);
        o.AddHeader("ETag", GetToken(new MemoryStream(data)));
    }
}

More Resources:

http://authors.aspalliance.com/aspxtreme/sys/Web/HttpResponseClassFilter.aspx
http://forums.asp.net/t/1380989.aspx/1