How to use ETag in Web API using action filter along with HttpResponseMessage

My requirement was to cache my web api JSON responses... And all the solutions provided don't have an easy "link" to where the data is generated - ie in the Controller...

So my solution was to create a wrapper "CacheableJsonResult" which generated a Response, and then added the ETag to the header. This allows a etag to be passed in when the controller method is generated and wants to return the content...

public class CacheableJsonResult<T> : JsonResult<T>
{
    private readonly string _eTag;
    private const int MaxAge = 10;  //10 seconds between requests so it doesn't even check the eTag!

    public CacheableJsonResult(T content, JsonSerializerSettings serializerSettings, Encoding encoding, HttpRequestMessage request, string eTag)
        :base(content, serializerSettings, encoding, request)
    {
        _eTag = eTag;
    }

    public override Task<HttpResponseMessage> ExecuteAsync(System.Threading.CancellationToken cancellationToken)
    {
        Task<HttpResponseMessage> response = base.ExecuteAsync(cancellationToken);

        return response.ContinueWith<HttpResponseMessage>((prior) =>
        {
            HttpResponseMessage message = prior.Result;

            message.Headers.ETag = new EntityTagHeaderValue(String.Format("\"{0}\"", _eTag));
            message.Headers.CacheControl = new CacheControlHeaderValue
            {
                Public = true,
                MaxAge = TimeSpan.FromSeconds(MaxAge)
            };

            return message;
        }, cancellationToken);
    }
}

And then, in your controller - return this object:

[HttpGet]
[Route("results/{runId}")]
public async Task<IHttpActionResult> GetRunResults(int runId)
{               
    //Is the current cache key in our cache?
    //Yes - return 304
    //No - get data - and update CacheKeys
    string tag = GetETag(Request);
    string cacheTag = GetCacheTag("GetRunResults");  //you need to implement this map - or use Redis if multiple web servers

    if (tag == cacheTag )
            return new StatusCodeResult(HttpStatusCode.NotModified, Request);

    //Build data, and update Cache...
    string newTag = "123";    //however you define this - I have a DB auto-inc ID on my messages

    //Call our new CacheableJsonResult - and assign the new cache tag
    return new CacheableJsonResult<WebsiteRunResults>(results, GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings, System.Text.UTF8Encoding.Default, Request, newTag);

    }
}

private static string GetETag(HttpRequestMessage request)
{
    IEnumerable<string> values = null;
    if (request.Headers.TryGetValues("If-None-Match", out values))
        return new EntityTagHeaderValue(values.FirstOrDefault()).Tag;

    return null;
}

You need to define how granular to make your tags; my data is user-specific, so I include the UserId in the CacheKey (etag)

a good solution for ETag and in ASP.NET Web API is to use CacheCow . A good article is here.

It's easy to use and you don't have to create a custom Attribute. Have fun .u