public class YourController : Controller

{

    public ActionResult MyPage(string id)

    {

        var entity = _db.Get(id);

        var headerValue = context.HttpContext.Request.Headers['If-Modified-Since'];

        if (headerValue != null)

        {

            var modifiedSince = DateTime.Parse(headerValue).ToLocalTime();

            if (modifiedSince >= entity.UpdatedAt)

            {

               context.HttpContext.Response.StatusCode = 304;

               return new EmptyResult();

            }

        }

        // page has been changed.

        // generate a view ...

        // .. and set last modified in the date format specified in the HTTP rfc.

        context.HttpContext.Response.AddHeader('Last-Modified', entity.UpdatedAt.ToUniversalTime().ToString("R"));

    }

}