A long while back I wrote about GZip compression in ASP.NET. In that article I describe two generic helper methods that I've used in all sorts of ASP.NET application from WebForms apps to HttpModules and HttpHandlers that require gzip or deflate compression. The same static methods also work in ASP.NET MVC.

Here are the two routines:

/// <summary>
/// Determines if GZip is supported
/// </summary>
/// <returns></returns>
public static bool IsGZipSupported()
{
    string AcceptEncoding = HttpContext.Current.Request.Headers["Accept-Encoding"];
    if (!string.IsNullOrEmpty(AcceptEncoding) &&
            (AcceptEncoding.Contains("gzip") || AcceptEncoding.Contains("deflate")))
        return true;
    return false;
}

/// <summary>
/// Sets up the current page or handler to use GZip through a Response.Filter
/// IMPORTANT:  
/// You have to call this method before any output is generated!
/// </summary>
public static void GZipEncodePage()
{
    HttpResponse Response = HttpContext.Current.Response;

    if (IsGZipSupported())
    {
        string AcceptEncoding = HttpContext.Current.Request.Headers["Accept-Encoding"];

        if (AcceptEncoding.Contains("gzip"))
        {
            Response.Filter = new System.IO.Compression.GZipStream(Response.Filter,
                                        System.IO.Compression.CompressionMode.Compress);
            Response.Headers.Remove("Content-Encoding");
            Response.AppendHeader("Content-Encoding", "gzip");
        }
        else
        {
            Response.Filter = new System.IO.Compression.DeflateStream(Response.Filter,
                                        System.IO.Compression.CompressionMode.Compress);
            Response.Headers.Remove("Content-Encoding");
            Response.AppendHeader("Content-Encoding", "deflate");
        }
    }

    // Allow proxy servers to cache encoded and unencoded versions separately
    Response.AppendHeader("Vary", "Content-Encoding");
}

The first method checks whether the client sending the request includes the accept-encoding for either gzip or deflate, and if if it does it returns true. The second function uses IsGzipSupported() to decide whether it should encode content and uses an Response Filter to do its job. Basically response filters look at the Response output stream as it's written and convert the data flowing through it. Filters are a bit tricky to work with but the two .NET filter streams for GZip and Deflate Compression make this a snap to implement.

In my old code and even now in MVC I can always do:

public ActionResult List(string keyword=null, int category=0)
{
    WebUtils.GZipEncodePage();    …}

to encode my content. And that works just fine.

The proper way: Create an ActionFilterAttribute

However in MVC this sort of thing is typically better handled by an ActionFilter which can be applied with an attribute. So to be all prim and proper I created an CompressContentAttribute ActionFilter that incorporates those two helper methods and which looks like this:

/// <summary>
/// Attribute that can be added to controller methods to force content
/// to be GZip encoded if the client supports it
/// </summary>
public class CompressContentAttribute : ActionFilterAttribute
{

    /// <summary>
    /// Override to compress the content that is generated by
    /// an action method.
    /// </summary>
    /// <param name="filterContext"></param>
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        GZipEncodePage();
    }

    /// <summary>
    /// Determines if GZip is supported
    /// </summary>
    /// <returns></returns>
    public static bool IsGZipSupported()
    {
        string AcceptEncoding = HttpContext.Current.Request.Headers["Accept-Encoding"];
        if (!string.IsNullOrEmpty(AcceptEncoding) &&
                (AcceptEncoding.Contains("gzip") || AcceptEncoding.Contains("deflate")))
            return true;
        return false;
    }

    /// <summary>
    /// Sets up the current page or handler to use GZip through a Response.Filter
    /// IMPORTANT:  
    /// You have to call this method before any output is generated!
    /// </summary>
    public static void GZipEncodePage()
    {
        HttpResponse Response = HttpContext.Current.Response;

        if (IsGZipSupported())
        {
            string AcceptEncoding = HttpContext.Current.Request.Headers["Accept-Encoding"];

            if (AcceptEncoding.Contains("gzip"))
            {
                Response.Filter = new System.IO.Compression.GZipStream(Response.Filter,
                                            System.IO.Compression.CompressionMode.Compress);
                Response.Headers.Remove("Content-Encoding");
                Response.AppendHeader("Content-Encoding", "gzip");
            }
            else
            {
                Response.Filter = new System.IO.Compression.DeflateStream(Response.Filter,
                                            System.IO.Compression.CompressionMode.Compress);
                Response.Headers.Remove("Content-Encoding");
                Response.AppendHeader("Content-Encoding", "deflate");
            }


        }

        // Allow proxy servers to cache encoded and unencoded versions separately
        Response.AppendHeader("Vary", "Content-Encoding");
    }
}

It's basically the same code wrapped into an ActionFilter attribute, which intercepts requests MVC requests to Controller methods and lets you hook up logic before and after the methods have executed. Here I want to override OnActionExecuting() which fires before the Controller action is fired.

With the CompressContentAttribute created, it can now be applied to either the controller as a whole:

[CompressContent]
public class ClassifiedsController : ClassifiedsBaseController
{ … } 

or to one of the Action methods:

[CompressContent]    
public ActionResult List(string keyword=null, int category=0)
{ … }

The former applies compression to every action method, while the latter is selective and only applies it to the individual action method.

Is the attribute better than the static utility function? Not really, but it is the standard MVC way to hook up 'filter' content and that's where others are likely to expect to set options like this. In fact,  you have a bit more control with the utility function because you can conditionally apply it in code, but this is actually much less likely in MVC applications than old WebForms apps since controller methods tend to be more focused.

Compression Caveats

Http compression is very cool and pretty easy to implement in ASP.NET but you have to be careful with it - especially if your content might get transformed or redirected inside of ASP.NET. A good example, is if an error occurs and a compression filter is applied. ASP.NET errors don't clear the filter, but clear the Response headers which results in some nasty garbage because the compressed content now no longer matches the headers. Another issue is Caching, which has to account for all possible ways of compression and non-compression that the content is served. Basically compressed content and caching don't mix well. I wrote about several of these issues in an old blog post and I recommend you take a quick peek before diving into making every bit of output Gzip encoded.

None of these are show stoppers, but you have to be aware of the issues.

Related Posts

• GZip Compression with ASP.NET Content
• ASP.NET GZip Encoding Caveats