How to I replace URLs in rendered HTML using an ASP.NET MVC ActionFilter

Question

I'm trying to create an ActionFilter to replace some text in my HTML. Basically when the server is using SSL I want to replace references to my CDN (http://cdn.example.com) with references directly to my server (https://www.example.com). So the structure is something like this (I assume OnResultExecuted is where I should start):

public class CdnSslAttribute : ActionFilterAttribute
{
    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        if(filterContext.HttpContext.Request.IsSecureConnection)
        {
            // when the connection is secure,
            // somehow replace all instances of http://cdn.example.com
            // with https://www.example.com
        }
    }
}

This would be used in my secure controllers:

[CdnSsl] 
public class SecureController : Controller
{
}

The reason I want to do this is my CDN doesn't support SSL. And there are references in the Master pages to CDN resources. Example:

<link href="http://cdn.example.com/Content/base.css" rel="stylesheet" type="text/css" />

Answer 1

I ended up using a variation on this blog post:

http://arranmaclean.wordpress.com/2010/08/10/minify-html-with-net-mvc-actionfilter/

with my own filter:

public class CdnSslAttribute : ActionFilterAttribute
{
    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        if (filterContext.HttpContext.Request.IsSecureConnection)
        {
            var response = filterContext.HttpContext.Response;
            response.Filter = new CdnSslFilter(response.Filter);
        }
    }
}

Then the filter looks like this (some code omitted for brevity):

public class CdnSslFilter : Stream
{
    private Stream _shrink;
    private Func<string, string> _filter;

    public CdnSslFilter(Stream shrink)
    {
        _shrink = shrink;
        _filter = s => Regex.Replace(s,@"http://cdn\.","https://www.", RegexOptions.IgnoreCase);
    }

    //overridden functions omitted for clarity. See above blog post.

    public override void Write(byte[] buffer, int offset, int count)
    {
        // capture the data and convert to string 
        byte[] data = new byte[count];
        Buffer.BlockCopy(buffer, offset, data, 0, count);
        string s = Encoding.Default.GetString(buffer);

        // filter the string
        s = _filter(s);

        // write the data to stream 
        byte[] outdata = Encoding.Default.GetBytes(s);
        _shrink.Write(outdata, 0, outdata.GetLength(0));
    }
}