MVC4 StyleBundle: Can you add a cache-busting query string in Debug mode?

You just need a unique string. It doesn't have to be Hash. We use the LastModified date of the file and get the Ticks from there. Opening and reading the file is expensive as @Todd noted. Ticks is enough to output a unique number that changes when the file is changed.

internal static class BundleExtensions
{
    public static Bundle WithLastModifiedToken(this Bundle sb)
    {
        sb.Transforms.Add(new LastModifiedBundleTransform());
        return sb;
    }
    public class LastModifiedBundleTransform : IBundleTransform
    {
        public void Process(BundleContext context, BundleResponse response)
        {
            foreach (var file in response.Files)
            {
                var lastWrite = File.GetLastWriteTime(HostingEnvironment.MapPath(file.IncludedVirtualPath)).Ticks.ToString();
                file.IncludedVirtualPath = string.Concat(file.IncludedVirtualPath, "?v=", lastWrite);
            }
        }
    }
}

and how to use it:

bundles.Add(new StyleBundle("~/bundles/css")
    .Include("~/Content/*.css")
    .WithLastModifiedToken());

and this is what MVC writes:

<link href="bundles/css/site.css?v=635983900813469054" rel="stylesheet"/>

works fine with Script bundles too.

You can create a custom IBundleTransform class to do this. Here's an example that will append a v=[filehash] parameter using a hash of the file contents.

public class FileHashVersionBundleTransform: IBundleTransform
{
    public void Process(BundleContext context, BundleResponse response)
    {
        foreach(var file in response.Files)
        {
            using(FileStream fs = File.OpenRead(HostingEnvironment.MapPath(file.IncludedVirtualPath)))
            {
                //get hash of file contents
                byte[] fileHash = new SHA256Managed().ComputeHash(fs);

                //encode file hash as a query string param
                string version = HttpServerUtility.UrlTokenEncode(fileHash);
                file.IncludedVirtualPath = string.Concat(file.IncludedVirtualPath, "?v=", version);
            }                
        }
    }
}

You can then register the class by adding it to the Transforms collection of your bundles.

new StyleBundle("...").Transforms.Add(new FileHashVersionBundleTransform());

Now the version number will only change if the file contents change.