VaryByCustom.cs

public override String GetVaryByCustomString(HttpContext context, String args)
{
	//context.Items["VaryByCustom"] = args;
	args = args?.Trim(';');
	if (string.IsNullOrWhiteSpace(args)) return string.Empty;

	return String.Join(';', args.Split(';').Select(a => VaryByCustomStringOptionSingle(context, a)).ToArray());
}


private static readonly Regex KeyWithParams = new Regex(@"(?<Key>.+)(?:\[(?<Param>[^\[]+)\])?", RegexOptions.RightToLeft);
private String VaryByCustomStringOptionSingle(HttpContext context, String arg)
{
	Match result = KeyWithParams.Match(arg);
	var keyM = result.Groups["Key"];
	var paramM = result.Groups["Param"];

	var key = keyM.Value;
	var param = paramM.Value;

	var hasParam = !String.IsNullOrWhiteSpace(param);
	switch (key)
	{
		case "HttpMethod":
			return "HttpMethod="+Request.HttpMethod;
		case "Locale":
			return "Locale="+context.Request.Headers["Accept-Language"]; // Request.UserLanguages
		case "IsAuthenticated":
			return "IsAuthenticated=" + context.Request.IsAuthenticated;
		case "IsSecureConnection":
			return "IsSecureConnection=" + Request.IsSecureConnection;
		case "Roles":
			if(context.Request.IsAuthenticated)
			{
				string[] roles = RolesCache.GetRolesForUser();
				if(roles!=null && roles.Length!=0)
					return "Roles=" + roles.Join(',');
			}
			return "Roles=none";
		case "UserNegatedId":
			if (context.Request.QueryString.IsBlank(hasParam ? param : "id") && context.User != null && context.User.Identity!=null)
				return "UserName=" + context.User.Identity.Name;

			return "UserNegatedId";
		case "AbsolutePath":
			return "AbsolutePath=" + context.Request.Url.AbsolutePath;
		case "AbsoluteFolderPath":
			string ap = context.Request.Url.AbsolutePath;
			int lastSlash = ap.LastIndexOf("/", StringComparison.Ordinal);
			return "AbsoluteFolderPath=" + (lastSlash<0 ? "/" : ap.Substring(0, lastSlash) );
		case "Param":
			if (hasParam)
				return key + "=" + (context.Request.Params[param] ?? String.Empty);
			break;

	}

	return base.GetVaryByCustomString(context, arg);
}

protected void Application_EndRequest(object sender, EventArgs e)
{
    if (Request.Form.Count != 0) Response.Cache.SetNoServerCaching();
}