Response is not available in this context in event

I have a ASP.NET webpage call page1. In Page_Load(), I created a watcher that watches change of score.txt file:

    var watcher = new FileSystemWatcher(@"D:\");
    watcher.EnableRaisingEvents = true;
    watcher.IncludeSubdirectories = true;
    watcher.Filter = "score.txt";

I want the page to refresh when score.txt file changes, so I add an event like this:

watcher.Changed += (o, a) => 
{
    Response.Redirect(Request.RawUrl);
};

Unfortunately, when I changed the score.txt file manually and save it, the VS2017 throws an exception right at Response.Redirect(Request.RawUrl): System.Web.HttpException: 'Response is not available in this context.'

I try googling but there is still no solution. Could anyone point out what's wrong with my program? Thanks in advance!