A WebForms user control to add a query string to prevent browser caching when the file changes. Works with JavaScript files (script tags) and CSS stylesheets (link tags).

FileVersioner.ascx.cs

/*
Usage:
For JavaScript Files
<asp:FileVersioner Path="~/js/plugins/jQuery-1.7.2.js" Type="script" runat="server"></asp:FileVersioner>

For CSS stylesheets
<asp:FileVersioner Path="~/stylesheets/CustomerPortal.css" Type="style" runat="server"></asp:FileVersioner>
*/

using System;
using System.Diagnostics;
using System.IO;
using System.Security.Cryptography;
using System.Web.UI;

namespace Site.Controls
{
    public partial class FileVersioner : UserControl
    {
        public string Type { get; set; }
        public string Path { get; set; }

        private const string _scriptTemplate = "<script src=\"{0}?ver={1}\" type=\"text/javascript\"></script>";
        private const string _styleTemplate = "<link href=\"{0}?ver={1}\" rel=\"stylesheet\" type=\"text/css\" />";

        public FileVersioner()
        {
            Type = string.Empty;
            Path = string.Empty;
        }

        protected void Page_Load(object sender, EventArgs e)
        {
            var checksum = GetChecksum();
            if (Type.ToLower() == "script")
            {
                var scriptTag = new LiteralControl { Text = string.Format(_scriptTemplate, Path, checksum) };
                Controls.Add(scriptTag);
                return;
            }

            if (Type.ToLower() == "style")
            {
                var scriptTag = new LiteralControl { Text = string.Format(_styleTemplate, Path, checksum) };
                Controls.Add(scriptTag);
                return;
            }

            throw new ArgumentException("The Type attribute must be 'script' or 'style'.");
        }

        private string GetChecksum()
        {
            var physicalFile = MapPath(Path);

            if (!File.Exists(physicalFile))
                return "0";

            var lastModified = File.GetLastWriteTime(physicalFile);

            if (Cache["FileVersioner:" + Path] != null)
            {
                var cachedInfo = Cache["FileVersioner:" + Path] as FileVersionInfo;
                if (lastModified == cachedInfo.LastModified)
                {
                    return cachedInfo.Checksum;
                }

                Debug.WriteLine("Cached Checksum is invalid for {0}", Path, 0);
                Cache.Remove("FileVersioner:" + Path);
            }

            Debug.WriteLine("Caclulating Checksum for {0}", Path, 0);

            using (var stream = File.OpenRead(physicalFile))
            {
                var sha = new SHA1Managed();
                var checksum = sha.ComputeHash(stream);
                var fileChecksum = BitConverter.ToString(checksum).Replace("-", String.Empty);

                Cache.Insert("FileVersioner:" + Path, new FileVersionInfo { Checksum = fileChecksum, LastModified = lastModified });
                return fileChecksum;
            }
        }

        private class FileVersionInfo
        {
            public string Checksum { get; set; }
            public DateTime LastModified { get; set; }
        }
    }
}