Simple way to throttle parts of your Asp.Net web app

using (new Throttle()) {
  ... do my expensive operation here ...
}

using System;
using System.Collections;
using System.Web;
using System.Web.Caching;

/// <summary>
/// This is used to throttle anything really, it throws an exception if too many are running simultaneously
/// It works by storing the runs in the http cache
/// Use this with a 'using' statement surrounding your work, eg:
/// 
/// using (new Throttle()) {
///   ... work to be throttled ...
/// }
/// </summary>
public class Throttle : IDisposable
{
  // Used to lock all instances of the throttle, so they don't walk over each other when using the http cache
  static object cacheLock = new object();
  // Max simultaneous users
  const int max = 10;
  // The timeout in seconds at which to consider each run to have finished anyway, eg in case the throttle forgets to clear it
  const int timeout = 30;
  // What prefix to give the names in the cache
  const string cacheEntryNamePrefix = "Throttle";
  // The name of the cache entry used by this particular throttled run
  string thisCacheEntryName;

  /// <summary>
  /// Checks that we haven't hit the throttle, and if so it throws an exception
  /// Then it registers the current run in the cache, to be forgotten when finished
  /// </summary>
  public Throttle() {
    lock (cacheLock) {
      // Check we haven't hit the maximum
      int runners = 0;
      foreach (DictionaryEntry kv in HttpContext.Current.Cache)
        if (kv.Key.ToString().StartsWith(cacheEntryNamePrefix))
          runners++;
      if (runners >= max)
        throw new Exception("Too many people accessing the system right now. Please retry soon.");

      // Register this one into the cache
      thisCacheEntryName = cacheEntryNamePrefix + System.Guid.NewGuid().ToString();
      HttpContext.Current.Cache.Add(thisCacheEntryName, 1, null, DateTime.Now.AddSeconds(timeout), Cache.NoSlidingExpiration, CacheItemPriority.Normal, null);
    }
  }

  /// <summary>
  /// Removes the entry from the cache
  /// </summary>
  public void Dispose() {
    lock (cacheLock) {
      HttpContext.Current.Cache.Remove(thisCacheEntryName);
    }
  }
}