Elmah email rate limiter

ElmahRateLimiter.cs

using System;
using System.Collections.Concurrent;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading;
using Elmah;

public class ElmahRateLimiter
{
    private readonly static ConcurrentDictionary<string, DateTime> Exceptions = new ConcurrentDictionary<string, DateTime>();
    private readonly static ThreadLocal<HashAlgorithm> Hash = new ThreadLocal<HashAlgorithm>(() => new MD5CryptoServiceProvider());
    private readonly TimeSpan rateLimitTime = TimeSpan.FromMinutes(5);

    public ElmahRateLimiter(TimeSpan rateLimitTime)
    {
        this.rateLimitTime = rateLimitTime;
    }

    /// <summary>
    /// Throttle errors to avoid flooding log sinks with the same error
    /// </summary>
    public void RateLimit(ExceptionFilterEventArgs e)
    {
        var key = Fingerprint(e.Exception);
        
        // If an email for this event has already been sent within the rate limiting period, don't send another
        DateTime expiry;
        if (Exceptions.TryGetValue(key, out expiry))
        {
            if (expiry > DateTime.UtcNow)
            {
                e.Dismiss();
                return;
            }
        }

        expiry = DateTime.UtcNow.Add(this.rateLimitTime);
        Exceptions.AddOrUpdate(key, expiry, (k, v) => expiry);

        // Remove expired exceptions
        var keysToRemove = Exceptions
            .Where(x => x.Value < DateTime.UtcNow)
            .Select(x => x.Key)
            .ToList();

        foreach (var keyToRemove in keysToRemove)
        {
            DateTime tmp;
            Exceptions.TryRemove(keyToRemove, out tmp);
        }
    }

    /// <summary>
    /// Create a unique fingerprint from an exception
    /// </summary>
    /// <remarks>
    /// Note that we only consider the exception message, as the full stacktrace is likely to contain
    /// text that we expect to change in every exception instance, such as timestamps
    /// </remarks>
    public string Fingerprint(Exception e)
    {
        var bytes = Encoding.Unicode.GetBytes(e.Message.ToCharArray());
        var hash = Hash.Value.ComputeHash(bytes);

        return Convert.ToBase64String(hash);
    }
}