SimpleRateLimiter.cs

using System;
using System.Threading;
using System.Threading.Tasks;

namespace Oz.RateLimiting {
    public class SimpleRateLimiter {
        // Fields
        private int _requestCount;
        private DateTime _timeWindowStartUtc;
        
        // Properties
        public int MaxRequests { get; set; }
        public TimeSpan TimeWindow { get; set; }

        /// <summary>
        /// Instantiates a simple rate limiter.
        /// </summary>
        /// <param name="maxRequests">The maximum number of requests per <paramref name="timeWindow"/>.</param>
        /// <param name="timeWindow">A <see cref="TimeSpan"/> representing the amount of time <paramref name="maxRequests"/> can be made.</param>
        public SimpleRateLimiter(int maxRequests, TimeSpan timeWindow) {
            _timeWindowStartUtc = DateTime.UtcNow;
            
            MaxRequests = maxRequests;
            TimeWindow = timeWindow;
        }

        /// <summary>
        /// Checks if the specified rate limit has been reached and if so, waits until we're safe to continue.
        /// </summary>
        /// <param name="cancellationToken">Cancellation token allowing for the interruption of this method.</param>
        /// <returns>Returns true when safe to continue. Returns false if cancelled.</returns>
        public async Task<bool> WaitForAvailabilityAsync(CancellationToken cancellationToken) {
            while (true) {
                lock (this) {
                    var utcNow = DateTime.UtcNow;

                    if ((utcNow - _timeWindowStartUtc) >= TimeWindow) {
                        _timeWindowStartUtc = utcNow;
                        _requestCount = 0;
                    }

                    if (_requestCount < MaxRequests) {
                        _requestCount++;
                        return true;
                    }
                }

                await Task.Delay(100, cancellationToken);

                if (cancellationToken.IsCancellationRequested)
                    return false;
            }
        }
    }
}