rate-limited-client.cs

public class MyLimitedApiClient
{
    private static readonly object _syncRoot = new object();
    private DateTime? _nextRequestTimeSlot = null;
    private readonly HttpClient _http;
    
    private static readonly TimeSpan RateLimitedInterval = TimeSpan.FromSeconds(0.1); // 10 per sec
    
    public async Task<T> GetJson<T>(string url) where T : class
    {
        var serialiser = new DataContractJsonSerializer(typeof(T));

    retry: // just for the "goto" controversy!
        DateTime slot;
        lock (_syncRoot)
        {
            slot = _nextRequestTimeSlot ?? DateTime.UtcNow;
            _nextRequestTimeSlot = slot + TimeSpan.FromSeconds(RateLimitedInterval);
        }
        var toWait = (slot - DateTime.UtcNow).TotalSeconds;
        if (toWait > 0)
        {
            // wait with a super-efficient async delay
            await Task.Delay((int)(toWait * 1000));
        }

        // do the request!
        var response = await _http.GetAsync(url, HttpCompletionOption.ResponseHeadersRead);
 
        // check for limited response
        var limited = response.StatusCode == (System.Net.HttpStatusCode)429;
        if (limited)
        {
            // get a new slot
            goto retry;
        }

        // read the response.. as an example, we'll deserialise as an object
        using (var stream = await response.Content.ReadAsStreamAsync())
        {
            var result = serialiser.ReadObject(stream) as T;
            return result;
        }
    }
}