Predict gender from names in C# with .NET 8

using System.Net;
using System.Net.Http.Headers;
using System.Net.Http.Json;
using System.Text.Json;

// Finowl gender fields are snake_case (result_status, confidence_kind...).
var json = new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower };

using var http = new HttpClient
{
    BaseAddress = new Uri("https://api.finowl.co"),
    Timeout = TimeSpan.FromSeconds(15),
};
http.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer",
    Environment.GetEnvironmentVariable("FINOWL_API_KEY") ?? throw new InvalidOperationException("Set FINOWL_API_KEY."));

foreach (var (name, country) in new[] { ("Andrea", "IT"), ("Andrea", "US"), ("Kim", "KR") })
{
    var prediction = await PredictAsync(name, country);
    Console.WriteLine(prediction.ResultStatus == "unknown"
        ? $"{name}/{country}: unknown ({prediction.Reason})"
        : $"{name}/{country}: {prediction.Gender} {prediction.Confidence:P0} via {prediction.Source}");
}

async Task<GenderPrediction> PredictAsync(string name, string country)
{
    while (true)
    {
        using var response = await http.PostAsJsonAsync("/v1/gender/name", new { name, country }, json);

        if (response.IsSuccessStatusCode)
            return (await response.Content.ReadFromJsonAsync<GenderEnvelope>(json))!.Data;

        if (response.StatusCode == HttpStatusCode.TooManyRequests && response.Headers.RetryAfter?.Delta is { } wait)
        {
            await Task.Delay(wait); // rate_limited: safe to retry after the header value
            continue;
        }

        throw response.StatusCode switch
        {
            HttpStatusCode.BadRequest => new ArgumentException($"Invalid name or country code: {name}, {country}"),
            HttpStatusCode.Unauthorized => new InvalidOperationException("FINOWL_API_KEY was rejected."),
            HttpStatusCode.TooManyRequests => new InvalidOperationException("Out of Finowl credits (quota_exceeded)."),
            _ => new HttpRequestException($"Finowl returned {(int)response.StatusCode}."),
        };
    }
}

record GenderEnvelope(GenderPrediction Data);
record GenderPrediction(
    string ResultStatus, string? Gender, double? Confidence, string? ConfidenceKind,
    int? SampleCount, string? Source, string? Reason);