Program.cs

using System.Net.Sockets;
using System.Net.WebSockets;
using System.Reflection;
using ConsoleAppFramework;
using Mastonet;
using Mastonet.Entities;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using Sentry.Extensions.Logging.Extensions.DependencyInjection;

var configuration = new ConfigurationBuilder()
    .AddEnvironmentVariables()
    .Build();

var services = new ServiceCollection()
    .Configure<ConsoleOptions>(configuration)
    .AddLogging(b => b
        .AddConfiguration(configuration)
        .AddSentry(op =>
        {
            op.Dsn = "";
            op.SampleRate = 0.25f;
            op.CaptureFailedRequests = true;
            op.FailedRequestStatusCodes.Clear();
            op.FailedRequestStatusCodes.Add((400, 499));
            op.FailedRequestStatusCodes.Add((501, 599));
            op.SetBeforeSend(BeforeSend);
        })
        .AddConsole())
    .AddHttpClient();

static SentryEvent? BeforeSend(SentryEvent ev, SentryHint hint)
{
    // ConnectionRefusedの場合はサーバー起動してないので無視
    if (ev.Exception is HttpRequestException &&
        ev.Exception.InnerException is SocketException se &&
        se.SocketErrorCode == SocketError.ConnectionRefused)
    {
        return null;
    }
    // ConnectionClosedPrematurelyの場合はたぶんサーバーが再起動したので、再接続する
    else if (ev.Exception is WebSocketException wse &&
        wse.WebSocketErrorCode == WebSocketError.ConnectionClosedPrematurely)
    {
        return null;
    }
    return ev;
}

using var serviceProvider = services.BuildServiceProvider();
ConsoleApp.ServiceProvider = serviceProvider;

using (var sp = serviceProvider.CreateScope())
{
    var logger = sp.ServiceProvider.GetRequiredService<ILogger<Program>>();
    using var scope = logger.BeginScope("startup");
    var assembly = Assembly.GetExecutingAssembly();
    var version = assembly.GetCustomAttribute<AssemblyInformationalVersionAttribute>()?.InformationalVersion
        ?? assembly.GetName().Version?.ToString();
    logger.LogInformation($"Ver: {version}");
}

await ConsoleApp.RunAsync(args, Run);

static async Task Run([FromServices] ILogger<Program> logger, [FromServices] IOptions<ConsoleOptions> options, [FromServices] HttpClient http)
{
    var (mastodonUrl, mastodonToken, followerThreshold) = options.Value;
    var client = new MastodonClient(mastodonUrl, mastodonToken, http);

    var me = await client.GetCurrentUser();
    var follows = (await client.GetAccountFollowing(me.Id)).Select(x => x.Id).ToHashSet();

    var pub = client.GetPublicStreaming();
    pub.OnUpdate += async (_, e) =>
    {
        logger.LogInformation($"Update: {e.Status.Account.AccountName} {e.Status.Id}");
        await FollowIfTarget(logger, client, e.Status, follows, followerThreshold);
    };
    var home = client.GetUserStreaming();
    home.OnUpdate += async (_, e) =>
    {
        logger.LogInformation($"Home: {e.Status.Account.AccountName} {e.Status.Id}");
        var status = e.Status.Reblog;
        if (status is null)
        {
            return;
        }
        await FollowIfTarget(logger, client, status, follows, followerThreshold);
    };
    await Task.WhenAll(pub.Start(), home.Start());
}

static async Task FollowIfTarget(ILogger logger, MastodonClient client, Status status, ISet<string> follows, int followerThreshold)
{
    // フォロワー数が閾値未満ならフォローしない
    var follow = status.Account.FollowersCount > followerThreshold;
    // 既にフォローしているならフォローしない
    follow &= !follows.Contains(status.Account.Id);
    // botはフォローしない
    follow &= !status.Account.Bot ?? false;
    // 日本語以外の投稿はフォローしない
    follow &= status.Language == "ja";
    // フォロワーよりフォローしているならフォローしない
    follow &= status.Account.FollowingCount <= status.Account.FollowersCount;
    if (!follow)
    {
        return;
    }
    await client.Follow(status.Account.Id, true);
    follows.Add(status.Account.Id);
    logger.LogInformation($"Followed: {status.Account.AccountName}");
}

class ConsoleOptions
{
    public required string MastodonUrl { get; init; }
    public required string MastodonToken { get; init; }
    public int FollowerThreshold { get; init; } = 1000;

    public void Deconstruct(out string mastodonUrl, out string mastodonToken, out int followerThreshold)
    {
        mastodonUrl = MastodonUrl;
        mastodonToken = MastodonToken;
        followerThreshold = FollowerThreshold;
    }
}