Usage.cs

using System;
using System.Threading.Tasks;
using Telegram.Bot;
using Telegram.Bot.Exceptions;
using Telegram.Bot.Extensions.Polling;
using Telegram.Bot.Types;
using Telegram.Bot.Types.Enums;

namespace AsyncBot
{
    class Program
    {
        public static TelegramBotClient Bot = new TelegramBotClient("123:abc");

        public static async Task Main()
        {
            await Bot.ReceiveAsync(new DefaultUpdateHandler(HandleUpdateAsync, HandleExceptionAsync));
            // OR
            await Bot.ReceiveAsync<CustomUpdateHandler>();
        }

        public class CustomUpdateHandler : IUpdateHandler
        {
            public UpdateType[]? AllowedUpdates => null;

            public Task ErrorOccurred(Exception exception)
            {
                return HandleExceptionAsync(exception);
            }

            public Task UpdateReceived(Update update)
            {
                return HandleUpdateAsync(update);
            }
        }

        public static async Task HandleUpdateAsync(Update update)
        {
            if (update.Message is Message message && message.Text is string text)
            {
                await Bot.SendTextMessageAsync(message.Chat, "Echo:\n" + text);
            }
        }

        public static async Task HandleExceptionAsync(Exception exception)
        {
            if (exception is ApiRequestException apiRequestException)
            {
                Console.WriteLine(apiRequestException.ToString());
            }
            else
            {
                await Bot.SendTextMessageAsync(123, "Something went wrong:\n" + exception.ToString());
            }
        }
    }
}