BotHandler.cs

using System.IO;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;
using Telegram.Bot.Types;

namespace DotNetGroup
{
    public static class BotHandler
    {
        [FunctionName("BotHandler")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = null)] HttpRequest req,
            ILogger log)
        {
            var content = await new StreamReader(req.Body).ReadToEndAsync();
            var body = JsonConvert.DeserializeObject<Update>(content);

            if (body?.Message is null)
            {
                log.LogInformation("Not a message");
                return new OkResult();
            }

            var newMembers = body.Message.NewChatMembers;

            if (newMembers is null || newMembers.Length == 0)
            {
                log.LogInformation("No user joined");
                return new OkResult();
            }

            if (newMembers.Length > 1)
            {
                log.LogInformation($"Multiple users joined");
                const string multiUserText = "Welcome to the group, please **read the rules** in the group description and remember job posts **are not allowed** and will cause an immediate kick+ban";
                return TextMessageResult(body.Message, multiUserText);
            }

            var tagString = GetTagString(body.Message.NewChatMembers.First());
            log.LogInformation("Single user joined");
            var text =
                $"Welcome to the group {tagString}, please **read the rules** in the group description and remember job posts **are not allowed** and will cause an immediate kick+ban";
            return TextMessageResult(body.Message, text);
        }

        private static string GetTagString(User user)
        {
            if (!string.IsNullOrWhiteSpace(user.Username))
                return $"@{user.Username}";
            
            if (!string.IsNullOrWhiteSpace(user.LastName))
                return $"[{user.FirstName} {user.LastName}](tg://user?id={user.Id})";
            
            return $"[{user.FirstName}](tg://user?id={user.Id})";
        }

        private static IActionResult TextMessageResult(Message message, string text)
        {
            var response = new
            {
                chat_id = message.Chat.Id,
                text,
                parse_mode = "Markdown",
                disable_notification = true,
                reply_to_message_id = message.MessageId,
                method = "sendMessage"
            };
            
            return new OkObjectResult(response);
        }
    }
}