telegram-bot/Core/Extensions/UserExtension.cs

using System.Text;
using Telegram.Bot.Types;

namespace Kruzya.TelegramBot.Core.Extensions
{
    public static class UserExtension
    {
        public static string ToHtml(this User user, bool byUsername = true)
        {
            var text = new StringBuilder();
            text.Append($"<a href=\"tg://user?id={user.Id}\">");
           
            if (byUsername && !string.IsNullOrWhiteSpace(user.Username))
            {
                text.Append(user.Username);
            }
            else
            {
                text.Append($"{user.FirstName} {user.LastName}".Trim().HtmlEncode());
            }
            return text.Append("</a>").ToString();
        }
    }
}