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)
        {
            var text = new StringBuilder();
            text.Append($"<a href=\"tg://user?id={user.Id}\">");
            if (!string.IsNullOrWhiteSpace(user.Username))
                text.Append(user.Username);
            else
                text.Append($"{user.FirstName} {user.LastName}".Trim().HtmlEncode());
            text.Append("</a>");

            return text.ToString();
        }
    }
}