MarkdownEntitiesTransformer.cs

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

public class MarkdownEntitiesTransformer
{
    public string Transform(string message, MessageEntity[] entities)
    {
        var currentEntityIndex = 0;
        var currentCharacter = 0;

        var builder = new StringBuilder();

        while (currentCharacter < message.Length)
        {
            if (entities.Length == 0 || currentEntityIndex >= entities.Length)
            {
                builder.Append(message.Substring(currentCharacter));
                break;
            }

            var currentEntity = entities[currentEntityIndex];

            if (currentEntity.Offset == currentCharacter)
            {
                var currentEntityBody = message.Substring(currentEntity.Offset, currentEntity.Length);
                var transformedEntity = string.Empty;

                switch (currentEntity.Type)
                {
                    case MessageEntityType.TextLink:
                        transformedEntity = $"[{currentEntityBody}]({currentEntity.Url})";
                        break;
                    case MessageEntityType.Pre:
                        transformedEntity = $"```\n{currentEntityBody}```";
                        break;
                    case MessageEntityType.Bold:
                        transformedEntity = $"*{currentEntityBody}*";
                        break;
                    case MessageEntityType.Italic:
                        transformedEntity = $"_{currentEntityBody}_";
                        break;
                    case MessageEntityType.Code:
                        transformedEntity = $"`{currentEntityBody}`";
                        break;
                    case MessageEntityType.TextMention:
                        transformedEntity = $"[{currentEntityBody}](tg://user?id={currentEntity.User.Id})";
                        break;
                    case MessageEntityType.Mention:
                    case MessageEntityType.Hashtag:
                    case MessageEntityType.BotCommand:
                    case MessageEntityType.Url:
                    case MessageEntityType.Email:
                        transformedEntity = currentEntityBody;
                        break;
                    
                }

                builder.Append(transformedEntity);
                currentCharacter += currentEntity.Length;
            }
            else
            {
                builder.Append(message[currentCharacter]);
                currentCharacter += 1;
            }

            if (currentEntity.Offset < currentCharacter)
            {
                currentEntityIndex += 1;
            }
        }

        return builder.ToString();
    }
}