TelegramMessageMarkup

public static string GetMarkupMessage(this string plainText, MessageEntity[] entities, ParseMode parseMode)
        {
            var dict = new Dictionary<(MessageEntityType, ParseMode), (string open, string close)>
            {
                [(MessageEntityType.Bold, ParseMode.Html)] = ("<b>", "</b>"),
                [(MessageEntityType.Italic, ParseMode.Html)] = ("<i>", "</i>"),
                [(MessageEntityType.Underline, ParseMode.Html)] = ("<u>", "</u>"),
                [(MessageEntityType.Strikethrough, ParseMode.Html)] = ("<s>", "</s>"),
                [(MessageEntityType.Code, ParseMode.Html)] = ("<code>", "</code>"),
                [(MessageEntityType.Pre, ParseMode.Html)] = ("<pre>", "</pre>"),

                [(MessageEntityType.TextLink, ParseMode.Html)] = ("<a href='{{url}}'>", "</a>"),
                [(MessageEntityType.TextLink, ParseMode.MarkdownV2)] = ("[", "]({{url}})"),
            };

            var formatEntities = entities
                .Where(e => e.Type.In(MessageEntityType.Code, MessageEntityType.Pre,
                    MessageEntityType.Bold, MessageEntityType.Italic,
                    MessageEntityType.Underline, MessageEntityType.Strikethrough,
                    MessageEntityType.Url, MessageEntityType.TextLink))
                .Select(e => new
                {
                    StartIndex = e.Offset,
                    EndIndex = e.Offset + e.Length - 1,
                    OpenBrace = dict[(e.Type, parseMode)].open,
                    CloseBrace = dict[(e.Type, parseMode)].close
                })
                .ToArray();

            var insertBefore = formatEntities.ToLookup(e => e.StartIndex, e => e.OpenBrace);
            var insertAfter = formatEntities.Reverse().ToLookup(e => e.EndIndex, e => e.CloseBrace);

            var markupText = new StringBuilder();

            for (int i = 0; i < plainText.Length; i++)
            {
                var currentInsertBefore = insertBefore[i].ToArray();
                var currentInsertAfter = insertAfter[i].ToArray();
                if (currentInsertBefore.Any())
                {
                    markupText.Append(string.Join("", currentInsertBefore));
                }

                markupText.Append(plainText[i]);

                if (currentInsertAfter.Any())
                {
                    markupText.Append(string.Join("", currentInsertAfter));
                }
            }

            return markupText.ToString();
        }