I am making a application which can convert a excel file to CSV file.
However, some string contain a quotation mark, so when the CSV exported, some of the String was spl
You aren't assigning the result of the Trim or replace to a string.
You need to do something like
HtmlTable = HtmlTable.Replace("\"", string.Empty);
Looking at your comment, if you don't like string.Empty then you need to do
HtmlTable = HtmlTable.Replace("\"", "");