How to read and add lines in the html?

Question

Hi! I have been searching for quite sometime but did not find any close answer. I need to basically add iframe line to an html file dynamically through asp.net code and the html file is in the asp.net directory. But after searching sometime, all the codes that i found is to read the html file not to edit it. Is there any link or tutorial that can help to do this?

Any help will be really appreciated.

 

Thank You