Web Aplication to android, ios

I have a web site, and I will start to create a aplication of it to android and ios. My web site its in c# so it will speed up things a bit (I think so).

I have my dll which have all my methods that talks to the data base, user structure, etc...

So my question is: Can I use my dlls to connect to the database and everything else, or I have to create a web service to tranfers the data?

I dont know which is used, or which is best to use.

Thanks in advance.