Writing a script to the first line of a file without overwriting it?

Hey. I need to write a string to a new line at the top of a file without overwriting the file. It's a .cs file. Currently I can write to the string to a file, but it overwrites the contents of the file. This is using the openforwrite and writeline commands.

Here's an example of what I need; this is what the file contains already:

"//This is the file
function foo()
{"

I need it to write to the file, and come out like this:

"//New line here
//This is the file
function foo()
{"

Thanks.