After importing the System.IO namespace:
and then went on to other code where it adds/modifies the file. But this threw a runtime error like "The file is being used by another process", which was reasonable because the above code while creating the file generates a handle which is still being held.
After a couple of workarounds, I came up with the following piece of code, where all the necessary modifications could be done and then release the handle:
using(FileStream fs = new FileStream("test.txt",FileMode.CreateNew,FileAccess.ReadWrite))
//do whatever preferred to the file........
//this releases the handle