System.IO.IOException: The process cannot access the file because it is being used by another process.

The process is being used by another process. An error in executing another file before closing the previous one.

The solution is - You simple need to close the previous file TextIO class. For closing the file you need to use this syntax:

TextIo _file;

_file.finalize(); 

This will close the file and then you can proceed with your remaining operation on the file. Just doing equal to null will not do the trick.