I wouldn't use WriteAllLines all though it sounds as if it would be useful. I would simply create a foreach loop that goes through the lines of text in 'ta' and writes it to the fie.
foreach (string line in ta.Text)
{
//Put writing code here
}
But that's just me; I don't generally deal with System.IO so I can't be too much help, but with a little reading on Google about how to save your files using System.IO you can implement it into a foreach, or find exact information on what you're looking for. I hope this gives you a slight push in the right direction.
Good Luck,
Jamie
***EDIT***
Okay, I revised your post (as I usually do after replying to ensure my reply is accurate) and I thought you were using a text box control. But seeing as you're using TextArray to do this I have no clue because I've never heard of the namespace. Probably have used it without knowing, but never have actually caught the name in full view like that. Sorry I can't be more assistance. I also don't see anything trying to save the file in there, but I may be just glimpsing too quickly.
Jamie
***EDIT***
Look into StreamReader/StreamWriter. It might help out:
Read
// Specify file, instructions, and privelegdes
file = new FileStream("test.txt", FileMode.OpenOrCreate, FileAccess.Read);
// Create a new stream to read from a file
StreamReader sr = new StreamReader(file);
// Read contents of file into a string
string s = sr.ReadToEnd();
// Close StreamReader
sr.Close();
// Close file
file.Close();
Write
// Specify file, instructions, and privelegdes
FileStream file = new FileStream("test.txt", FileMode.OpenOrCreate, FileAccess.Write);
// Create a new stream to write to the file
StreamWriter sw = new StreamWriter(file);
// Write a string to the file
sw.Write("Hello file system world!");
// Close StreamWriter
sw.Close();
// Close file
file.Close();
For more examples check MSDN for Stream Reader and Stream Writer. Also can be implemented with Open and Save File Dialogs to ease reading and writing.
Jamie