This article will show you how to merge the text of text files located in a directory.
txtFiles = Directory.GetFiles(txtFileDestination.Text, "*.txt");
using (StreamWriter writer = new StreamWriter(txtFileDestination.Text + @"\allfiles.txt"))
for (int i = 0; i < txtFiles.Length; i++)
using (StreamReader reader = File.OpenText(txtFiles[i]))
[Modified: it really doesn't take much to fix the tabbing...]