comparing the contents of two huge text files quickly

Question

what i\'m basically trying to do is compare two HUGE text files and if they match write out a string, i have this written but it\'s extremely slow. I was hoping you guys might

Answer 1

First I'd suggest normalizing both files and putting one of them in a set. This allows you to quickly test whether a specific line is present and reduces the complexity from O(n*n) to O(n).

Also you shouldn't open and close the file every time you write a line:

File.AppendAllText(...); // This causes the file to be opened and closed.

Open the output file once at the start of the operation, write lines to it, then close it when all lines have been written.