While File.ReadAllLines() is one of the simplest ways to read a file, it is also one of the slowest.
If you're just wanting to read lines in a file without doing much, according to these benchmarks, the fastest way to read a file is the age old method of:
using (StreamReader sr = File.OpenText(fileName))
{
string s = String.Empty;
while ((s = sr.ReadLine()) != null)
{
//do minimal amount of work here
}
}
However, if you have to do a lot with each line, then this article concludes that the best way is the following (and it's faster to pre-allocate a string[] if you know how many lines you're going to read) :
AllLines = new string[MAX]; //only allocate memory here
using (StreamReader sr = File.OpenText(fileName))
{
int x = 0;
while (!sr.EndOfStream)
{
AllLines[x] = sr.ReadLine();
x += 1;
}
} //Finished. Close the file
//Now parallel process each line in the file
Parallel.For(0, AllLines.Length, x =>
{
DoYourStuff(AllLines[x]); //do your work here
});