File.ReadLines vs File.ReadAllLines

Which is the best was to read all the lines of a text file? The ReadLines and ReadAllLines methods differ as follows: When you use ReadLines, you can start enumerating the collection of strings before the whole collection is returned; when you use ReadAllLines, you must wait for the whole array of strings be returned before you can access the array.

Therefore, when you are working with very large files, ReadLines can be more efficient.

You can use the ReadLines method to do the following:

Perform LINQ to Objects queries on a file to obtain a filtered set of its lines.

Write the returned collection of lines to a file with the File.WriteAllLines(String, IEnumerable<String>) method, or append them to an existing file with the File.AppendAllLines(String, IEnumerable<String>) method.

Create an immediately populated instance of a collection that takes an IEnumerable<T> collection of strings for its constructor, such as a IList<T> or a Queue<T>.

Examples:

foreach (string line in File.ReadLines(@"d:\data\episodes.txt"))
{
if (line.Contains("episode") & line.Contains("2006"))
{
Console.WriteLine(line);
}
}

The following example uses the ReadLines method in a LINQ query that enumerates all directories for files that have a .txt extension, reads each line of the file, and displays the line if it contains the string "Microsoft".

using System;
using System.IO;
using System.Linq;

class Program
{
static void Main(string[] args)
{

try
{
var files = from file in Directory.EnumerateFiles(@"c:\",
"*.txt", SearchOption.AllDirectories)
from line in File.ReadLines(file)
where line.Contains("Microsoft")
select new
{
File = file,
Line = line
};

foreach (var f in files)
{
Console.WriteLine("{0}\t{1}", f.File, f.Line);
}
Console.WriteLine("{0} files found.",
files.Count().ToString());
}
catch (UnauthorizedAccessException UAEx)
{
Console.WriteLine(UAEx.Message);
}
catch (PathTooLongException PathEx)
{
Console.WriteLine(PathEx.Message);
}
}
}