File.ReadAllLines

This C# method receives a file name, and returns an array. This array contains a string for each line in the file specified.

We use this C# method. We then look inside its .NET implementation to see how it works. It is found in the System.IO namespace.

Example

This example program specifies a file on the disk. ReadAllLines() gets a string array from the file. The string array returned can be used as any other string array.

using System;
using System.IO;

class Program
{
    static void Main()
    {
        string[] lines = File.ReadAllLines("C:\\rearrange.txt");

        Console.WriteLine("Length: {0}", lines.Length);
        Console.WriteLine("First: {0}", lines[0]);

        int count = 0;
        foreach (string line in lines)
        {
            count++;
        }

        int c = 0;
        for (int i = 0; i < lines.Length; i++)
        {
            c++;
        }

        Console.WriteLine("Count: {0}", count);
        Console.WriteLine("C: {0}", c);
    }
}
Length: 430
First: /_1=D1
Count: 430
C: 430

Implementation

File.ReadAllLines() uses a List and the StreamReader type, and then ToArray, to return the array. This is inefficient if you want to put a file into a List.

Notes, optimize

If you need to optimize a File.ReadAllLines call, you could estimate the capacity of the List in its constructor. You could also avoid ToArray if you want to keep the List.

We looked at the File.ReadAllLines method. This is a convenience method that uses StreamReader and List internally. It can cause excess memory use.