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.

C# method info

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.

A summary

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