C# File.ReadAllLines

File.ReadAllLines returns an array. This array contains a string for each line in the file specified. We use this method. We then look inside its .NET Framework implementation to see how it works. It is found in the System.IO namespace.

This C# example program demonstrates the File.ReadAllLines method from System.IO.

Example

This example program specifies a file on the disk. To have it run correctly, change the filename to one that exists. The string array returned by the File.ReadAllLines call can be used as any other string array. We can use the foreach-loop or a for-loop on it.

Program that calls File.ReadAllLines [C#]

using System;
using System.IO;

class A
{
    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);
    }
}

Output

Length: 430
First: /_1=D1
Count: 430
C: 430

Implementation

The File.ReadAllLines method is not a low-level implementation. Instead, it 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. If you need to optimize a File.ReadAllLines call, you could estimate the capacity of the List in its constructor, which would reduce resizes. You could also avoid ToArray if you want to keep the List.

Summary

We looked at the File.ReadAllLines method in the .NET Framework version 4.0. This is a convenience method that uses StreamReader and List internally. It can be useful in quick programs where you simply want all the lines in a file. If you need to save memory, reading in the lines one-by-one and processing them as you read them would be effective.