.NET Array Dictionary List String 2D Async DataTable Dates DateTime Enum File For Foreach Format IEnumerable If IndexOf Lambda LINQ Parse Path Process Property Regex Replace Sort Split Static StringBuilder Substring Switch Tuple

C#: File

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.

Based on:

.NET 4.5

File: text page

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.

Then: We can use "foreach" or "for" to loop over the file's data in a clean and intuitive way.

C# program that calls File.ReadAllLines

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)

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

	Console.WriteLine("Count: {0}", count);
	Console.WriteLine("C: {0}", c);


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

Framework: NET

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.

C# programming language

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.

But: If you need to save memory, reading in the lines one-by-one and processing them as you read them would be effective.