C# - How to read a file

The simplest way to read a file is to use a high-level method in the .NET File API (in System.IO), such as File.ReadAllText(). These high-level methods abstract away the details of opening a file stream, reading it with StreamReader, and closing the file.

Here’s an example of reading a text file’s content using File.ReadAllText():

using System.IO;

var content = File.ReadAllText("hello.txt");

//Using absolute path
content = File.ReadAllText(@"C:\temp\hello.txt");


Console.WriteLine(content);
Code language: C# (cs)

Notice that you can use a relative or absolute file path when reading file.

This outputs the file’s contents:

Hello world!
Blah blah etc etc
Well, bye.
Code language: plaintext (plaintext)

In this article, I’ll explain how to read files in different scenarios.

Table of Contents

Read a text file all at once

If the file is small, then you can read it into memory all at once. You can either read all of the text into a single string with File.ReadAllText(), or read the lines into a string array with File.ReadAllLines().

Read a text file into a string

To read a text file into a string, use File.ReadAllText(), like this:

using System.IO;

var content = File.ReadAllText(@"C:\temp\animals.txt");

Console.WriteLine(content);
Code language: C# (cs)

This outputs the file’s contents:

blue dog
red cat
white fish
Code language: plaintext (plaintext)

If you’d benefit from reading the file asynchronously in your situation, you can use File.ReadAllTextAsync(). Here’s an example of asynchronously reading the text of a file into a string asynchronously:

using System.IO;

var content = await File.ReadAllTextAsync(@"C:\temp\animals.txt");
Code language: C# (cs)

Read all lines in a text file into an array

To load the text file into a string array, where each line is a separate string in the array, use File.ReadAllLines() like this:

using System.IO;

string[] animals = File.ReadAllLines(@"C:\temp\animals.txt");

//Process the array
foreach (var line in animals)
{
    Console.WriteLine(line);
}
Code language: C# (cs)

Note: The async version is File.ReadAllLinesAsync().

This outputs the file’s lines (from the string array):

blue dog
red cat
white fish
Code language: plaintext (plaintext)

Reading the file lines into an array would be useful if you need random access to specific lines later on (i.e. line 0 is lines[0]). That should be pretty rare. More likely, you’ll want to process one line at a time. In that case, use File.ReadLines() instead. I’ll show that next.

Read a text file line by line

Use File.ReadLines() when you want to read and process a text file one line at a time. This is a memory-efficient approach that reads a small amount of the file into memory at a time (buffering) and gives you one line at a time. Use this approach if you’re reading a big file (to avoid OutOfMemoryException) or if you’re doing a forward-only read.

Here’s an example of using File.ReadLines() to read and process a file line by line:

using System.IO;

foreach (var line in File.ReadLines(@"C:\temp\animals.txt"))
{
    Console.WriteLine(line);
}
Code language: C# (cs)

This outputs the file’s lines:

blue dog
red cat
white fish
Code language: plaintext (plaintext)

Read lines asynchronously with ReadLinesAsync() (.NET 7+)

If you want to read lines asynchronously, you can use ReadLinesAsync(). This was added in .NET 7. It returns an IAsyncEnumerable. Here’s an example of how to read a file with ReadLinesAsync():

using System.IO;

await foreach (var line in File.ReadLinesAsync(@"C:\temp\animals.txt"))
{
    Console.WriteLine(line);
}
Code language: C# (cs)