Read data from files

Completed

C# gives you several ways to read text from a file. The best choice depends on the size of the file and what you want to do with it.

Read the entire file at once

File.ReadAllText() returns the entire file as a single string.

string contents = File.ReadAllText("notes.txt");
Console.WriteLine(contents);

This approach works well for small files, but for very large files it loads everything into memory at once.

Note

If the file doesn't exist, both File.ReadAllText() and StreamReader throw a FileNotFoundException. Be sure the file exists before you try to read it.

Read the file as an array of lines

File.ReadAllLines() returns each line as a separate string in an array.

string[] lines = File.ReadAllLines("notes.txt");

foreach (string line in lines)
{
    Console.WriteLine(line);
}

Read one line at a time

The most memory-friendly way is to use a StreamReader and call .ReadLine() in a loop. It returns null once there are no more lines to read.

using StreamReader reader = new StreamReader("notes.txt");
string line;

while ((line = reader.ReadLine()) != null)
{
    Console.WriteLine(line);
}

This pattern is recommended for large files because only one line is held in memory at a time.

Choosing the right approach

If you want to...	Use...
Work with the entire file as one string	File.ReadAllText()
Get an array of lines you can index into	File.ReadAllLines()
Process each line one at a time efficiently	StreamReader with ReadLine()