C# Program to read all the lines of a file at once

The File.ReadAllText() method in C# reads all the text from a file and returns it as a single string. This method is part of the System.IO namespace and provides a simple way to read entire file contents at once.

When you need to read all lines from a text file in one operation, ReadAllText() is more efficient than reading line by line, especially for smaller files.

Syntax

Following is the syntax for the File.ReadAllText() method −

public static string ReadAllText(string path);

Parameters

• path − The file path to read from. Can be relative or absolute path.

Return Value

Returns a string containing all the text in the file, including newline characters between lines.

Using File.ReadAllText() Method

Let's say we have a file "hello.txt" with the following content −

One
Two
Three

Example

using System;
using System.IO;

public class Demo {
   public static void Main() {
      string myPath = "hello.txt";
      string allLines;
      
      // Create the file first for demonstration
      File.WriteAllText(myPath, "One\nTwo\nThree");
      
      // Read all text from the file
      allLines = File.ReadAllText(myPath);
      Console.WriteLine("File contents:");
      Console.WriteLine(allLines);
      
      // Clean up
      File.Delete(myPath);
   }
}

The output of the above code is −

File contents:
One
Two
Three

Using File.ReadAllLines() for Line Array

If you need each line as a separate array element, use File.ReadAllLines() instead −

Example

using System;
using System.IO;

public class Demo {
   public static void Main() {
      string myPath = "data.txt";
      
      // Create file with sample data
      File.WriteAllText(myPath, "Line 1\nLine 2\nLine 3\nLine 4");
      
      // Read all lines into string array
      string[] lines = File.ReadAllLines(myPath);
      
      Console.WriteLine("Total lines: " + lines.Length);
      for (int i = 0; i < lines.Length; i++) {
         Console.WriteLine($"Line {i + 1}: {lines[i]}");
      }
      
      // Clean up
      File.Delete(myPath);
   }
}

The output of the above code is −

Total lines: 4
Line 1: Line 1
Line 2: Line 2
Line 3: Line 3
Line 4: Line 4

Comparison of File Reading Methods

Conclusion

The File.ReadAllText() method provides a simple way to read entire file contents as a single string. For processing individual lines, use File.ReadAllLines() to get an array of strings instead.