The File class provides two static methods to read a text file in C#. The File.ReadAllText() method opens a text file, reads all the text in the file into a string, and then closes the file.
The following code reads a text file into a string.
// Read entire text file content in one string  
string text = File.ReadAllText(textFile);  
Console.WriteLine(text);

The File.ReadAllLines() method opens a text file, reads all lines of the file into a string array, and then closes the file. The following code snippet reads a text file into an array of strings.

// Read a text file line by line.  
string[] lines = File.ReadAllLines(textFile);  
  
foreach (string line in lines)  
Console.WriteLine(line);

One more way to read a text file is using a StreamReader class that implements a TextReader and reads characters from a byte stream in a particular encoding. The ReadLine method of StreamReader reads one line at a time.

// Read file using StreamReader. Reads file line by line    
using(StreamReader file = new StreamReader(textFile)) {  
 int counter = 0;  
 string ln;  
  
 while ((ln = file.ReadLine()) != null) {  
  Console.WriteLine(ln);  
  counter++;  
 }  
 file.Close();  
 Console.WriteLine($ "File has {counter} lines.");  
}

The complete code sample uses the above-discussed methods to read a text file and display its content to the console. To test this code, find a text file (or create one with some text in it) on your machine and change the "textFile" variable to the full path of your .txt file.

using System;  
using System.IO;  
  
namespace ReadATextFile {  
 class Program {  
  // Default folder    
  static readonly string rootFolder = @ "C:\Temp\Data\";    
  //Default file. MAKE SURE TO CHANGE THIS LOCATION AND FILE PATH TO YOUR FILE   
  static readonly string textFile = @ "C:\Temp\Data\Authors.txt";  
  
  static void Main(string[] args) {  
   if (File.Exists(textFile)) {  
    // Read entire text file content in one string    
    string text = File.ReadAllText(textFile);  
    Console.WriteLine(text);  
   }  
  
   if (File.Exists(textFile)) {  
    // Read a text file line by line.  
    string[] lines = File.ReadAllLines(textFile);  
    foreach(string line in lines)  
    Console.WriteLine(line);  
   }  
  
   if (File.Exists(textFile)) {  
    // Read file using StreamReader. Reads file line by line  
    using(StreamReader file = new StreamReader(textFile)) {  
     int counter = 0;  
     string ln;  
  
     while ((ln = file.ReadLine()) != null) {  
      Console.WriteLine(ln);  
      counter++;  
     }  
     file.Close();  
     Console.WriteLine($ "File has {counter} lines.");  
    }  
   }  
  
   Console.ReadKey();  
  }  
 }  
}

Summary

The code example in this article taught you how to use the File class to read a text file in C#.

Here is a detailed article on the File class in C#: Working With File Class In C# (c-sharpcorner.com)