Reading text files in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reading a text file line by line

What will be the output of this C# code snippet that reads a text file line by line and prints each line?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "test.txt";
        File.WriteAllText(path, "Hello\nWorld\nCSharp");
        using StreamReader reader = new StreamReader(path);
        string? line;
        while ((line = reader.ReadLine()) != null) {
            Console.WriteLine(line);
        }
    }
}

A

Hello
World
CSharp

B Hello World CSharp

C HelloWorldCSharp

D Throws an exception

💡 Hint

Remember that ReadLine reads one line at a time and Console.WriteLine prints each line on a new line.

The code writes three lines to the file, then reads each line and prints it. Each line is printed on its own line, so the output is three lines: Hello, World, and CSharp.

❓ Predict Output

intermediate

Reading entire file content at once

What will this C# code print when reading the entire content of a text file at once?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "data.txt";
        File.WriteAllText(path, "Line1\nLine2\nLine3");
        string content = File.ReadAllText(path);
        Console.WriteLine(content);
    }
}

A Line1 Line2 Line3

B

Line1
Line2
Line3

C Line1Line2Line3

D Throws an exception

💡 Hint

ReadAllText reads the whole file as one string including newlines.

The file contains three lines separated by newline characters. ReadAllText returns the entire content including newlines, so printing it shows three lines.

❓ Predict Output

advanced

Reading file with incorrect path

What happens when this C# code tries to read a file that does not exist?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "nofile.txt";
        string content = File.ReadAllText(path);
        Console.WriteLine(content);
    }
}

A Throws NullReferenceException

B Prints null

C Prints empty string

D Throws FileNotFoundException

💡 Hint

What happens if you try to read a file that does not exist?

File.ReadAllText throws a FileNotFoundException if the file path does not exist.

❓ Predict Output

advanced

Reading file lines with LINQ

What is the output of this C# code that reads all lines from a file and filters lines containing 'a'?

C Sharp (C#)

using System;
using System.IO;
using System.Linq;

class Program {
    static void Main() {
        string path = "words.txt";
        File.WriteAllLines(path, new[] {"apple", "banana", "cherry", "date"});
        var lines = File.ReadLines(path).Where(line => line.Contains('a'));
        foreach (var line in lines) {
            Console.WriteLine(line);
        }
    }
}

A

apple
banana
date

B

banana
date

C cherry

D

apple
banana
cherry
date

💡 Hint

Check which words contain the letter 'a'.

The words 'apple', 'banana', and 'date' contain 'a'. 'cherry' does not. So only those three lines are printed.

❓ Predict Output

expert

Reading file asynchronously

What will this C# async code print when reading a file asynchronously?

C Sharp (C#)

using System;
using System.IO;
using System.Threading.Tasks;

class Program {
    static async Task Main() {
        string path = "async.txt";
        await File.WriteAllTextAsync(path, "Async\nRead\nTest");
        string content = await File.ReadAllTextAsync(path);
        Console.WriteLine(content);
    }
}

A Compilation error

B Async Read Test

C

Async
Read
Test

D AsyncReadTest

💡 Hint

Async file methods read and write the full content including newlines.

The file is written with three lines separated by newlines. Reading asynchronously returns the full content including newlines, so printing it shows three lines.