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.

Practice

1. What does the File.ReadAllLines method do in C#?

easy

A. Reads the entire file content as a single string.

B. Reads all lines from a text file and returns them as a string array.

C. Writes lines to a text file.

D. Deletes the specified text file.

Solution

1. Step 1: Understand the method purpose

   File.ReadAllLines reads a text file and returns each line as an element in a string array.

2. Step 2: Compare with other methods

   File.ReadAllText returns the whole file as one string, not an array of lines.

3. Final Answer:

   Reads all lines from a text file and returns them as a string array. -> Option B

4. Quick Check:

   ReadAllLines = string array [OK]

Hint: ReadAllLines returns array of lines, not one big string [OK]

Common Mistakes:

• Confusing ReadAllLines with ReadAllText
• Thinking it writes to a file
• Assuming it deletes files

2. Which of the following is the correct syntax to read all text from a file named "data.txt" using File.ReadAllText?

easy

A. string content = File.ReadAllText("data.txt");

B. string content = File.ReadAllLines("data.txt");

C. string[] content = File.ReadAllText("data.txt");

D. string[] content = File.ReadAllLines("data.txt");

Solution

1. Step 1: Identify method return types

   File.ReadAllText returns a single string, so the variable must be string.

2. Step 2: Match syntax with variable type

   string content = File.ReadAllText("data.txt"); uses string variable with ReadAllText correctly. Options B and D mismatch method and variable types. string[] content = File.ReadAllText("data.txt"); tries to assign string to string array.

3. Final Answer:

   string content = File.ReadAllText("data.txt"); -> Option A

4. Quick Check:

   ReadAllText returns string, so variable is string [OK]

Hint: Match method return type with variable type [OK]

Common Mistakes:

• Assigning ReadAllText to string array
• Using ReadAllLines but expecting string
• Wrong variable type for method

3. What will be the output of this C# code if the file "example.txt" contains three lines: "Hello", "World", "!"?

string[] lines = File.ReadAllLines("example.txt");
Console.WriteLine(lines.Length);

medium

A. 3

B. Hello World !

C. 1

D. Error at runtime

Solution

1. Step 1: Understand ReadAllLines output

   The method returns an array with one element per line in the file. Here, 3 lines means array length is 3.

2. Step 2: Analyze Console.WriteLine output

   Printing lines.Length outputs the number of lines, which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Array length = number of lines = 3 [OK]

Hint: ReadAllLines length equals number of lines in file [OK]

Common Mistakes:

• Expecting content printed instead of length
• Confusing ReadAllText with ReadAllLines
• Assuming runtime error without cause

4. Identify the error in this code snippet that reads all lines from "notes.txt":

string[] lines = File.ReadAllLines(notes.txt);
foreach (string line in lines)
{
    Console.WriteLine(line);
}

medium

A. Console.WriteLine cannot print strings.

B. File.ReadAllLines returns a string, not string array.

C. foreach loop syntax is incorrect.

D. Missing quotes around the file name in ReadAllLines.

Solution

1. Step 1: Check file path syntax

   The file name must be a string literal, so it needs quotes: "notes.txt".

2. Step 2: Verify other code parts

   File.ReadAllLines returns string array, foreach syntax is correct, and Console.WriteLine can print strings.

3. Final Answer:

   Missing quotes around the file name in ReadAllLines. -> Option D

4. Quick Check:

   File path must be in quotes [OK]

Hint: File names must be in quotes in method calls [OK]

Common Mistakes:

• Forgetting quotes around file path
• Thinking ReadAllLines returns string
• Misunderstanding foreach syntax

5. You want to read a text file and count how many lines contain the word "error" (case insensitive). Which code snippet correctly does this?

hard

A. var lines = File.ReadAllLines("log.txt");
int count = 0;
foreach(var line in lines) {
if(line.Contains("error")) count++;
}

B. string content = File.ReadAllText("log.txt");
int count = content.Split('\n').Count(line => line.Contains("error"));

C. var lines = File.ReadAllLines("log.txt");
int count = lines.Count(line => line.ToLower().Contains("error"));

D. string[] lines = File.ReadAllText("log.txt").Split('\n');
int count = lines.Count(line => line.Contains("error"));

Solution

1. Step 1: Choose method to read lines

   File.ReadAllLines returns an array of lines, perfect for line-by-line processing.

2. Step 2: Count lines with "error" case-insensitive

   var lines = File.ReadAllLines("log.txt"); int count = lines.Count(line => line.ToLower().Contains("error")); converts each line to lowercase and checks if it contains "error", then counts matches using LINQ.

3. Step 3: Check other options

   string content = File.ReadAllText("log.txt"); int count = content.Split('\n').Count(line => line.Contains("error")); uses ReadAllText but misses case-insensitive check. var lines = File.ReadAllLines("log.txt"); int count = 0; foreach(var line in lines) { if(line.Contains("error")) count++; } misses case-insensitive check. string[] lines = File.ReadAllText("log.txt").Split('\n'); int count = lines.Count(line => line.Contains("error")); incorrectly assigns ReadAllText to string array without ToLower.

4. Final Answer:

   var lines = File.ReadAllLines("log.txt"); int count = lines.Count(line => line.ToLower().Contains("error")); -> Option C

5. Quick Check:

   Use ReadAllLines + ToLower + Count for case-insensitive search [OK]

Hint: Use ReadAllLines and ToLower for case-insensitive line checks [OK]

Common Mistakes:

• Ignoring case sensitivity
• Using ReadAllText but treating as array
• Not converting lines to lowercase