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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C# code writing to a file?

Consider the following C# code that writes text to a file and then reads it back. What will be printed on the console?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "testfile.txt";
        File.WriteAllText(path, "Hello World!");
        string content = File.ReadAllText(path);
        Console.WriteLine(content);
    }
}

A File not found exception

B testfile.txt

C Hello World!

D Compilation error

💡 Hint

Think about what File.WriteAllText and File.ReadAllText do.

The code writes the string "Hello World!" to the file named "testfile.txt" and then reads it back. The console prints the content read, which is "Hello World!".

❓ Predict Output

intermediate

What happens if you try to write to a file in a non-existent directory?

What will happen when running this C# code?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "nonexistent_dir/output.txt";
        File.WriteAllText(path, "Data");
        Console.WriteLine("Done");
    }
}

A Done

B Compilation error

C FileNotFoundException

D DirectoryNotFoundException

💡 Hint

Does File.WriteAllText create missing directories automatically?

The method File.WriteAllText does not create directories. If the directory does not exist, it throws a DirectoryNotFoundException.

🔧 Debug

advanced

Why does this code throw an exception when writing to a file?

The following code throws an exception. Identify the cause.

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "output.txt";
        using (StreamWriter writer = new StreamWriter(path)) {
            writer.WriteLine("Line 1");
        }
        writer.WriteLine("Line 2");
    }
}

A The file path is invalid causing a FileNotFoundException

B The StreamWriter is used outside its using block causing an ObjectDisposedException

C The StreamWriter constructor throws an IOException

D No exception, code runs fine

💡 Hint

Check where the writer variable is accessible.

The writer variable is declared inside the using block and disposed at the end of it. Trying to use it outside causes an ObjectDisposedException.

🧠 Conceptual

advanced

Which method appends text to an existing file without overwriting?

Which of the following C# methods appends text to a file instead of overwriting it?

A File.AppendAllText(path, text)

B File.WriteAllText(path, text)

C File.ReadAllText(path)

D File.Create(path)

💡 Hint

Look for the method name that suggests adding to the end.

File.AppendAllText adds text to the end of the file without deleting existing content. File.WriteAllText overwrites the file.

❓ Predict Output

expert

What is the content of the file after running this code?

Given the code below, what will be the content of log.txt after execution?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "log.txt";
        File.WriteAllText(path, "Start\n");
        using (StreamWriter sw = new StreamWriter(path, append: true)) {
            sw.WriteLine("Middle");
        }
        File.WriteAllText(path, "End");
    }
}

A End

B Start\nMiddle\nEnd

C Start\nEnd

D Start\nMiddle

💡 Hint

Consider the order of file writes and whether content is overwritten.

The first WriteAllText writes "Start\n". Then StreamWriter appends "Middle\n". Finally, WriteAllText overwrites the entire file with "End". So the file only contains "End".

Practice

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

easy

A. It creates a new text file or overwrites an existing one with the specified content.

B. It reads all text from a file and returns it as a string.

C. It appends text to the end of an existing file without overwriting.

D. It deletes a specified text file from the disk.

Solution

1. Step 1: Understand the purpose of File.WriteAllText

   This method is designed to write text to a file, creating it if it doesn't exist or overwriting it if it does.

2. Step 2: Compare with other file methods

   Reading text uses File.ReadAllText, appending uses File.AppendAllText, and deleting uses File.Delete, so these are different methods.

3. Final Answer:

   It creates a new text file or overwrites an existing one with the specified content. -> Option A

4. Quick Check:

   File.WriteAllText writes or overwrites text [OK]

Hint: WriteAllText creates or overwrites files with given text [OK]

Common Mistakes:

• Confusing WriteAllText with ReadAllText
• Thinking it appends instead of overwriting
• Assuming it deletes files

2. Which of the following is the correct syntax to write "Hello World" to a file named "greeting.txt" using File.WriteAllText?

easy

A. File.WriteText("greeting.txt", "Hello World");

B. File.WriteAllText("Hello World", "greeting.txt");

C. WriteAllText.File("greeting.txt", "Hello World");

D. File.WriteAllText("greeting.txt", "Hello World");

Solution

1. Step 1: Check method name and parameters

   The correct method is File.WriteAllText with the first parameter as the file path and the second as the text content.

2. Step 2: Validate syntax correctness

   File.WriteAllText("greeting.txt", "Hello World"); matches the correct method name and parameter order. Options A, B, and D have incorrect method names or parameter order.

3. Final Answer:

   File.WriteAllText("greeting.txt", "Hello World"); -> Option D

4. Quick Check:

   Correct method and parameter order = File.WriteAllText("greeting.txt", "Hello World"); [OK]

Hint: Method name is File.WriteAllText(path, content) [OK]

Common Mistakes:

• Swapping parameters order
• Using incorrect method names like WriteText
• Calling method on WriteAllText instead of File

3. What will be the content of the file "notes.txt" after running this code?

string path = "notes.txt";
File.WriteAllText(path, "Line 1\nLine 2");
File.WriteAllText(path, "New Line");

medium

A. New Line

B. Line 1\nLine 2

C. Empty file

D. Line 1\nLine 2\nNew Line

Solution

1. Step 1: Analyze first File.WriteAllText call

   The first call writes "Line 1\nLine 2" to "notes.txt", creating or overwriting the file.

2. Step 2: Analyze second File.WriteAllText call

   The second call overwrites the entire file content with "New Line", replacing previous text.

3. Final Answer:

   New Line -> Option A

4. Quick Check:

   Second WriteAllText overwrites file content [OK]

Hint: WriteAllText overwrites file, last call wins [OK]

Common Mistakes:

• Assuming text appends instead of overwriting
• Thinking both texts combine in file
• Ignoring the second WriteAllText call

4. Identify the error in this code snippet that tries to write "Data" to "output.txt":

File.WriteAllText(output.txt, "Data");

medium

A. File.WriteAllText cannot write strings

B. Missing quotes around the file name output.txt

C. The method name should be WriteTextAll

D. The file path must be an absolute path

Solution

1. Step 1: Check the file path parameter

   The file path must be a string, so it needs quotes around it. Here, output.txt is unquoted, causing a syntax error.

2. Step 2: Verify method name and parameter type

   The method name is correct, and it accepts strings. Absolute path is not mandatory; relative paths work fine.

3. Final Answer:

   Missing quotes around the file name output.txt -> Option B

4. Quick Check:

   File path must be a string literal [OK]

Hint: File path must be in quotes as a string [OK]

Common Mistakes:

• Forgetting quotes around file path
• Mixing method name order
• Thinking absolute path is required

5. You want to write multiple lines from a string array lines to a file named "log.txt" so that each element appears on its own line. Which code snippet correctly does this using File.WriteAllText?

hard

A. File.WriteAllText("log.txt", lines);

B. File.WriteAllText("log.txt", lines.ToString());

C. File.WriteAllText("log.txt", string.Join("\n", lines));

D. File.WriteAllText("log.txt", string.Concat(lines));

Solution

1. Step 1: Understand the input and desired output

   We have a string array lines and want each element on its own line in the file.

2. Step 2: Check how to convert array to single string with line breaks

   Using string.Join("\n", lines) joins array elements with newline characters, creating the correct multiline string.

3. Step 3: Validate other options

   lines.ToString() returns type name, not content; passing array directly is invalid; string.Concat joins without separators.

4. Final Answer:

   File.WriteAllText("log.txt", string.Join("\n", lines)); -> Option C

5. Quick Check:

   Join array with \n for multiline text [OK]

Hint: Join array with \n before writing text file [OK]

Common Mistakes:

• Passing array directly instead of a string
• Using ToString() on array expecting content
• Concatenating without separators