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".