File Handling in C#

# CHAPTER 22

File Handling in C# #

1. Introduction #

2. Learning Objectives #

• Use the System.IO namespace.

• Use the static File class for quick reads/writes.

• Use StreamWriter and StreamReader for handling large files.

• Understand basic file paths.

3. The Static File Class (The Easy Way) #

using System;
using System.IO; // CRITICAL: Required for file handling

class Program
{
    static void Main()
    {
        string filePath = "notes.txt";

        // 1. WRITE to a file (creates the file, or overwrites if it exists)
        File.WriteAllText(filePath, "Hello, this is my first saved file!");

        // 2. APPEND to a file (adds to the end without deleting existing content)
        File.AppendAllText(filePath, "\nHere is a second line.");

        // 3. READ from a file
        string content = File.ReadAllText(filePath);
        Console.WriteLine("File Content:\n" + content);
    }
}

4. Streams (StreamWriter / StreamReader) #

Instead, we use Streams. Streams read/write data in small chunks (line by line).

using System.IO;

// Writing with StreamWriter
using (StreamWriter writer = new StreamWriter("log.txt", append: true))
{
    writer.WriteLine("User logged in at 10:00 AM");
} // The 'using' block automatically CLOSES and flushes the file when finished!

// Reading with StreamReader
using (StreamReader reader = new StreamReader("log.txt"))
{
    string line;
    // ReadLine() returns null when it reaches the End Of File (EOF)
    while ((line = reader.ReadLine()) != null)
    {
        Console.WriteLine(line);
    }
}

5. The using Statement (Memory Management) #

When you open a file, the Operating System locks it. If your program crashes before you manually call writer.Close(), the file remains locked! Wrapping file operations in a using block guarantees that the file is safely closed and disposed of, even if an exception occurs.

6. File Paths #

• Relative Path: "notes.txt" saves the file exactly where the .exe is currently running from (usually bin/Debug/net8.0/).

• Absolute Path: "C:\\Users\\Admin\\Desktop\\notes.txt" saves it to a specific hard drive location. Remember to use \\ or verbatim strings @"C:\..." to avoid escape character errors.

7. Checking if a File Exists #

if (File.Exists("config.txt"))
{
    string config = File.ReadAllText("config.txt");
}
else
{
    Console.WriteLine("Configuration file missing!");
}

8. Mini Project: Notes Manager #

using System;
using System.IO;

namespace NotesManager
{
    class Program
    {
        static void Main()
        {
            string path = "my_notes.txt";
            
            Console.WriteLine("Enter a note (Type EXIT to quit):");
            while (true)
            {
                string input = Console.ReadLine();
                if (input == "EXIT") break;
                
                File.AppendAllText(path, input + Environment.NewLine);
                Console.WriteLine("Note saved.");
            }

            Console.WriteLine("\n--- ALL NOTES ---");
            if (File.Exists(path))
            {
                Console.WriteLine(File.ReadAllText(path));
            }
        }
    }
}

9. Common Mistakes #

• File In Use Error: Trying to read a file while a StreamWriter still holds it open. Always close writers or use using blocks.

• UnauthorizedAccessException: Trying to write a file to a protected system directory (like C:\Windows) without administrator privileges.

10. Best Practices #

• Always wrap StreamReader and StreamWriter in a using block.

• Always check File.Exists() before attempting to read.

11. Exercises #

1. 1. Write a program that asks the user for their name and age, and writes it to a file called profile.txt.

1. 2. Write a program that reads profile.txt and prints it to the console.

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the purpose of the using block in the context of IDisposable and file streams.

• Q: Differentiate between File.WriteAllText() and StreamWriter.

14. Summary #

15. Next Chapter Recommendation #