💾 C# File I/O / C# File Handling – A Complete Guide for Beginners

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🧲 Introduction – Why Learn C# File Handling?

Modern software often needs to read, write, and manipulate files—whether it’s processing user data, generating logs, or storing persistent information. File handling is a fundamental feature in C# programming that empowers developers to interact with the file system efficiently.

🔍 In this guide, you’ll learn:

• How to perform basic file operations (create, read, write, delete)
• How to use StreamReader, StreamWriter, and File classes
• Error handling during file I/O
• Best practices and performance tips

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🔍 Core Concepts – Understanding File I/O in C#

C# provides multiple classes and namespaces for file handling under the System.IO namespace. Key classes include:

Class	Description
File	Provides static methods for file operations like Create, Delete, Exists, etc.
StreamWriter	Writes characters to a stream (commonly used for text files).
StreamReader	Reads characters from a stream.
FileStream	Used for both reading and writing to files at a lower level (binary or text).
BinaryReader / BinaryWriter	Used for reading/writing primitive data types in binary form.

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💻 Code Examples – With Step-by-Step Explanations

✅ 1. Writing to a Text File

using System.IO;

File.WriteAllText("sample.txt", "Hello, World!");

📘 Explanation:

• WriteAllText creates a file if it doesn’t exist or overwrites it if it does.
• Writes the specified string content into the file.

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✅ 2. Reading a Text File

using System.IO;

string content = File.ReadAllText("sample.txt");
Console.WriteLine(content);

📘 Explanation:

• ReadAllText reads the entire content of a text file and returns it as a string.

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✅ 3. Appending to a Text File

File.AppendAllText("sample.txt", "\nWelcome to C# File I/O!");

📘 Explanation:

• AppendAllText adds content to the end of the existing file.

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✅ 4. Reading Line-by-Line Using StreamReader

using (StreamReader reader = new StreamReader("sample.txt"))
{
    string line;
    while ((line = reader.ReadLine()) != null)
    {
        Console.WriteLine(line);
    }
}

📘 Explanation:

• Uses StreamReader inside a using block for safe disposal.
• Reads each line until EOF (null).

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✅ 5. Writing Line-by-Line Using StreamWriter

using (StreamWriter writer = new StreamWriter("log.txt", append: true))
{
    writer.WriteLine("Log Entry at " + DateTime.Now);
}

📘 Explanation:

• append: true keeps existing data intact.
• Logs timestamped data line-by-line.

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💡 Best Practices & Tips

📘 Best Practice: Always close streams using using blocks to avoid resource leaks.

💡 Tip: Prefer File.ReadAllText() or File.WriteAllText() for small, simple files.

⚠️ Pitfall: Avoid reading huge files entirely into memory. Use StreamReader for large files.

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📊 Functional Comparison Table

Method	Use Case	Synchronous	Reads All	Appends
File.ReadAllText()	Small text files	✔️	✔️	❌
File.AppendAllText()	Add to end of a text file	✔️	❌	✔️
StreamReader	Line-by-line processing	✔️	❌	❌
StreamWriter	Line-by-line writing	✔️	❌	✔️/❌
FileStream	Low-level binary/text I/O	✔️	❌	✔️/❌

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🛠️ Real-World Use Cases

• ✅ Logging systems (StreamWriter)
• ✅ Configuration loaders (File.ReadAllText)
• ✅ File migration tools (FileStream)
• ✅ Import/export CSV, TXT, JSON

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📌 Summary – Recap & Next Steps

Efficient file handling is essential in all software—from desktop applications to web APIs and services.

🔍 Key Takeaways:

• Use File for simple file operations
• StreamReader / StreamWriter for line-based access
• Always handle exceptions for robust code

⚙️ Explore next: Binary file access, file I/O performance optimization, directory traversal using Directory class.

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❓ FAQ Section

❓ How do I check if a file exists in C#?
✅ Use File.Exists(path). Returns true if the file exists.

❓ How to delete a file in C#?
✅ Use File.Delete(path) to remove a file.

❓ How do I read a file line-by-line efficiently?
✅ Use StreamReader.ReadLine() in a while loop for large files.

❓ What’s the difference between File and FileStream?
✅ File is for high-level operations; FileStream gives low-level control (binary mode).

❓ Can I read/write binary data using File classes?
✅ Yes, use BinaryReader and BinaryWriter for reading/writing primitives in binary format.

❓ What happens if the file doesn’t exist during reading?
✅ An exception is thrown. Always check with File.Exists() before reading.

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