Mastering C# File Streams: StreamReader & StreamWriter – Step‑by‑Step Guide

In C# file I/O, streams serve as an intermediary between your application and the underlying file system. They allow you to read from or write to files in a memory‑efficient way, especially when dealing with large data sets.

Instead of loading an entire 100 MB file into memory—which can cause your app to freeze—streams break the data into manageable chunks. This approach keeps performance smooth and prevents crashes.

Below you’ll find concise examples that demonstrate how to read and write files using StreamReader and StreamWriter. Follow the step‑by‑step instructions to integrate these patterns into your own projects.

StreamReader – Reading Files

The StreamReader class reads text from a file one line at a time, passing the data through a buffer before your application processes it.

Assume you have a file named Example.txt on the D drive containing two lines:

• Guru99 – .Net
• Guru99 – C#

Create a console application and add the following code to Program.cs:

using System;
using System.IO;

namespace DemoApplication
{
    class Tutorial
    {
        static void Main(string[] args)
        {
            string path = @"D:\Example.txt";

            using (StreamReader sr = File.OpenText(path))
            {
                string line;
                while ((line = sr.ReadLine()) != null)
                {
                    Console.WriteLine(line);
                }
            }

            Console.ReadKey();
        }
    }
}

Explanation:

1. Instantiate StreamReader with File.OpenText, opening the file in read‑only mode.
2. Read each line with ReadLine(), which transfers data from the file to the buffer and then to the line variable.
3. Output each line to the console.

Running this program in Visual Studio displays:

The two lines from Example.txt appear correctly, proving that the reader works as intended.

StreamWriter – Writing to Files

Conversely, StreamWriter writes text from your application to a file via a buffer. Below is a simple example that appends a line to the same Example.txt file.

using System;
using System.IO;

namespace DemoApplication
{
    class Tutorial
    {
        static void Main(string[] args)
        {
            string path = @"D:\Example.txt";

            using (StreamWriter sw = File.AppendText(path))
            {
                sw.WriteLine("Guru99 – ASP.Net");
            }

            Console.WriteLine(File.ReadAllText(path));
            Console.ReadKey();
        }
    }
}

Explanation:

1. Instantiate StreamWriter with File.AppendText, opening the file in append mode.
2. Write a new line using WriteLine(), which pushes the text into the buffer and then to the file.
3. Close the writer automatically via the using block.
4. Read the entire file content with File.ReadAllText to confirm the write operation.

Executing this program shows:

The console displays all three lines, confirming that the new entry was appended successfully.

Key Takeaways

• Streams decouple file I/O from the application logic, enabling efficient handling of large files.
• Use StreamReader to read text line‑by‑line.
• Use StreamWriter to write or append text, ensuring you close the stream to release file handles.