Streams — StreamReader, StreamWriter and FileStream

A stream is an abstraction over a sequence of bytes — it could come from a file, a network socket, a memory buffer, or an HTTP request body. The Stream base class provides Read(), Write(), Seek(), and Position, and derived classes provide the source. StreamReader and StreamWriter layer text encoding/decoding on top of a byte stream. Understanding streams is essential for efficient file processing in ASP.NET Core — handling large uploads, serving large downloads, and processing log files without loading everything into memory.

StreamReader and StreamWriter #

// ── StreamReader — text reading with encoding control ─────────────────────
using var reader = new StreamReader("data.csv", Encoding.UTF8);

// Read entire file
string all = reader.ReadToEnd();

// Read line by line (memory-efficient for large files)
while (!reader.EndOfStream)
{
    string? line = reader.ReadLine();
    if (line is not null)
        ProcessLine(line);
}

// ── StreamWriter — text writing ───────────────────────────────────────────
using var writer = new StreamWriter("output.csv", append: false, Encoding.UTF8);
writer.WriteLine("Id,Name,Email");
writer.WriteLine("1,Alice,alice@example.com");
writer.Flush();   // ensure buffered data is written to the underlying stream

// ── File.OpenText / File.CreateText — convenience factory methods ─────────
using var reader2 = File.OpenText("input.txt");   // → StreamReader with UTF-8
using var writer2 = File.CreateText("output.txt"); // → StreamWriter, create/overwrite

FileStream — Binary File Access #

// FileStream — direct byte-level access
using var fs = new FileStream(
    "binary.dat",
    FileMode.OpenOrCreate,   // create if not exists, open if exists
    FileAccess.ReadWrite,    // open for both reading and writing
    FileShare.None);         // no other process can open this file

// Write bytes
byte[] header = new byte[] { 0x89, 0x50, 0x4E, 0x47 };  // PNG magic bytes
fs.Write(header, 0, header.Length);

// Seek to position
fs.Seek(0, SeekOrigin.Begin);   // go to start

// Read bytes into buffer
var buffer = new byte[4];
int bytesRead = fs.Read(buffer, 0, buffer.Length);
Console.WriteLine($"Read {bytesRead} bytes");

MemoryStream — In-Memory Buffer #

// MemoryStream — acts like a file stream but stored in RAM
// Useful for building byte arrays, piping data between components

using var ms = new MemoryStream();
using var writer = new StreamWriter(ms, Encoding.UTF8, leaveOpen: true);

writer.WriteLine("Line 1");
writer.WriteLine("Line 2");
writer.Flush();

ms.Position = 0;   // rewind before reading
using var reader = new StreamReader(ms);
string content = reader.ReadToEnd();
Console.WriteLine(content);

// Get bytes without a reader
byte[] bytes = ms.ToArray();   // independent copy of all bytes in the stream

Common Mistakes #

Mistake 1 — Forgetting using on StreamReader/StreamWriter (resource leak) #

❌ Wrong — file handle stays open indefinitely:

var reader = new StreamReader("file.txt");
string content = reader.ReadToEnd();
// Dispose never called — file handle leaked!

✅ Correct:

using var reader = new StreamReader("file.txt");
string content = reader.ReadToEnd();   // ✓ disposed at end of scope

Mistake 2 — Not rewinding MemoryStream before reading (reads 0 bytes) #

❌ Wrong — position is at the end after writing, reading returns empty:

ms.Write(data);
var bytes = ms.ToArray();   // ✓ works (ToArray ignores position)
var text  = new StreamReader(ms).ReadToEnd();  // ✗ returns "" — position is at end!

✅ Correct — reset position before reading:

ms.Position = 0;   // or ms.Seek(0, SeekOrigin.Begin)
var text = new StreamReader(ms).ReadToEnd();   // ✓