Mahmoud Abas, Full-Stack Software Engineer & AI

Operating Systems 1 (2025-2026) - Lab: File Streams in C#

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Operating Systems 1 - Lab

File Streams in C#

Building a Linux-style file emulator on top of FileStream, StreamReader and StreamWriter, one shell command at a time.

What Is a Stream

A stream: An abstraction representing a sequence of bytes, used to read and write data. System.IO.Stream is the base class for every stream in .NET.

FileStream reads and writes files on disk
MemoryStream holds the same bytes in memory instead
NetworkStream carries them over a network connection
This lab maps shell-style commands onto those stream operations

touch - Create an Empty File

csharp

var fs = File.Create(name);
fs.Close();  // Important: always close the stream after use

File.Create hands back an open FileStream, and the handle stays held until you close it.

rm - Remove a File

csharp

private static void Rm()
{
    var name = Console.ReadLine();
    if (!ValidateExists(name)) return;
    File.Delete(name);
}

Deletion has no stream to manage, so the only work is checking the file is really there first.

cat and nano-append

csharp

var content = File.ReadAllText(name);
Console.WriteLine(content);

File.AppendAllText(name, text);

ReadAllText returns the whole file as one string
AppendAllText writes to the end without disturbing what is already there
Both are convenience methods that open and close a stream for you

Reading with Streams Directly

csharp

FileStream fs = new FileStream(name, FileMode.OpenOrCreate, FileAccess.ReadWrite);
StreamReader sr = new StreamReader(fs);
sr.BaseStream.Seek(0, SeekOrigin.Begin);
string str = sr.ReadLine();

Writing with Streams Directly

csharp

FileStream fs = new FileStream(name, FileMode.Create, FileAccess.ReadWrite);
StreamWriter w = new StreamWriter(fs);
w.Write(text);
w.Flush();
w.Close();
fs.Close();

Close in wrapper-first order: the StreamWriter, then the FileStream underneath it.

FileMode Options

Mode	What it does
`CreateNew`	creates a new file, and throws if one already exists
`Create`	creates a new file, overwriting an existing one
`Open`	opens an existing file, and throws if it is missing
`OpenOrCreate`	opens it if present, creates it if not
`Truncate`	opens it and clears the existing content
`Append`	opens it and moves the position to the end

FileAccess Options

Read grants read-only access to the stream
Write grants write-only access
ReadWrite grants both on the same stream

Validation Helper

csharp

private static bool ValidateExists(string? name)
{
    if (string.IsNullOrEmpty(name)) return false;
    if (!File.Exists(name)) return false;
    return true;
}

One shared check keeps the same two conditions out of every single command handler.

Best Practices

Always close streams, because an unclosed one locks the file and leaks the handle
Prefer a using statement so the close happens when the block ends, exception or not
Call Flush when a write must reach disk immediately
Validate inputs before any file operation, and catch exceptions in production code

Key Takeaways

A stream is one unified abstraction for reading and writing sequential data
FileStream is the low-level file class, and the reader and writer add text convenience on top
File.ReadAllText, File.AppendAllText and File.Create manage a stream internally for you
FileMode decides how the file is opened, and FileAccess decides what the stream may do
Buffering helps performance but needs Flush when a write has to hit disk now
Every stream is closed or disposed, or the resource stays held

Open this lesson

Mahmoud AbasLab: File Streams in C#

Mode

What it does

CreateNew

creates a new file, and throws if one already exists

Create

creates a new file, overwriting an existing one

Open

opens an existing file, and throws if it is missing

OpenOrCreate

opens it if present, creates it if not

Truncate

opens it and clears the existing content

Append

opens it and moves the position to the end