BitTorrent Client In C#

A compact BitTorrent client written in C# and .NET 8, originally built as part of the CodeCrafters "Build Your Own BitTorrent" challenge.

This is a learning-oriented networking project. It focuses on the mechanics behind torrent metadata, tracker communication, peer handshakes, piece requests, integrity checks, and magnet links rather than trying to be a polished replacement for a full BitTorrent client.

Why This Project Exists

This project explores the lower-level pieces that a BitTorrent client needs before a user interface, persistence layer, or long-running session manager can be useful.

It was built to work directly with:

• bencode decoding and encoding
• .torrent metainfo parsing
• SHA-1 info hash and piece hash validation
• HTTP tracker announce requests
• compact peer list parsing
• BitTorrent peer handshakes
• interested, unchoke, bitfield, and piece messages
• block-level piece downloading
• concurrent piece downloading across discovered peers
• magnet URI parsing
• BitTorrent extension handshakes for ut_metadata

What It Currently Supports

The CLI exposes the challenge stages as direct commands:

• decode decodes bencoded values
• info prints tracker, file, info hash, piece length, and piece hash metadata from a .torrent file
• peers asks the tracker for peers and prints compact peer addresses
• handshake performs a BitTorrent peer handshake
• download_piece downloads and verifies one piece from a torrent
• download downloads all pieces from a torrent and writes the assembled file
• magnet_parse parses a magnet URI
• magnet_handshake performs a peer handshake from a magnet URI
• magnet_info fetches torrent metadata through the extension protocol
• magnet_download_piece downloads one piece from a magnet URI
• magnet_download downloads all pieces from a magnet URI

Design Notes

The code deliberately keeps the architecture direct:

• src/Program.cs maps CLI commands to command handlers
• src/Bencoding contains the bencode reader and writer
• src/Metadata models torrent and magnet metadata
• src/Connection contains peer connection and protocol message types
• src/Extensions/TrackerExtensions.cs handles tracker announce requests and compact peer parsing
• src/Extensions/NetworkStreamExtensions.cs handles peer protocol reads, writes, piece downloads, and piece hash verification

The implementation favors traceable protocol code over framework-style abstraction. For a project meant to demonstrate systems and protocol work, I kept the path from CLI input to network I/O easy to follow.

Scope And Limitations

This is not a production BitTorrent client.

Important limitations:

• only single-file torrents are in scope
• tracker support is HTTP-oriented and minimal
• there is no DHT, PEX, peer choking strategy, resume data, or long-running session state
• peer selection and retry behavior are intentionally simple
• magnet metadata fetching currently targets the challenge-sized happy path rather than arbitrary large metadata responses
• diagnostics are written directly to the console instead of using structured logging
• there is no comprehensive automated test suite

The code is best read as a protocol implementation exercise, not as an application meant for everyday torrenting.

Prerequisites

• Docker

You do not need a local .NET SDK for the Docker workflow below.

Build The Image

Run this once from the repository root:

$ docker build -t codecrafters-bittorrent-csharp .

Run Commands In Docker

The container runs the BitTorrent CLI directly. Bind-mount the repository into /workspace so the container can read torrent files from your working tree.

$ docker run --rm -v "$PWD:/workspace" codecrafters-bittorrent-csharp info sample.torrent

Ask the tracker for peers:

$ docker run --rm -v "$PWD:/workspace" codecrafters-bittorrent-csharp peers sample.torrent

Parse a magnet URI:

$ docker run --rm -v "$PWD:/workspace" codecrafters-bittorrent-csharp magnet_parse "magnet:?xt=urn:btih:d69f91e6b2ae4c542468d1073a71d4ea13879a7f&dn=sample&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce"

The magnet example uses the info hash and tracker from sample.torrent. The peers command contacts the CodeCrafters tracker, so it needs outbound network access from Docker.

Local .NET Alternative

If you do have the .NET 8 SDK installed, the CodeCrafters helper script still works:

$ ./your_bittorrent.sh info sample.torrent
$ ./your_bittorrent.sh peers sample.torrent
$ ./your_bittorrent.sh download -o tmp/output.bin sample.torrent

You can also run the project directly:

$ dotnet run --project . --configuration Release -- info sample.torrent

Authorship And AI Usage

The BitTorrent implementation in this repository was written by me as part of hands-on CodeCrafters practice.

AI assistance was used later for repository analysis and documentation work, including this README. The protocol implementation itself should be evaluated as a compact learning project rather than as AI-generated application scaffolding.

What I Would Improve Next

If I continue developing this project, the next areas I would focus on are:

• support multi-file torrents
• make peer retry behavior bounded and easier to reason about
• fetch multi-piece magnet metadata correctly
• add focused tests around bencode parsing, tracker response parsing, and piece assembly
• introduce clearer command-line validation and help output
• replace console diagnostics with structured logging