VT100Sharp

A VT100/ANSI terminal emulator and telnet client written from scratch in C# — no terminal library, no telnet library. It implements the escape-sequence state machine, the telnet option-negotiation protocol, and the keyboard-to-terminal encoding by hand, then renders the screen with GDI+ on a WinForms panel.

It was built to match the behavior of the classic Windows telnet client byte-for-byte when talking to line-oriented hosts, which meant getting the protocol details exactly right rather than "close enough."

Platform: Windows (WinForms UI), .NET 9.

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What's actually in here

Three self-contained pieces, each interesting on its own:

1. VT100Terminal — the terminal emulator

A character-cell screen model (char[,] plus per-cell foreground/background color planes) driven by a byte-at-a-time state machine:

• Escape-sequence parser that buffers ESC [ … <final> sequences and dispatches on the final byte, guarding against unterminated sequences.
• Cursor control: absolute position (H/f), relative moves (A/B/C/D), each with parameter parsing and clamping to screen bounds.
• Erase operations: erase-display and erase-line (J/K) in all three modes (to-cursor, from-cursor, whole).
• SGR color (m): the 30–37 / 40–47 foreground/background palette and reset.
• Control characters: BEL, backspace, tab-to-8, CR, LF, with scroll-up when the cursor runs past the last row.
• Cursor visibility via ?25h / ?25l.
• A logical/physical split: the emulator always presents a logical 80×25 to the host (so server-side formatting stays correct) while the panel can be any physical size, with content re-flowed and re-centered on resize.

2. TelnetNegotiation — the telnet protocol

A stateless processor for the telnet command stream (RFC 854 and friends):

• Full IAC command handling — WILL / WONT / DO / DONT with correct accept/refuse responses per option.
• Options supported: ECHO, Suppress-Go-Ahead, Terminal-Type, Window-Size (NAWS).
• Subnegotiation (SB … SE): replies to a terminal-type request with VT100, and emits NAWS window-size updates encoded as high/low byte pairs.
• Correct escaped-IAC handling (a doubled 0xFF is literal data, not a command) so binary bytes survive intact.
• Sends a sensible initial negotiation on connect and exposes live window-size updates.

3. AutomationCommand — scripted input

Turns high-level intents (CreateText, CreateKeyPress, CreateKeyCombination) into the exact byte sequences a VT100 host expects — including the ESC O P-style function-key encodings and ESC [ arrow/navigation keys — so sessions can be driven programmatically, not just typed.

How the pieces fit

   TCP socket  ──bytes──▶  TelnetNegotiation.ProcessTelnetData
                              │  (strips & answers IAC commands,
                              │   writes responses back to the stream)
                              ▼
                        display bytes  ──▶  VT100Terminal.ProcessData
                                               │ (escape-sequence state machine,
                                               │  updates the cell/color buffer)
                                               ▼
                                          Panel.Paint ──▶ GDI+ render

   keyboard / AutomationCommand ──encode──▶ socket

The two protocols share one inbound stream, so the design keeps them cleanly separated: telnet commands are consumed and answered first, and only the remaining display bytes reach the terminal emulator.

Build & run

Requires the .NET 9 SDK on Windows.

git clone https://github.com/astralmaster/VT100Sharp.git
cd VT100Sharp
dotnet build -c Release
dotnet run -c Release

Enter a host and port, connect, and interact with the remote session in the terminal panel.

Notes & scope

• Targets the VT100/ANSI subset needed for line-oriented and full-screen text hosts; it is not a full xterm (no 256-color, no mouse reporting, no alternate screen buffer).
• Rendering is intentionally simple GDI+ DrawString per cell — clear over clever. A glyph atlas would be the obvious next step for very high-throughput screens.
• The emulator reports a fixed logical 80×25 to the host by design; see the logical/physical split above.

License

MIT — see LICENSE.