C4/Diagram/4 - Code

• Code diagram (C4 diagram)

  • Overview

    • Level 4 static-structure view: inside a C4/Abstraction/3 - Component, showing language-level elements—classes, interfaces, files, modules—and key relationships.
    • The most detailed C4 diagram; often omitted when IDEs and generators already answer the same question.

  • Context

    • Still structural (not a sequence chart); behavioural stories use C4/Diagram/Dynamic instead.

  • Key principles

    • Selective projection — Omit noise (simple DTOs, utilities) unless they carry architectural meaning.
    • Maintainability — Prefer generated or tool-backed views so the picture does not rot.

  • Mechanism

    • Feeds from reverse-engineering pipelines (e.g. Java/C# analysis) in tools like Structurizr for large codebases.

  • Examples

    • A trimmed UML class diagram for the core of one service; a module graph for a front-end bundle.

  • Misconceptions

    • “C4 level 4 = mandatory” — False; many successful C4 adoptions stop at container or component.
    • “Code diagram replaces good naming in code” — Unwise; the diagram should echo intentional module boundaries, not substitute for them.