Grasshopper C# Script Component Rules

For create/rename/port workflow → script-component-lifecycle.md.

Scope

Grasshopper C# script component inside Rhino — not a standalone app.

Rules

• Inputs: typed (double x, List<Point3d> pts) or object with cast in RunScript.
• Outputs: ref parameters — assign directly.
• Name I/O params correctly; keep code minimal and recomputation-safe.
• Helpers below RunScript only when needed.
• Prefer List<T>; DataTree<T> only when trees are required.

Agent workflow (preferred)

Do not emit the class wrapper or using lines. Use gh_edit_script with scriptParts — the server assembles the full Grasshopper script.

{
  "action": "setCode",
  "targetId": "<guid>",
  "scriptParts": {
    "references": ["System", "Rhino.Geometry"],
    "runScript": "private void RunScript(\n  double x,\n  ref double a\n)\n{\n  a = x * 2;\n}"
  }
}

• references: namespace strings only (no using, no ;). Omit to use the default GH set.
• runScript: the full private void RunScript(...) method.
• helpers: optional methods placed below RunScript inside the class.

Small edits

Use patchCode instead of rewriting everything. Line numbers are 1-based within the chosen scope (default runScriptBody):

{
  "action": "patchCode",
  "targetId": "<guid>",
  "scope": "runScriptBody",
  "patches": [
    { "op": "replace", "startLine": 1, "endLine": 1, "lines": ["a = x * 3;"] }
  ]
}

Scopes: runScriptBody (default), runScript, helpers, references, full.

Read structured code with getCodeParts (returns references, runScript, runScriptBody, helpers, lineMap).

Legacy full-code template

code with the full script still works, but prefer scriptParts / patchCode.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using Rhino;
using Rhino.Geometry;
using Grasshopper;
using Grasshopper.Kernel;
using Grasshopper.Kernel.Data;
using Grasshopper.Kernel.Types;

public class Script_Instance : GH_ScriptInstance
{
  private void RunScript(
    double x,
    ref double a
  )
  {
      // Do works
      // Assign to output params
  }
}

mds/reference/csharp-boilerplate.md / View source on GitHub ↗