awesome-copilot/skills/rhino3d-scripts/references/vbscript-quirks.md

VBScript Quirks for RhinoScript

Things that bite when writing .rvb / .vbs files and aren’t obvious to anyone whose mental model is C-family or Python.

Always Start With Option Explicit

Without it, mistyping a variable name silently creates a new Variant set to Empty. Every .rvb file should begin:

Option Explicit

No Block Scope

All Dim declarations inside a Sub/Function are hoisted to the top. A Dim inside an If block is visible after the If ends. Loop counters survive the loop.

Nothing vs Empty vs Null

Sentinel	Test with	Meaning
Empty	IsEmpty(x)	Dim’d but never assigned
Null	IsNull(x)	Explicit “no value” — what Rhino.GetObject returns on cancel
Nothing	x Is Nothing	An object reference that points to nothing — only for Set variables

Wrong sentinel → silently false.

Parentheses Change Semantics

Foo a, b              ' Call a Sub or Function (return value discarded)
Call Foo(a, b)        ' Call a Sub or Function (return value discarded)
x = Foo(a, b)         ' Call a Function and capture the return
Foo(a, b)             ' SYNTAX ERROR for multi-arg subs

Foo(x)                ' Calls Foo passing x BY VALUE, even if Foo declares ByRef
Foo x                 ' Honors Foo's ByRef declaration

If a Sub modifies its argument and your change isn’t taking effect — you wrapped the argument in parentheses.

ByRef Is the Default

Unlike most languages, VBScript passes arguments by reference by default. Functions can mutate their callers’ variables. Be explicit:

Sub Increment(ByRef n)
    n = n + 1
End Sub

Arrays Are 0-Based But Have UBound, Not Length

Dim arr(2)            ' Three elements: arr(0), arr(1), arr(2)
For i = 0 To UBound(arr)
    arr(i) = i * 10
Next

Dim arr(n) creates n+1 elements. ReDim Preserve arr(newSize) resizes (only the last dimension of a multi-dim array).

Set Is Required for Object Assignment

Set fso = CreateObject("Scripting.FileSystemObject")    ' correct
fso = CreateObject("Scripting.FileSystemObject")        ' RUNTIME ERROR

Any time the right-hand side is an object (COM object, RegExp, Dictionary), you must use Set.

Error Handling Is Manual

On Error Resume Next
Rhino.AddCircle Array(0,0,0), -1
If Err.Number <> 0 Then
    Rhino.Print "Failed: " & Err.Description
    Err.Clear
End If
On Error GoTo 0           ' Restore normal error behavior

On Error Resume Next suppresses all errors until On Error GoTo 0. Forgetting to restore is a common bug.

Points Are 3-Element Arrays

Rhino expects Array(x, y, z). A 2-element array (Array(x, y)) raises a type-mismatch error.

Dim pt
pt = Array(1.0, 2.0, 3.0)
Rhino.AddPoint pt

String Concatenation Uses &, Not +

+ on strings works only if both sides are strings. If one side is a number, + does numeric addition and throws a type-mismatch. Always use &:

Rhino.Print "Count: " & n

For Each Needs a Variant

Dim item
For Each item In someCollection
    ' ...
Next

The loop variable must be Variant. You cannot Dim item As Long (VBScript has no typed Dim).