Creates an arc length dimension for an arc.
Supported platforms: Windows only
VBA:
RetVal = object.AddDimArc(ArcCenter, FirstEndPoint, SecondEndPoint, ArcPoint)
Type: Block, ModelSpace, PaperSpace
The objects this method applies to.
Access: Input-only
Type: Variant (three-element array of doubles)
The 3D WCS coordinates specifying the center of the arc.
Access: Input-only
Type: Variant (three-element array of doubles)
The 3D WCS coordinates specifying the point through which the first extension line passes.
Access: Input-only
Type: Variant (three-element array of doubles)
The 3D WCS coordinates specifying the point through which the second extension line passes.
Access: Input-only
Type: Variant (three-element array of doubles)
The 3D WCS coordinates specifying a point on the arc.

The ArcCenter is the center of the arc being dimensioned. FirstEndPoint and SecondEndPoint are the points through which the two extension lines pass.
VBA:
Sub Example_AddDimArc()
Dim PI As Double: PI = 3.141592
Dim oMS As AcadModelSpace
Set oMS = ThisDrawing.ModelSpace
Dim ptCenter(2) As Double
Dim oA As AcadArc
Set oA = oMS.AddArc(ptCenter, 10, PI / 3, PI * 3 / 4)
Dim ptArcPoint(2) As Double
ptArcPoint(0) = 0: ptArcPoint(1) = 15
Dim oAcadDimArcLength As AcadDimArcLength
Set oAcadDimArcLength = oMS.AddDimArc(oA.Center, oA.startPoint, oA.endPoint, ptArcPoint)
Update
ZoomExtents
End Sub
Visual LISP:
(vl-load-com)
(defun c:Example_AddDimArc()
;; This example creates an arc and arc length dimension in model space.
(setq acadObj (vlax-get-acad-object))
(setq doc (vla-get-ActiveDocument acadObj))
(setq modelSpace (vla-get-ModelSpace doc))
;; Define the arc
(setq center (vlax-3d-point 0 0 0))
;; Create the arc in model space
(setq arc (vla-AddArc modelSpace center 10 (/ PI 3) (/ (* PI 3) 4)))
;; Define the arc length dimension
(setq arcPoint (vlax-3d-point 0 15 0)
startPoint (vla-get-StartPoint arc)
endPoint (vla-get-EndPoint arc))
;; Create the arc length dimension in model space
(setq dimArcLength (vla-AddDimArc modelSpace center startPoint endPoint arcPoint))
(vla-ZoomExtents acadObj)
)