Arc.md

Arc Class

Overview

The Arc class represents a circular arc in AutoCAD, defined by a center point, radius, start angle, and end angle.

Namespace

Autodesk.AutoCAD.DatabaseServices

Inheritance Hierarchy

System.Object
  └─ RXObject
      └─ DBObject
          └─ Entity
              └─ Curve
                  └─ Arc

Key Properties

Property	Type	Description
Center	Point3d	Gets/sets the center point
Radius	double	Gets/sets the radius
StartAngle	double	Gets/sets the start angle (radians)
EndAngle	double	Gets/sets the end angle (radians)
TotalAngle	double	Gets the total angle swept (radians)
StartPoint	Point3d	Gets the start point
EndPoint	Point3d	Gets the end point
Length	double	Gets the arc length
Normal	Vector3d	Gets/sets the normal vector
Thickness	double	Gets/sets the thickness

Code Examples

Example 1: Creating an Arc

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    BlockTableRecord btr = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) as BlockTableRecord;
    
    // Create an arc: center (50,50), radius 25, from 0° to 90°
    Arc arc = new Arc();
    arc.Center = new Point3d(50, 50, 0);
    arc.Radius = 25.0;
    arc.StartAngle = 0; // 0 degrees
    arc.EndAngle = Math.PI / 2; // 90 degrees
    
    btr.AppendEntity(arc);
    tr.AddNewlyCreatedDBObject(arc, true);
    
    tr.Commit();
}

Example 2: Creating Arc from 3 Points

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    BlockTableRecord btr = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) as BlockTableRecord;
    
    Point3d startPt = new Point3d(0, 0, 0);
    Point3d midPt = new Point3d(50, 50, 0);
    Point3d endPt = new Point3d(100, 0, 0);
    
    // Use CircularArc3d to calculate arc parameters
    CircularArc3d arc3d = new CircularArc3d(startPt, midPt, endPt);
    
    Arc arc = new Arc();
    arc.Center = arc3d.Center;
    arc.Radius = arc3d.Radius;
    arc.StartAngle = arc3d.ReferenceVector.AngleOnPlane(new Plane(arc3d.Center, arc3d.Normal));
    arc.EndAngle = arc.StartAngle + (arc3d.EndAngle - arc3d.StartAngle);
    arc.Normal = arc3d.Normal;
    
    btr.AppendEntity(arc);
    tr.AddNewlyCreatedDBObject(arc, true);
    
    tr.Commit();
}

Example 3: Getting Arc Properties

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    Arc arc = tr.GetObject(arcId, OpenMode.ForRead) as Arc;
    
    double startAngleDeg = arc.StartAngle * (180.0 / Math.PI);
    double endAngleDeg = arc.EndAngle * (180.0 / Math.PI);
    double totalAngleDeg = arc.TotalAngle * (180.0 / Math.PI);
    double arcLength = arc.Length;
    
    ed.WriteMessage($"\nStart Angle: {startAngleDeg:F2}°");
    ed.WriteMessage($"\nEnd Angle: {endAngleDeg:F2}°");
    ed.WriteMessage($"\nTotal Angle: {totalAngleDeg:F2}°");
    ed.WriteMessage($"\nArc Length: {arcLength:F2}");
    
    tr.Commit();
}

Related Objects

• Circle - Full circle
• Ellipse - Elliptical arc
• Curve - Base class

References

• Autodesk Official Documentation