Circle.md

Circle Class

Overview

The Circle class represents a circular entity in AutoCAD, defined by a center point and radius.

Namespace

Autodesk.AutoCAD.DatabaseServices

Inheritance Hierarchy

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

Key Properties

Property	Type	Description
Center	Point3d	Gets/sets the center point of the circle
Radius	double	Gets/sets the radius
Diameter	double	Gets/sets the diameter
Circumference	double	Gets the circumference (read-only)
Area	double	Gets the area (read-only)
Normal	Vector3d	Gets/sets the normal vector (defines plane)
Thickness	double	Gets/sets the thickness (for 3D extrusion)

Key Methods

Method	Return Type	Description
GetClosestPointTo(Point3d, bool)	Point3d	Gets the closest point on the circle
GetPointAtParameter(double)	Point3d	Gets a point at parameter (0-2π)
GetDistAtPoint(Point3d)	double	Gets the distance along the circle to a point

Code Examples

Example 1: Creating a Circle

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    BlockTableRecord btr = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) as BlockTableRecord;
    
    // Create a circle at (50, 50, 0) with radius 25
    Circle circle = new Circle();
    circle.Center = new Point3d(50, 50, 0);
    circle.Radius = 25.0;
    
    // Set properties
    circle.Layer = "0";
    circle.ColorIndex = 2; // Yellow
    
    // Add to database
    btr.AppendEntity(circle);
    tr.AddNewlyCreatedDBObject(circle, true);
    
    tr.Commit();
}

Example 2: Getting Circle Properties

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    Circle circle = tr.GetObject(circleId, OpenMode.ForRead) as Circle;
    
    Point3d center = circle.Center;
    double radius = circle.Radius;
    double diameter = circle.Diameter;
    double circumference = circle.Circumference;
    double area = circle.Area;
    
    ed.WriteMessage($"\nCenter: ({center.X:F2}, {center.Y:F2})");
    ed.WriteMessage($"\nRadius: {radius:F2}");
    ed.WriteMessage($"\nDiameter: {diameter:F2}");
    ed.WriteMessage($"\nCircumference: {circumference:F2}");
    ed.WriteMessage($"\nArea: {area:F2}");
    
    tr.Commit();
}

Example 3: Getting Points on Circle

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    Circle circle = tr.GetObject(circleId, OpenMode.ForRead) as Circle;
    
    // Get points at 0°, 90°, 180°, 270°
    Point3d pt0 = circle.GetPointAtParameter(0); // 0 degrees
    Point3d pt90 = circle.GetPointAtParameter(Math.PI / 2); // 90 degrees
    Point3d pt180 = circle.GetPointAtParameter(Math.PI); // 180 degrees
    Point3d pt270 = circle.GetPointAtParameter(3 * Math.PI / 2); // 270 degrees
    
    tr.Commit();
}

Example 4: Creating Concentric Circles

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    BlockTableRecord btr = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) as BlockTableRecord;
    
    Point3d center = new Point3d(100, 100, 0);
    
    // Create 5 concentric circles
    for (int i = 1; i <= 5; i++)
    {
        Circle circle = new Circle();
        circle.Center = center;
        circle.Radius = i * 10.0;
        
        btr.AppendEntity(circle);
        tr.AddNewlyCreatedDBObject(circle, true);
    }
    
    tr.Commit();
}

Example 5: Modifying Circle

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    Circle circle = tr.GetObject(circleId, OpenMode.ForWrite) as Circle;
    
    // Move center
    circle.Center = new Point3d(75, 75, 0);
    
    // Change radius
    circle.Radius = 50.0;
    
    // Or set diameter instead
    circle.Diameter = 100.0; // Same as radius 50
    
    tr.Commit();
}

Common Patterns

Creating a Circle from 3 Points

// Calculate circle from 3 points (requires geometry calculations)
Point3d p1 = new Point3d(0, 0, 0);
Point3d p2 = new Point3d(100, 0, 0);
Point3d p3 = new Point3d(50, 50, 0);

CircularArc3d arc3d = new CircularArc3d(p1, p2, p3);
Point3d center = arc3d.Center;
double radius = arc3d.Radius;

Circle circle = new Circle();
circle.Center = center;
circle.Radius = radius;

Checking if Point is Inside Circle

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    Circle circle = tr.GetObject(circleId, OpenMode.ForRead) as Circle;
    
    Point3d testPoint = new Point3d(60, 60, 0);
    double distance = circle.Center.DistanceTo(testPoint);
    
    bool isInside = distance <= circle.Radius;
    
    ed.WriteMessage($"\nPoint is {(isInside ? "inside" : "outside")} the circle");
    
    tr.Commit();
}

Related Objects

• Arc - Partial circle
• Ellipse - Oval shape
• Curve - Base class
• Entity - Base class for all entities

References

• Autodesk Official Documentation