AnimationDeveloperSamples.MapGraphicKeyframe.RotateElement C# (CSharp) Method

        private void RotateElement(IElement elem, double new_angle, IAGAnimationContainer pContainer)
        {
            ITransform2D transform2D = elem as ITransform2D;
            IPoint rotateOrigin;

            if (elem.Geometry.GeometryType == esriGeometryType.esriGeometryPoint)
            {
                rotateOrigin = elem.Geometry as IPoint;
            }
            else
            {
                IEnvelope elementEnvelope = elem.Geometry.Envelope;
                IArea elementArea = elementEnvelope as IArea;
                rotateOrigin = elementArea.Centroid;
            }

            AddPropertySet(elem);

            IElementProperties prop = (IElementProperties)elem; //record the old properties
            IPropertySet propSet;
            propSet = (IPropertySet)prop.CustomProperty;
            double old_angle;
            old_angle = (double)propSet.GetProperty("Angle");

            propSet.SetProperty("Angle", new_angle); //update old angle

            transform2D.Rotate(rotateOrigin, new_angle-old_angle);
        }