...
    bool largeArc = WedgeAngle>180.0;
               
    Size outerArcSize = new Size(Radius, Radius);
    Size innerArcSize = new Size(InnerRadius, InnerRadius);

    context.BeginFigure(innerArcStartPoint, true, true);
    context.LineTo(outerArcStartPoint, true, true);
    context.ArcTo(outerArcEndPoint, outerArcSize, 0, largeArc, 
                  SweepDirection.Clockwise, true, true);
    context.LineTo(innerArcEndPoint, true, true);
    context.ArcTo(innerArcStartPoint, innerArcSize, 0, largeArc, 
                  SweepDirection.Counterclockwise, true, true);
...

        void DrawDialStrip(float p)
        {
            if (dialPath == null)
            { 
                dialPath = new Path();
                dialPath.SetFill(new SolidColorBrush(stripColor));
                AssociatedObject.GetChildren().Add(dialPath);
            }

            float a = startAngleRad + (finishAngleRad - startAngleRad) * p;
            bool isLargeArc = Mathf.Abs(a - startAngleRad) > Mathf.PI;
                        
            var geometry = new StreamGeometry();
            using (var context = geometry.Open())
            {
                context.BeginFigure(startPoint1, true);
                context.ArcTo(GetPointFromPolarCoord(a, r1), new Size(r1, r1), 0, isLargeArc, isClockwise);
                context.LineTo(GetPointFromPolarCoord(a, r2));
                context.ArcTo(startPoint2, new Size(r2, r2), 0, isLargeArc, !isClockwise);
            }
            dialPath.SetData(geometry);
        }

        Point GetPointFromPolarCoord(float a, float r)
        {
            return new Point(r * Mathf.Cos(a), -r * Mathf.Sin(a)) + centerPoint;
        }