/// <param name="angleRadians">The angle, in radians, from 3-o'clock going counter-clockwise.</param>
public static void DrawLine(this DrawingContext drawingContext, Pen pen, Point startPoint, double angleRadians, double length)
{
Contract.Requires <ArgumentNullException>(drawingContext != null);
Contract.Requires <ArgumentException>(startPoint.IsValid());
Contract.Requires <ArgumentNullException>(pen != null);
drawingContext.DrawLine(pen, startPoint, startPoint + GeoHelper.GetVectorFromAngle(angleRadians, length));
}