根据以下代码,按要求填空。 enum Shape { Square=1, Rectangle } struct ShapeArg { public double Length; public double Width; } class Program { static void Main(string[] args) { ShapeArg sa = new ShapeArg(); sa.Length = 10; sa.Width = 20; double d1 = GetArea(Shape.Rectangle, sa); Console.WriteLine(d1); double d2 = GetArea(Shape.Square, sa); Console.WriteLine(d2); sa.Length = 20; double d3 = GetArea(Shape.Rectangle, sa); Console.WriteLine(d3); } static double GetArea(Shape shape, ShapeArg shapeArg) { double result = -1; switch (shape) { case Shape.Square: if (shapeArg.Length == shapeArg.Width) { result = shapeArg.Length * shapeArg.Width; } else { Console.WriteLine("不是正方形"); } break; case Shape.Rectangle: result = shapeArg.Length * shapeArg.Width; break; } return result; } } 以上代码输出依次分别是( 1 )、( 2 )、( 3 )、( 4 )。