求问一道 C#笔试题，想知道最好的解答是什么

题目如下：

这是一段用于计算多个几何形状面积之和的程序

1. 指出程序段存在的不妥之处，并重构代码
2. 在 1 的基础上增加一种可计算的几何形状类型三角形，其面积为 底×高×1/2

public class Shape
{
    public int Type { get; set; }
    public int Radius { get; set; }
    public int Width { get; set; }
    public int Length { get; set; }
}

public class ShapeCalculator
{
    public static double CalculateTotalArea(List<Shape> shapes)
    {
        double totalArea = 0;
        if (shapes != null && shapes.Count() > 0)
        {
            foreach (Shape shape in shapes)
            {
                if (shape.Type == 1)
                {
                    totalArea += Math.PI * shape.Radius * shape.Radius;
                }
                else if (shape.Type == 2)
                {
                    totalArea += shape.Length * shape.Width;
                }
            }
        }
        return totalArea;
    }
}

我的回答是这样的：

interface ICalc
{
    double GetAera();
}

class Circle : ICalc
{
    public int Radius { get; set; }

    public double GetAera()
    {
        return Math.PI * Radius * Radius;
    }
}

class Rectangle : ICalc
{
    public int Length { get; set; }
    public int Width { get; set; }

    public double GetAera()
    {
        return Length * Width;
    }
}

class Triangle : ICalc
{
    public int Bottom { get; set; }
    public int Height { get; set; }

    public double GetAera()
    {
        return Bottom * Height * 0.5;
    }
}

class ShapeCalculator
{
    public static double CalculateTotalArea(List<ICalc> shapes)
    {
        double totalArea = 0;
        if (shapes != null)
        {
            totalArea = shapes.Sum(shape => shape.GetAera());
        }
        return totalArea;
    }
}

凭我的水平和理解能力就只有写到这种程度了……有什么更好的写法吗？