使用LINQ获取具有自定义对象的列表的总和/平均值

I have a class:

我有一节课:

public class PointD
{
    public double X
    { get; set; }
    public double Y
    { get; set; }

    public PointD(double x, double y)
    {
        X = x;
        Y=y;
    }
   //operator for +,-, * and / are overridden
 }
public class PointD