c# - 列表需要类型参数

Question

编译代码时出现三个错误。1.使用泛型类型List需要1个参数。2.使用泛型类型List需要1个参数。3. foreach 语句无法对类型变量进行操作，因为 List 不包含“GetEnumerator”的公共(public)定义

多态示例程序如下。

namespace PolymorExample
{
    abstract class Shape
    {
        public abstract void area();
    }

    class Rectangle : Shape
    {
        private double length;
        private double width;

        public Rectangle(double length, double width)
        {
            this.length = length;
            this.width = width;
        }

        public override void area()
        {
            Console.WriteLine("Rectangel Area: {0}", length * width);
        }
    }

    class Triangle : Shape
    {
        private double baseline;
        private double height;

        public Triangle(double baseline, double height)
        {
            this.baseline = baseline;
            this.height = height;
        }

        public override void area()
        {
            Console.WriteLine("Triangel Area: {0}", baseline * height / 2.0);
        }
    }

    class Circle : Shape
    {
        const double PI = 3.14;
        private double radius;

        public Circle(double radius)
        {
            this.radius = radius;
        }

        public override void area()
        {
            Console.WriteLine("Circle Area: {0}", radius * radius * PI);
        }
    }

    public class TestShape
    {
        static void Main()
        {
            List shapes = new List();
            Shape shape1 = new Rectangle(10, 10);
            shapes.Add(shape1);
            shapes.Add(new Circle(10));
            shapes.Add(new Triangle(10, 10));
            shapes.Add(new Circle(20));

            foreach (Shape s in shapes)
            {
                s.area();
            }

            Console.Read();
        }
    }
}

Answer 1

如果您查看 List<T> class 的文档，你会注意到 List是 generic类型(因此是 <T> )，泛型类型需要一个(或更多)参数来指定它将使用/包含的对象的类型。您必须指定某些类型，即使它只是 object 。

就您而言，您有一个列表 Shape对象，因此可以修改您的初始化代码(并通过使用集合初始值设定项语法进行简化)以指定该类型:

var shapes = new List<Shape>
{
    new Rectangle(10, 10),
    new Circle(10),
    new Triangle(10, 10),
    new Circle(20)
};