找不到合适的方法来覆盖c#

Question

我已经尝试了一些方法来修复错误,我似乎无法想出这个,我会非常感谢任何帮助.错误在Triangle和Square类中,Triangle中的错误是"不实现GeometricFigure的继承抽象成员"和"没有找到合适的方法来覆盖"而Square只是"找不到合适的方法来覆盖"错误.

namespace ShapesDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            Rectangle rec = new Rectangle(8,10);
            Square squ = new Square(11, 12);
            Triangle tri = new Triangle(10, 20);
            Console.WriteLine("Computed area is {0}" + "\n\n" + "Computed Triangle is: {1}"         + "\n", squ.ComputeArea(rec.Area), tri.ComputeArea(rec.Area));

        }
    }

    abstract class GeometricFigure
    {
        public decimal _height, _width, _area;


        public decimal Height
        {
            get { return _height; }
            set { _height = value; }
        }

        public decimal Width
        {
            get { return _width; }
            set { _width = value; }
        }

        public decimal Area
        {
            get { return _area; }
        }

        public abstract decimal ComputeArea();

    }

    class Rectangle : GeometricFigure
    {
        private decimal height, width;

        public Rectangle(decimal sideA, decimal sideB)
        {
            this.height = sideA;
            this.width = sideB;
        }

        public Rectangle()
        {
        }

        public override decimal ComputeArea()
        {
            Console.WriteLine("The Area is" + _width.ToString(), _height.ToString());
            return width * height;
        }

    }

    class Square : Rectangle
    {
        public Square(decimal sideA, decimal sideB)
        {
            this._width = sideA;
            this._height = sideB;
            if (sideA != sideB)
                this._height = this._width;
        }

        public Square(decimal xy)
        {
            this._width = xy;
            this._height = this._width;
        }

        public override decimal ComputeArea(decimal _area)
        { return _area = this._width * this._height; }
    }

    class Triangle : GeometricFigure
    {
        public Triangle(int x, int y)
        {
            this.Width = x;
            this.Height = y;
        }

        public override decimal ComputeArea(decimal _area)
        { return _area = (this.Width * this.Height) / 2; }
    }
}

Answer 1

每当你覆盖一个方法时,你必须使用与基类相同的签名覆盖(协方差和逆变的例外,但那些不适用于你的问题,所以我会在这里忽略它们).

在GeometricFigure,你有声明

public abstract decimal ComputeArea();

但在你Square和Triangle你有声明

public override decimal ComputeArea(decimal _area)
{
    // ...
}

假设其他一些类包含以下代码:

GeometricFigure fig = new Triangle(10, 10);
decimal area = fig.ComputeArea();

哪个ComputeArea叫？ Triangle没有定义ComputeArea没有参数,也没有定义GeometricFigure,所以没有有效ComputeArea的调用.因此,语言规范不允许这种情况,override只要求将其放在实际覆盖基类方法的方法上,并使用相同数量和类型的参数.由于ComputeArea(decimal)不覆盖ComputeArea(),编译器会输出错误并告诉您必须将override关键字放在ComputeArea()定义中Triangle,并且不能将override关键字置于其上ComputeArea(decimal).

这并不是说你不能定义一个方法ComputeArea(decimal)上Triangle和Square,但不能将其声明为覆盖ComputeArea()在GeometricFigure.