using System;
using System.Collections.Generic;
using System.Text;

namespace 面积
{
   public  class Class1
    {
        private double x;
        public double y;
        
        public Class1(double m, double n)
        {
            x = m;
            y = n;
        }

        public  double Aera();

    }
}
    public class rentangle : Class1

    {
        public rentangle(double _x, double _y)
        {

            x = _x;
            y = _y;


        }

        public override double Area()
        {
            return x * y;

        }
    }
   public  class circle : Class1
    {

        public circle(double _x)
        {

            x = _x;


        }

        public override double Area()
        {
            return x * x * 3.14;


        }
    }
错误    1    找不到类型或命名空间名称“Class1”(是否缺少 using 指令或程序集引用?)    E:\c#\新建文件夹\新建文件夹\面积\面积\Class1.cs    22    30    面积

using那里加一个using 面积;
另代码中也有问题，算了还是帮你改下吧，太乱了，如下：

using 面积;

namespace 面积
{
    public abstract class Class1
    {
        public double x;
        public double y;

        protected Class1(double m, double n)
        {
            x = m;
            y = n;
        }
        public abstract double Aera();
    }
}

public class rentangle : Class1
{
    public rentangle(double _x, double _y) : base(_x, _y)
    {
        x = _x;
        y = _y;
    }

    public override double Aera()
    {
        return x * y;
    }
}

public class circle : Class1
{
    public circle(double _x) : base(_x, _x)
    {
        x = _x;
    }

    public override double Aera()
    {
        return x * x * 3.14;
    }
}

你没有调用基类构造函数
也没有将Class1定义为抽象类，抽象方法Area的定义也不对

[ 本帖最后由 mmxo 于 2012-12-15 21:19 编辑 ]

谢谢版主啦！