我写的一个练习题  目的是 使用面向对象的方法求三角形面积，用到了海伦公式。貌似可以运行正确结果，但是奇怪在我把其中的<换成<=就不对了，全部都成了不能构成三角形，不知道问题出在哪里，程序如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace study1
{
    class Program
    {
        static void Main(string[] args)
        {
            tri tt = new tri();
            tt.A = 4;
            tt.B = 5;
            tt.C = 6;
            if (tt.ar(tt.A, tt.B, tt.C) == -1)
            {
                Console.WriteLine("没有面积");
            }
            else
            {
                Console.WriteLine("面积是{0}", tt.ar(tt.A, tt.B, tt.C));
            }
            Console.ReadKey();
        }
    }
    class tri
    {
        private float a,b,c;
        public float A
        {
            set
            {
                if (a < 0)
                {
                    return;
                }
                this.a = value;
            }
            get
            {
                return this.a;
            }
        }
        public float B
        {
            set
            {
                if (b < 0)
                {
                    return;
                }
                this.b = value;
            }
            get
            {
                return this.b;
            }
        }
        public float C
        {
            set
            {
                if (c < 0)
                {
                    return;
                }
                this.c = value;
            }
            get
            {
                return this.c;
            }
        }
        public float ar(float aa, float bb, float cc)
        {
            float s, d, p;
            if (aa > (cc + bb))
            {
                Console.WriteLine("不能构成三角形！");
                return -1;
            }
            if (bb > (aa + cc))
            {
                Console.WriteLine("不能构成三角形！");
                return -1;
            }
            if (cc > (bb + aa))
            {
                Console.WriteLine("不能构成三角形！");
                return -1;
            }
            p = aa / 2 + bb / 2 + cc / 2;
            d = p*(p-aa)*(p-bb)*(p-cc);
            s = Convert.ToSingle(System.Math.Sqrt(d));
            return s;
        }
    }
}

只能在于，等于就不是三角形了！

比如说：
 set
            {
                if (b < 0)
                {
                    return;
                }
                this.b = value;
            }
按理说这个<换成<=没啥问题么，但是一换结果就成了不能构成三角形 - -！

   class tri
    {
        private float a,b,c;
        public float A
        {
            set
            {
                if (value <= 0)
                {
                    return;
                }
                this.a = value;
            }
            get
            {
                return this.a;
            }
        }
        public float B
        {
            set
            {
                if (value <= 0)
                {
                    return;
                }
                this.b = value;
            }
            get
            {
                return this.b;
            }
        }
        public float C
        {
            set
            {
                if (value <= 0)
                {
                    return;
                }
                this.c = value;
            }
            get
            {
                return this.c;
            }
        }
        public float ar(float aa, float bb, float cc)
        {
            float s, d, p;
            if (aa > (cc + bb))
            {
                Console.WriteLine("不能构成三角形！");
                return -1;
            }
            if (bb > (aa + cc))
            {
                Console.WriteLine("不能构成三角形！");
                return -1;
            }
            if (cc > (bb + aa))
            {
                Console.WriteLine("不能构成三角形！");
                return -1;
            }
            p = aa / 2 + bb / 2 + cc / 2;
            d = p*(p-aa)*(p-bb)*(p-cc);
            s = Convert.ToSingle(System.Math.Sqrt(d));
            return s;
        }
    }
}

在楼主的三角形的类中。 a ,b ,c 的没有初始化， 默认值是0。所以"a <= 0"就不对。会直接返回。正确的应该是（Value <= 0).