关于排序的问题

一、int 和string 类型的的集合的排序

static void Main(string[] args)
        {
            //ArrayList list = new ArrayList() {1,8,3,992,888,33,77,38,2,34 };//数字
            //ArrayList list = new ArrayList() {"Joney","Tindy","Bruce","Wesley","Candy","Alice","Tom"};//字符
            ArrayList list = new ArrayList() {"张三","李四","王五","赵六","刘七","钱八"};//数字
            Console.WriteLine("排序前：");
            for (int i = 0; i < list.Count; i++)
            {
                Console.Write(list[i]+" ");
            }
            Console.WriteLine("");

            //排序
            list.Sort();
            list.Reverse();
            //排序后
            Console.WriteLine("排序后：");
            for (int i = 0; i < list.Count; i++)
            {
                Console.Write(list[i]+" ");
            }

            Console.ReadKey();
        }

int 类型，是根据数字大小，降序排列。string类型是根据首字符的汉语拼音排序，这个都比较好理解。如果改成类的实例呢？

 static void Main(string[] args)
        {
            //ArrayList list = new ArrayList() {1,8,3,992,888,33,77,38,2,34 };//数字
            //ArrayList list = new ArrayList() {"Joney","Tindy","Bruce","Wesley","Candy","Alice","Tom"};//字符
            //ArrayList list = new ArrayList() {"张三","李四","王五","赵六","刘七","钱八"};//数字
            ArrayList list = new ArrayList() {
                new Person() { Name="张三" },
                new Person() { Name="李四" },
                new Person() { Name="王五" },
                new Person() { Name="赵六" },
                new Person() { Name="刘七" },
                new Person() {Name="钱八" }
            };
            Console.WriteLine("排序前：");
            for (int i = 0; i < list.Count; i++)
            {
                Console.Write(list[i]+" ");
            }
            Console.WriteLine("");

            //排序
            list.Sort();
            list.Reverse();
            //排序后
            Console.WriteLine("排序后：");
            for (int i = 0; i < list.Count; i++)
            {
                Console.Write(list[i]+" ");
            }

            Console.ReadKey();
        }

上述代码在list.sort()那报错了（无法比较的对象，至少有一个对象实现IComparable）。

这里可以这样理解：前面排序的时候，根据数值大小，字母顺序 排序，但是到了给对象排序的时候，编译器很迷茫，不知道根据什么排序，所以程序报错了，卡在了这里。打开详细信息，提示需要实现IComparable接口。我们把上面的代码改写下：

 class Program
    {
        static void Main(string[] args)
        {
            //ArrayList list = new ArrayList() {1,8,3,992,888,33,77,38,2,34 };//数字
            //ArrayList list = new ArrayList() {"Joney","Tindy","Bruce","Wesley","Candy","Alice","Tom"};//字符
            //ArrayList list = new ArrayList() {"张三","李四","王五","赵六","刘七","钱八"};//数字
            ArrayList list = new ArrayList() {
                new Person() { Name="张三",Age=18 },
                new Person() { Name="李四" ,Age=48},
                new Person() { Name="王五",Age=14 },
                new Person() { Name="赵六" ,Age=33},
                new Person() { Name="刘七",Age=27 },
                new Person() {Name="钱八" ,Age=41}
            };
            Console.WriteLine("排序前：");
            for (int i = 0; i < list.Count; i++)
            {
                Console.Write(((Person)list[i]).Name+" ");
            }
            Console.WriteLine("");

            //排序
            list.Sort();
            list.Reverse();
            //排序后
            Console.WriteLine("排序后：");
            for (int i = 0; i < list.Count; i++)
            {
                Console.Write(((Person)list[i]).Name + " ");
            }

            Console.ReadKey();
        }
    }

    public class Person:IComparable
    {
        public string Name { get; set; }
        public int Age { get; set; }
        public int CompareTo(object obj)
        {
            Person p=obj as Person;
            if (p != null)
            {
                return p.Age - this.Age;
            }
            else
            {
                throw new InvalidOperationException("无效的操作");
            }
        }
    }

输出后的结果是根据年龄升序排列的：

排序前：
张三 李四 王五 赵六 刘七 钱八
排序后：
王五 张三 刘七 赵六 钱八 李四

其实我们看反编译后的代码，不管是int还是string，都实现了IComparable接口。也就是说，想调用ArrayList中sort（）方法排序，集合中的元素都必须实现IComparable接口。

接下来，我们把程序写得灵活点。上面我们把排序的字段写死了，如果更换排序的依据，需要更改方法中的代码。我们能否不改类中方法中的代码，也能根据用户的想法排序呢？这需要我们写个排序的比较器。

class Program
    {
        static void Main(string[] args)
        {
            //ArrayList list = new ArrayList() {1,8,3,992,888,33,77,38,2,34 };//数字
            //ArrayList list = new ArrayList() {"Joney","Tindy","Bruce","Wesley","Candy","Alice","Tom"};//字符
            //ArrayList list = new ArrayList() {"张三","李四","王五","赵六","刘七","钱八"};//数字
            ArrayList list = new ArrayList() {
                new Person() { Name="张三",Age=18 },
                new Person() { Name="李四" ,Age=48},
                new Person() { Name="王五",Age=14 },
                new Person() { Name="赵六" ,Age=33},
                new Person() { Name="刘七",Age=27 },
                new Person() {Name="钱八" ,Age=41}
            };
            Console.WriteLine("排序前：");
            for (int i = 0; i < list.Count; i++)
            {
                Console.Write(((Person)list[i]).Name+" ");
            }
            Console.WriteLine("");

            //排序
            list.Sort(new SortPersonByName());
            //list.Reverse();
            //排序后
            Console.WriteLine("排序后：");
            for (int i = 0; i < list.Count; i++)
            {
                Console.Write(((Person)list[i]).Name + " ");
            }

            Console.ReadKey();
        }
    }

    /// <summary>
    /// 根据姓氏排序
    /// </summary>
    public class SortPersonByName : IComparer
    {
        public int Compare(object x, object y)
        {
            Person p1 = x as Person;
            Person p2 = y as Person;
            if (p1 != null && p2 != null)
            {
                return p1.Name.CompareTo(p2.Name);
            }
            else
            {
                throw new Exception("排序异常");
            }
        }
    }

    /// <summary>
    /// 根据年龄排序
    /// </summary>
    public class SortPersonByAge : IComparer
    {
        public int Compare(object x, object y)
        {
            Person p1 = x as Person;
            Person p2 = y as Person;
            if (p1!=null&&p2!=null)
            {
                return p1.Age - p2.Age;
            }
            else
            {
                throw new Exception("排序异常");
            }
        }
    }

如上代码，我们在调用list.sort()的时候传入自己编写的比较器，就可以个性化定制排序依据。需要强调的是，根据对象的Name排序的时候，由于字符串本身有compareto方法，可以直接调用。