创建对象时需要考虑是否实现比较器

1、基础

　　有对象的地方，一般都会存在比较。在此根据一个Salary列表实现对象比较的例子。Salary类需要实现接口IComparable。

class Salary:IComparable
{
    public string Name { get; set; }
    public int BaseSalary { get; set; }
    public int Bonus { get; set; }

    public int CompareTo(object obj)
    {
        Salary salary = obj is Salary ? obj as Salary : null;
        if(salary==null)
        {
            throw new Exception("类型不同，无法比较");
        }
        if(BaseSalary>salary.BaseSalary)
        {
            return 1;
        }
        else if(BaseSalary==salary.BaseSalary)
        {
            return 0;
        }
        else
        {
            return -1;
        }
        //以上代码可有以下代码代替，使用整型默认比较方法
        //return BaseSalary.CompareTo(salary.BaseSalary);
    }
}

　　实现接口IComparable后，可以根据BaseSalary对Salary集合对象排序。在Main函数中代码如下：

public static void Main()
{
    ArrayList companySalary = new ArrayList();
    companySalary.Add(new Salary() { Name = "Mike", BaseSalary = 3000,Bonus=1000 });
    companySalary.Add(new Salary() { Name = "Rose", BaseSalary = 2000, Bonus = 6000 });
    companySalary.Add(new Salary() { Name = "Tom", BaseSalary = 5000, Bonus = 5000 });
    companySalary.Add(new Salary() { Name = "Jeffry", BaseSalary = 1000, Bonus = 2000 });
    companySalary.Add(new Salary() { Name = "Steve", BaseSalary = 4500, Bonus = 1900 });
    companySalary.Sort();
    foreach (Salary salary in companySalary)
    {
        Console.WriteLine("{0}\t BaseSalary: {1}\tBonus: {2}", salary.Name, salary.BaseSalary,salary.Bonus);
    }
}

　　但是，此处实现相当于比较局限，局限与只能使用BaseSalary排序，不能使用其他方式排序。如果使用Bonus进行排序，就需要改变源码。接口IComparer的作用可以避免这类问题。

2、IComparer接口

　　使用IComparer接口实现一个自定义比较器：

class BonusComparer : IComparer
{
     int IComparer.Compare(object x, object y)
    {
        Salary salary1 = x is Salary ? x as Salary : null;
        Salary salary2 = y is Salary ? y as Salary : null;
        return salary1.Bonus.CompareTo(salary2.Bonus);
    }
}

　　在主控端调用时排序时，需要在Sort方法中传入一个实现了IComparer接口的比较器对象：companySalary.Sort(new BonusComparer())。以此实现可以根据需要，实现多种比较。

3、泛型实现

　　在前面，使用了性能不高的ArrayList类。在泛型出来之后，需要尽量使用泛型集合。在CompareTo和Compare和函数中，需要对对象进行转型，比较影响性能，尤其是对大型集合进行排序时，性能影响非常明显。而泛型的出现，可以避免运行时的转型。因此，上面的代码全部更换为泛型类型：

class Program
{
    public static void Main()
    {
        List<Salary> companySalary = new List<Salary>();
        companySalary.Add(new Salary() { Name = "Mike", BaseSalary = 3000, Bonus = 1000 });
        companySalary.Add(new Salary() { Name = "Rose", BaseSalary = 2000, Bonus = 6000 });
        companySalary.Add(new Salary() { Name = "Tom", BaseSalary = 5000, Bonus = 5000 });
        companySalary.Add(new Salary() { Name = "Jeffry", BaseSalary = 1000, Bonus = 2000 });
        companySalary.Add(new Salary() { Name = "Steve", BaseSalary = 4500, Bonus = 1900 });
        companySalary.Sort();
        foreach (Salary salary in companySalary)
        {
            Console.WriteLine("{0}\t BaseSalary: {1}\tBonus: {2}", salary.Name, salary.BaseSalary, salary.Bonus);
        }
        companySalary.Sort(new BonusComparer());//提供一个默认比较器
        foreach (Salary salary in companySalary)
        {
            Console.WriteLine("{0}\t BaseSalary: {1}\tBonus: {2}", salary.Name, salary.BaseSalary, salary.Bonus);
        }
    }
}
class Salary : IComparable<Salary>
{
    public string Name { get; set; }
    public int BaseSalary { get; set; }
    public int Bonus { get; set; }

    public int CompareTo(Salary other)
    {
        if (other == null)
        {
            throw new Exception("类型不同，无法比较");
        }
        if (BaseSalary > other.BaseSalary)
        {
            return 1;
        }
        else if (BaseSalary == other.BaseSalary)
        {
            return 0;
        }
        else
        {
            return -1;
        }
        //以上代码可有以下代码代替，使用整型默认比较方法
        //return BaseSalary.CompareTo(other.BaseSalary);
    }
}

class BonusComparer : IComparer<Salary>
{
    public int Compare(Salary x, Salary y)
    {
        return x.Bonus.CompareTo(y.Bonus);
    }
}

 4、Linq实现

　　针对前面实现排序方法时，需要实现多个接口，新增了方法使得代码入侵性太高。可扩展性差，对于有新的排序要求，就必须实现新的比较器。使用Linq，可以实现遍历、筛选、投影集合等功能。

class Program
{
    public static void Main()
    {
        List<Salary> companySalary = new List<Salary>();
        companySalary.Add(new Salary() { Name = "Mike", BaseSalary = 3000, Bonus = 1000 });
        companySalary.Add(new Salary() { Name = "Rose", BaseSalary = 2000, Bonus = 6000 });
        companySalary.Add(new Salary() { Name = "Tom", BaseSalary = 5000, Bonus = 5000 });
        companySalary.Add(new Salary() { Name = "Jeffry", BaseSalary = 1000, Bonus = 2000 });
        companySalary.Add(new Salary() { Name = "Steve", BaseSalary = 4500, Bonus = 1900 });
        var baseSalarySort = from salary in companySalary orderby salary.BaseSalary select salary;
        foreach (Salary salary in baseSalarySort)
        {
            Console.WriteLine("{0}\t BaseSalary: {1}\tBonus: {2}", salary.Name, salary.BaseSalary, salary.Bonus);
        }
        var bonusSort = from salary in companySalary orderby salary.Bonus select salary;
        foreach (Salary salary in bonusSort)
        {
            Console.WriteLine("{0}\t BaseSalary: {1}\tBonus: {2}", salary.Name, salary.BaseSalary, salary.Bonus);
        }
    }
}
class Salary
{
    public string Name { get; set; }
    public int BaseSalary { get; set; }
    public int Bonus { get; set; }
}

　　Linq可以带来很大的便利性，但需要掌握比较器、迭代器、索引器的原理，以便更好地理解Linq的思想。