排序IComparable的对象，其中一些是空的 [英] Sorting IComparable objects some of which are null

问题描述

大多数人，当写refence类型（类），它实现IComparable的＆LT; T＆GT ;,使用空比任何实际对象较少约定。但是，如果你尝试用相反的约定，一些有趣的事情发生了：

Most people, when writing a refence type (class) which implements IComparable<T>, use the convention that null is LESS than any actual object. But if you try to use the opposite convention, something interesting happens:

using System;
using System.Collections.Generic;

namespace SortingNulls
{
  internal class Child : IComparable<Child>
  {
    public int Age;
    public string Name;

    public int CompareTo(Child other)
    {
      if (other == null)
        return -1; // what's your problem?

      return this.Age.CompareTo(other.Age);
    }

    public override string ToString()
    {
      return string.Format("{0} ({1} years)", this.Name, this.Age);
    }
  }

  internal static class Program
  {
    private static void Main()
    {
      var listOfChilds = new List<Child>
      {
        null,
        null,
        null,
        null,
        new Child { Age = 5, Name = "Joe" },
        new Child { Age = 6, Name = "Sam" },
        new Child { Age = 3, Name = "Jude" },
        new Child { Age = 7, Name = "Mary" },
        null,
        null,
        null,
        null,
        new Child { Age = 7, Name = "Pete" },
        null,
        new Child { Age = 3, Name = "Bob" },
        new Child { Age = 4, Name = "Tim" },
        null,
        null,
      };

      listOfChilds.Sort();

      Console.WriteLine("Sorted list begins here");
      for (int i = 0; i < listOfChilds.Count; ++i)
        Console.WriteLine("{0,2}: {1}", i, listOfChilds[i]);
      Console.WriteLine("Sorted list ends here");
    }
  }
}

在运行上面的code，你看到的空引用不排序预期。显然，相比较A到B的时候，如果A是一个对象，B是零，用户自定义的比较时，但如果反过来A是零，B是一个对象，一些BCL对比来代替。

When running the above code, you see that the null references are not sorted as expected. Apparently, when comparing A to B, if A is an object and B is null, the user-defined comparision is used, but if conversely A is null and B is an object, some BCL comparision is used instead.

这是一个错误？

推荐答案

没有，这是不是一个错误。你的的CompareTo 方法，它实现 IComparable的＆LT;儿童＆GT; 是在儿童定义类。换句话说，如果你有才能做一个比较你的类型之一调用一个方法。

No, this is not a bug. Your CompareTo method which implements IComparable<Child> is defined on your Child class. In other words, if you have to invoke a method on one of your types in order to make a comparison.

如果的孩子项目之一，被比较是空，你怎么可以调用的CompareTo 就可以了？

If one of the Child items being compared is null, how can you invoke CompareTo on it?

请注意，从<一个定义href="http://msdn.microsoft.com/en-us/library/system.icomparable.compareto.aspx">IComparable:

根据定义，任何物体比较大于（或以下）空引用（在Visual Basic中为Nothing），和两个空引用比较彼此相等。的

这也解释了结果，你观察到。

Which explains the results you observe.

解决的办法是委托给一些其他的类来进行比较。请参阅<一href="http://msdn.microsoft.com/en-us/library/system.collections.icomparer.compare.aspx">IComparer接口。

The solution is to delegate to some other class to perform the comparison. See the IComparer interface.

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