无法在Array.BinarySearch中调用键的比较器

Question

为了观察System.Array.BinarySearch中键的比较器的调用（与涉及调用数组元素的比较器的典型方案相反），我们设置了以下测试：

class Many 
{
    public string data { get; set; }
}
class One : Many, IComparable
{
    public int CompareTo(object arg)
    {
        Many other = arg as Many;
        Console.WriteLine("Comparator of One invoked from: " + this.data);
        if (this.data.Length < other.data.Length) return -1;
        if (this.data.Length > other.data.Length) return 1;
        return 0;
    }
}

使用这些新的声明时，我运行以下命令：

   Many[] manies = new[] { new Many { data = "1" }, 
                           new Many { data = "22" }, 
                           new Many { data = "333" }, 
                           new Many { data = "4444" }, };
   One one = new One {data="???"};
   Console.WriteLine(Array.BinarySearch(manies, one));

我得到的输出：

Unhandled Exception: System.InvalidOperationException: Failed to compare two elements in the array. ---> System.ArgumentException: At least one object must implement IComparable.
   at System.Collections.Comparer.Compare(Object a, Object b)
   at System.Collections.Generic.ObjectComparer`1.Compare(T x, T y)
   at System.Collections.Generic.ArraySortHelper`1.InternalBinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)
   at System.Collections.Generic.ArraySortHelper`1.BinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)
   --- End of inner exception stack trace ---
   at System.Collections.Generic.ArraySortHelper`1.BinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)
   at System.Array.BinarySearch[T](T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)
   at System.Array.BinarySearch[T](T[] array, T value)
   at ConsoleApplication1.Prosram.Main(String[] args) in C:\Users\xxx\documents\visual studio 2010\Projects\ConsoleApplication1\ConsoleApplication1\Program.cs:line xxx

Answer 1

BinarySearch的行为始终存在一个问题，即密钥的比较器从未真正执行过，即使MSDN文档说如果数组元素未实现IComparable就是这种情况。 .NET版本4.5中解决了此问题。

错误的堆栈跟踪中有一些线索显示出源自System.Collections.Comparer.Compare()的错误。 在.NET 4.5和.NET 4.0中查看此方法的.NET源代码，可以发现对该方法的实现方式进行了重要更改：

.NET 4.5：

public int Compare(Object a, Object b) {
    if (a == b) return 0;
    ...
    IComparable ia = a as IComparable; 
    if (ia != null)
        return ia.CompareTo(b); 

    IComparable ib = b as IComparable;
    if (ib != null) 
        return -ib.CompareTo(a);

    throw new ArgumentException(Environment.GetResourceString(
                                       "Argument_ImplementIComparable"));

.NET 4.0：

public int Compare(Object a, Object b) {
    if (a == b) return 0;
    ...
    IComparable ia = a as IComparable; 
    if (ia != null)
        return ia.CompareTo(b); 

    throw new ArgumentException(Environment.GetResourceString(
                                      "Argument_ImplementIComparable"));

在我们的例子中，我们想观察到键的比较器正在被调用，在这种情况下，键为b而数组中的元素为a 。

这意味着对于4.5之前的.NET版本，BinarySearch方法的MSDN文档不正确。 如果数组中的元素未实现IComparable ，则该方法不考虑键的Comparator ，因为该实现取决于Comparer实际执行的操作，而不是这样做，而是抛出ArgumentException 。 但是，在4.5版中，它的工作原理始终如BinarySearch 。