获取在c#中转换为动态的数组的计数

Question

考虑以下代码:

  static void Main(string[] args)
  {
      var ints=new List<int> {10,11,22};
      Something(ints);//Output:Count is:3
      Something(new int[10]); //'System.Array' does not contain
                              //  a definition for 'Count'
      Console.ReadLine();     
  }
  static void Something(ICollection collection)
  {
      dynamic dynamic = collection;
      Console.WriteLine("Count is:{0}", dynamic.Count);
  }

通过List时,一切都还可以.但是当传递数组并转换为动态我得到这个错误:'System.Array' does not contain a definition for 'Count'.

我知道我的解决方案是什么,但我想知道为什么编译器有这种行为？

Answer 1

Something(new int[10]);

static void Something(ICollection collection)
{
    //The dynamic keyword tells the compilier to look at the underlying information
    //at runtime to determine the variable's type.  In this case, the underlying 
    //information suggests that dynamic should be an array because the variable you
    //passed in is an array.  Then it'll try to call Array.Count.
    dynamic dynamic = collection;
    Console.WriteLine("Count is:{0}", dynamic.Count);

    //If you check the type of variable, you'll see that it is an ICollection because
    //that's what type this function expected.  Then this code will try to call 
    //ICollection.Count
    var variable = collection;
    Console.WriteLine("Count is:{0}", variable.Count);
}

现在我们可以理解为什么dynamic.Count试图调用System.Array.Count.但是,当Array实现具有Count方法的System.Collections.ICollection时,仍然不清楚为什么没有定义Array.Count.实际上,Array确实正确地实现了ICollection,它确实有一个Count方法.但是,如果没有将Array显式地转换为ICollection,则Array.Count的使用者无权访问Count属性.Array.Count使用称为显式接口实现的模式实现,其中Array.Count显式实现为ICollection.并且您只能通过将变量转换为具有此模式的ICollection来访问count方法.这反映在Array的文档中.查找"显式接口实现"部分.

var myArray = new int[10];
//Won't work because Array.Count is implemented with explicit interface implementation
//where the interface is ICollection
Console.Write(myArray.Count);
//Will work because we've casted the Array to an ICollection
Console.Write(((ICollection)myArray).Count);