为什么我可以通过索引访问 KeyCollection/ValueCollection 中的项目，即使它没有实现 IList(Of Key)? [英] Why can I access an item in KeyCollection/ValueCollection by index even if it doesn't implement IList(Of Key)?

问题描述

我注意到一个奇怪的 VB.NET 事情.来自

解决方案

它调用 Enumerable.ElementAtOrDefault，而不是索引器.

//[10 13 - 10 31]IL_001f: ldloc.1//键IL_0020:ldc.i4.0IL_0021: call !!0/*valuetype [mscorlib]System.DateTime*/[System.Core]System.Linq.Enumerable::ElementAtOrDefault(class [mscorlib]System.Collections.Generic.IEnumerable`1, int32)IL_0026: stloc.2//firstKey

此行为记录在 Visual Basic 语言规范，11.21.3:

<块引用>

元素类型为 T 并且还没有默认属性的每个可查询集合类型都被认为具有以下一般形式的默认属性:

公共只读默认属性项(索引为整数)为T得到返回 Me.ElementAtOrDefault(index)结束获取最终财产

默认属性只能使用默认属性访问语法来引用；默认属性不能通过名称引用.例如:

Dim customers As IEnumerable(Of Customer) = ...昏暗的客户三 = 客户(2)' 错误，没有这样的属性Dim customerFour = customers.Item(4)

如果集合类型没有 ElementAtOrDefault 成员，则会发生编译时错误.

I've noticed a strange VB.NET thing. Coming from this question I've provided a way to access keys and values of dictionaries' KeysCollection and ValuesCollection via index to get, say, the first item. I know that it makes only sense in a SortedDictionary since a normal Dictionary is not ordered (well, you should not rely on its order).

Here's a simple example:

Dim sortedDict As New SortedDictionary(Of DateTime, String)
sortedDict.Add(DateTime.Now, "Foo")

Dim keys As SortedDictionary(Of DateTime, String).KeyCollection = sortedDict.Keys
Dim values As SortedDictionary(Of DateTime, String).ValueCollection = sortedDict.Values
Dim firstkey As DateTime = keys(0)
Dim firstValue As String = values(0)

But I was surprised that the question's asker said that it doesn't compile whereas it compiles and works for me without a problem:

System.Diagnostics.Debug.WriteLine("Key:{0} Value:{1}", firstkey, firstValue) ' Key:04/29/2016 10:15:23 Value:Foo

So why can I use it like there was an indexer if there isn't actually one in SortedDictionary(Of TKey, TValue).KeyCollection-class and also none in the ValueCollection. Both implement ICollection<T> which is the parent interface of IList<T>. So you can loop it and it has a Count property, but you can't access items via index as I do above.

Note that it's a fresh console application with no extensions inside. I can't go to the definition of the indexer either(also not with resharper). Why does it work for me?

Side-note: it doesn't work in C#. I get the expected compiler error:

Cannot apply indexing with [] to an expression of type 'SortedDictionary.KeyCollection'

var dict = new SortedDictionary<DateTime, string>();
dict.Add(DateTime.Now, "Foo");
DateTime dt = dict.Keys[0]; // here

Here's a screenshot of the compiling VB.NET code:

解决方案

It invokes Enumerable.ElementAtOrDefault, not the indexer.

// [10 13 - 10 31]
IL_001f: ldloc.1      // keys
IL_0020: ldc.i4.0     
IL_0021: call         !!0/*valuetype [mscorlib]System.DateTime*/ [System.Core]System.Linq.Enumerable::ElementAtOrDefault<valuetype [mscorlib]System.DateTime>(class [mscorlib]System.Collections.Generic.IEnumerable`1<!!0/*valuetype [mscorlib]System.DateTime*/>, int32)
IL_0026: stloc.2      // firstKey

This behavior is documented in the Visual Basic Language Specification, 11.21.3:

Every queryable collection type whose element type is T and does not already have a default property is considered to have a default property of the following general form:

Public ReadOnly Default Property Item(index As Integer) As T
    Get
        Return Me.ElementAtOrDefault(index)
    End Get
End Property

The default property can only be referred to using the default property access syntax; the default property cannot be referred to by name. For example:

Dim customers As IEnumerable(Of Customer) = ...
Dim customerThree = customers(2)

' Error, no such property
Dim customerFour = customers.Item(4)

If the collection type does not have an ElementAtOrDefault member, a compile-time error will occur.

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