Why can I access an item in KeyCollection/ValueCollection by index even if it doesn't implement IList(Of Key)?

Question

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 Values

Answer 1

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(class [mscorlib]System.Collections.Generic.IEnumerable`1, 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.