public static IEnumerable<KeyValuePair<TKey, int>> CountBy<TSource, TKey>(
	this IEnumerable<TSource> source,
	Func<TSource, TKey> keySelector
)

<ExtensionAttribute>
Public Shared Function CountBy(Of TSource, TKey) ( 
	source As IEnumerable(Of TSource),
	keySelector As Func(Of TSource, TKey)
) As IEnumerable(Of KeyValuePair(Of TKey, Integer))

public:
[ExtensionAttribute]
generic<typename TSource, typename TKey>
static IEnumerable<KeyValuePair<TKey, int>>^ CountBy(
	IEnumerable<TSource>^ source, 
	Func<TSource, TKey>^ keySelector
)

[<ExtensionAttribute>]
static member CountBy : 
        source : IEnumerable<'TSource> * 
        keySelector : Func<'TSource, 'TKey> -> IEnumerable<KeyValuePair<'TKey, int>>