public static 数据结构.IEnumerable <系统内置接口.IGrouping <TKey, TSource> > GroupBy <TSource, TKey>(this IEnumerable <TSource> source, 内置委托.Func <TSource, TKey> keyselector)
{
MyLookup <TKey, TSource> lookup = new MyLookup <TKey, TSource>(null);
lookup.Create(source, keyselector, IdentityFunction <TSource> .Instance);
return(lookup.GetPerGroup());
}