public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) {
            return new GroupedEnumerable<TSource, TKey, TSource>(source, keySelector, IdentityFunction<TSource>.Instance, null);
        }
 
        public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer) {
            return new GroupedEnumerable<TSource, TKey, TSource>(source, keySelector, IdentityFunction<TSource>.Instance, comparer);
        }
 
        public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector) {
            return new GroupedEnumerable<TSource, TKey, TElement>(source, keySelector, elementSelector, null);
        }
 
        public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer) {
            return new GroupedEnumerable<TSource, TKey, TElement>(source, keySelector, elementSelector, comparer);
        }
 
       public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IEnumerable<TSource>, TResult> resultSelector){
           return  new GroupedEnumerable<TSource, TKey, TSource, TResult>(source, keySelector, IdentityFunction<TSource>.Instance, resultSelector, null);
        }
 
        public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector){
           return new GroupedEnumerable<TSource, TKey, TElement, TResult>(source, keySelector, elementSelector, resultSelector, null);
        }
 
        public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IEnumerable<TSource>, TResult> resultSelector, IEqualityComparer<TKey> comparer){
            return  new GroupedEnumerable<TSource, TKey, TSource, TResult>(source, keySelector, IdentityFunction<TSource>.Instance, resultSelector, comparer);
        }
 
        public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector, IEqualityComparer<TKey> comparer){
            return  new GroupedEnumerable<TSource, TKey, TElement, TResult>(source, keySelector, elementSelector, resultSelector, comparer);
        }

--其他方法

 public GroupedEnumerable(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector, IEqualityComparer<TKey> comparer){
            if (source == null) throw Error.ArgumentNull("source");
            if (keySelector == null) throw Error.ArgumentNull("keySelector");
            if (elementSelector == null) throw Error.ArgumentNull("elementSelector");
            if (resultSelector == null) throw Error.ArgumentNull("resultSelector");
            this.source = source;
            this.keySelector = keySelector;
            this.elementSelector = elementSelector;
            this.comparer = comparer;
            this.resultSelector = resultSelector;
        }

这个好像实现复杂，仔细分析下。