もっと試してみた

昨日のを入力を破壊しないようにできた。

    class Program

    {

        static void Main( string[] args )

        {

            int result = SumOfSquares( new int[] { 1, 2, 3, 4, 5 } );

            Console.WriteLine( result );

            Console.Read();

        }
        []delegate[] T AccumulateAction<T>( Combiner<T> combiner, T value, IEnumerator<T> it );

        []delegate[] T Combiner<T>( T x, T y );
        static T Accumulate<T>( Combiner<T> combiner, T nullValue, IEnumerator<T> it )

        {

            if ( !it.MoveNext() )

                return nullValue;

            T first = it.Current;

            return combiner( first, Accumulate<T>( combiner, nullValue, it ) );

        }
        static int SumOfSquares( IEnumerable<int> list )

        {

            return Accumulate( []delegate[]( int x, int y ) { return x * x + y; }, 0, list.GetEnumerator() );

        }

    }