List<double[]> temp = list.Select((a, i) => new double[2] { a, i }).OrderByDescending(a => a[0]).Take(3).ToList();
                /*
                 * -        temp    Count = 3    System.Collections.Generic.List<double[]>
                    -        [0]    {double[2]}    double[]
                                [0]    19.35    double
                                [1]    5.0    double
                    -        [1]    {double[2]}    double[]
                                [0]    18.35    double
                                [1]    3.0    double
                    -        [2]    {double[2]}    double[]
                                [0]    17.35    double
                                [1]    4.0    double

                 */

------解决方案--------------------
C# code

            List<double> list = new List<double>();
            list.Add(2.55);
            list.Add(4.45);
            list.Add(12.35);
            list.Add(18.35);
            list.Add(17.35);
            list.Add(19.35);
            list.Add(6.355);
            list.Add(7.95);
            list.Add(4.35);
            var v = list.Select((m, index) => new { index, m }).OrderByDescending(n => n.m).Take(1);

------解决方案--------------------
List<double> list = new List<double>();
           list.Add(2.55);
           list.Add(4.45);
           list.Add(12.35);
           list.Add(18.35);
           list.Add(17.35);
           list.Add(19.35);
           list.Add(6.355);
           list.Add(7.95);
           list.Add(4.35);
           var v = list.Select((m, index) => new { index, m }).OrderByDescending(n => n.m).Take(1);