private static int[] DeleteSomethingInArray(int[] a)
        {
            Debug.Assert(a.Length > 1 && a.Any(x => x > 0) && a.Any(x => x < 0));  // это просто проверка, можно удалить или закоментировать
            if (a.Length == 2)
                return a;

            var max_i = -1;
            var min_i = -1;
            for (var i = 0; i < a.Length; i++)
            {
                if (a[i] < 0 && (max_i < 0 || a[max_i] < a[i]))
                    max_i = i;
                if (a[i] > 0 && (min_i < 0 || a[min_i] > a[i]))
                    min_i = i;
            }

            var result = new int[a.Length - Math.Abs(max_i - min_i) + 1];

            var j = 0;
            for (var i = 0; i < a.Length; i++)
            {
                if (i <= Math.Min(max_i, min_i) || i >= Math.Max(max_i, min_i))
                    result[j++] = a[i];
            }

            return result;
        }

На всякий случай в заголовках
using System;
using System.Diagnostics;
using System.Linq;