using System;

using System.Collections.Generic;

​

static class Program

{

    static void Main()

    {

        int[] a = { 1, 2, 3, };

        int[] b = { 3, 2, 1, };

        int[] c = { 2, 3, 1, };

        Console.WriteLine(a.IsInOrder(LessThanOrEqual));

        Console.WriteLine(c.IsInOrder(LessThanOrEqual));

        Console.WriteLine(b.IsInOrder(GreaterThanOrEqual));

        Console.WriteLine(c.IsInOrder(GreaterThanOrEqual));

    }

​

    public static bool LessThanOrEqual(int a, int b) => a <= b;

    public static bool GreaterThanOrEqual(int a, int b) => a >= b;

​

    public static bool IsInOrder<T>(this IEnumerable<T> ienumerable, Func<T, T, bool> check)

    {

        T previous = default;

        bool assigned = false;

        foreach (T value in ienumerable)

        {

            if (assigned)

            {

                if (!check(previous, value))

                {

                    return false;

                }

                previous = value;

            }

            else

            {

                previous = value;

                assigned = true;

            }

        }

        if (!assigned)

        {

            throw new ArgumentException("Empty IEnumerable");

        }

        return true;

    }

}