Ex, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Microsoft (R) Visual C# Compiler version 2.9.0.63208 (958f2354)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int[] numbers = { 0, 30, 20, 15, 90, 85, 40, 75 };
            //IEnumerable<int> query = MyOwnWhere(numbers, IsElementValid);
            IEnumerable<int> query = MyOwnWhere(numbers, (bob, simon) => IsElementValid(bob, simon));
            
            foreach (int number in query)
            {
                Console.WriteLine(number);
            }
        }
        
        public static bool IsElementValid(int number, int index)
        {
            bool result = number <= index * 10;
            Console.WriteLine($"Is {number} <= {index * 10}? {(result ? "Yes" : "No")}");
            return result;
        }

public static List<int> MyOwnWhere(int[] source, Func<int, int, bool> filter)
        {
            var result = new List<int>();
            for (int i = 0; i < source.Length; ++i)
            {
                Console.WriteLine($"Pass value {source[i]} at index {i} to filter.");
                if (filter(source[i], i) == true)
                {
                    Console.WriteLine($"Check passed. Adding {source[i]} to the result list.");
                    result.Add(source[i]);
                }
                else
                {
                    Console.WriteLine($"Check didn't pass. Skipping {source[i]}.");
                }
            }
            return result;
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,64 sec, absolute running time: 0,09 sec, cpu time: 0,09 sec, average memory usage: 14 Mb, average nr of threads: 3, absolute service time: 0,77 sec