Linq Func Action, C#

Language: Layout:

//Title of this code
//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

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

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Your code goes here
            IEnumerable<string> cities = new string[]{"Atlanta", "Warsaw", "Gliwice", "Atmonia"};
            
            var query = cities.Filter(s => s.StartsWith("At"));
            var q1 = cities.Where(c => c.StartsWith("Atl"));
            var q2 = cities.Or
            foreach (var c in q1)
            {
                Console.WriteLine(c);
            }
            DateTime date = new DateTime(2015, 1, 1);
            int days = date.DaysToEndOfMonth();
            Console.WriteLine(days);
            //funkcja która zwraca string i przyjmuje jeden parametr int
            Func<int, string> add = (x) => (x+1).ToString();
            
            //gdy nie ma zwracanej warości to zamiast func stosujemy Action<T>, T - input parameter
            Action<int> write = x => Console.WriteLine(x);
            write(4);
            Console.WriteLine(add(5));
            
        }
        static bool StrA(string input)
        {
            return input.StartsWith("At");
        }
        
    }
    
    
    //Extention method to datetime, musi być w static class
    public static class DateUtilities
    {
        public static int DaysToEndOfMonth(this DateTime date)
        {
            return DateTime.DaysInMonth(date.Year, date.Month) - date.Day;
        }
    }
    
    
    public static class FilterExtention
    {
        //Extention methor for IEnumerable<string>
        public static IEnumerable<string> StringsThatStartWith(this IEnumerable<string> input, string start)
        {
            foreach(var s in input)
                
            {
                if (s.StartsWith(start))
                {
                    yield return s;
                }
            }
        }
        
         public static IEnumerable<T> Filter<T>(this IEnumerable<T> input, Func<T, bool> predicate)
        {
            foreach(var item in input)
                
            {
                if (predicate(item))
                {
                    yield return item;
                }
            }
        }
        public delegate bool FilterDelegate<T> (T item);
    }
    
}

Show compiler warnings [ + ] Show input
Compilation time: 0.17 sec, absolute running time: 0.11 sec, cpu time: 0.12 sec, average memory usage: 16 Mb, average nr of threads: 3