public static  class 
{
public delegate bool CounterHashSetDelegate(int a);
public static int Function12(HashSet<int> intSet, CounterHashSetDelegate filter)
{
 
    int count = 0;
    foreach (int i in intSet)
    {
        if(filter(i))
                    count++;
}
return count;
}
}

public delegate bool CounterHashSetDelegate(int a);

class Program
{
   static void Main(string[] args)
   {
       CounterHashSetDelegate d = IsEvenNum;

       Console.WriteLine($"Количество четных элементов:{Function12(new HashSet<int> { 1, 2, 3, 4, 5 }, d)}");

       d = IsGreaterThen;

       Console.WriteLine($"Количество  элементов > 2:{Function12(new HashSet<int> { 1, 2, 3, 4, 5 }, d)}");

       Console.ReadKey();
   }

   public static bool IsEvenNum(int a)
   {
       return a % 2 == 0;// четное ли число например
   }

   public static bool IsGreaterThen(int a)
   {
       return a > 2;//больше  ли  например
   }


   public static int Function12(HashSet<int> intSet, CounterHashSetDelegate filter)
   {

       int count = 0;
       foreach (int i in intSet)
       {
           if (filter(i))
           {
               count++;
           }
       }
       return count;
   }

}

Print("Multiple each item in a list by 2", Enumerable.Range(1, 10).Select(i => i * 2));

Print("Sum a list of numbers", Enumerable.Range(1, 1000).Sum());

var wordlist = new[] { "C#", "and stuff" };
var tweet = "This is an example tweet talking about C# and stuff";
Print("Verify if a word exists in string", wordlist.Any(word => tweet.IndexOf(word) > -1));
Print("Show matched words in string", wordlist.Where(word => tweet.IndexOf(word) > -1));

Print("Read in a File", File.ReadAllBytes("oneliners.exe").Length);

Print("Happy Birthday", Enumerable.Range(1, 4).Select((i) => string.Format("Happy Birthday {0} ", i == 3 ? "dear NAME" : "to You")));

var passed = new List<int>();
var failed = new List<int>();

(from bucket in new[] { passed, failed } from i in new[] { 49, 58, 76, 82, 88, 90 } select new { bucket, i }).ToList().ForEach((tuple) => tuple.bucket.AddRange(Enumerable.Repeat(tuple, 1).Where((tup) => (tup.bucket == passed && tup.i > 60) || (tup.bucket == failed && tup.i <= 60)).Select((tup) => tup.i)));
Print("Filter list of numbers >60", (IEnumerable<int>)passed);
Print("Filter list of numbers <=60", (IEnumerable<int>)failed);

Print("Fetch and Parse an XML web service", XDocument.Load("https://search.twitter.com/search.atom?&q=scala"));

Print("Find minimum in a list", Enumerable.Min(new[] { 14, 35, -7, 46, 98 }));
Print("Find maximum in a list", Enumerable.Max(new[] { 14, 35, -7, 46, 98 }));

Print("Parallel Processing", Enumerable.Range(1, 10).AsParallel().Select((i)=>i*2).AsEnumerable());

Print("Fizzbuzz", Enumerable.Range(1, 15).Select((i)=>i + (i%3==0?"fizz":"") + (i%5==0?"buzz":"")));

var res = FirstMethod().SecondMethod().ThirdMethod();

var res2 = new[] { 1, 2, 3 }.Where(x => x > 10).Select(x => x);

private void func(out string value)
{
    value = "Hello World!";
}

private void func(ref string value)
{
    value = "Hello World!";
}

private void func1(out string value)
{
    Console.WriteLine(value); // нельзя, value не инициализировано
    if (false)
        return;               // нельзя, забыли установить значение value
    value = "Hello World!";
}

string s1;
func1(out s1);
private void func2(ref string value)
{
    Console.WriteLine(value); // можно
    if (false)
        return;               // не проблема, у value остаётся старое значение
    value = "Hello World!";
}

string s2;
func2(ref s2); // нельзя, функция имеет право использовать значение,
               // значит, оно должно быть инициализировано сначала