using System;

using System.Collections.Generic;

using static Fn;

​

public class Program

{

    public static void Main()

    {

        var shapes = new List<Shape> {

          new Shape { ShapeType = ShapeType.Circle },

          new Shape { ShapeType = ShapeType.Square },

          new Shape { ShapeType = ShapeType.Triangle },

          new Shape { ShapeType = ShapeType.Square },

          new Shape { ShapeType = ShapeType.Hexagon }

        };

        Func<Shape, bool> predicate = isSquare;

        IEnumerable<Shape> squares = filter (shapes, predicate);

        foreach(var square in squares) Console.WriteLine(square);

​

    }

    public static bool isSquare(Shape item) => item.ShapeType == ShapeType.Square;

}

​

public enum ShapeType

{

    Triangle,

    Square,

    Circle,

    Hexagon

}

​

public struct Shape

{

    public ShapeType ShapeType;

      public override string ToString(){

        return $"I am a {ShapeType}.";

      }

}

​

public static class Fn

{

    public static IEnumerable<T> filter<T>(IEnumerable<T> collection, Func<T, bool> predicate)

    {

        foreach (var element in collection)

            if (predicate(element))

                yield return element;

    }

    public static IEnumerable<T> filter<T>(IEnumerable<T> collection, Predicate<T> predicate)

    {

        foreach (var element in collection)

            if (predicate(element))

                yield return element;

    }   

}