using System;

using System.Linq;

using System.Collections.Generic;

public interface IXYPoint {

    double X { get; set; }

    double Y { get; set; }

}

​

public class Vector2 : IXYPoint

{

    public double X {get; set; }

    public double Y {get; set; }

​

    public Vector2(){

    }

    public Vector2(double x, double y)

    {

        X = x;

        Y = y;

    }

    public override string ToString()

    {

        return string.Format("X:{0},Y:{1}",X,Y);

    }

}

​

public class Program

{

    public static void Main()

    {

        var field = new List<Vector2>() {

            new Vector2(-1,1),  new Vector2(1,1), 

            new Vector2(-1,-1), new Vector2(1,-1)

        };

        Console.WriteLine(string.Join("; ", field.IsInSector(new Vector2(0,0), new Vector2(2,2), 45.0)));

        Console.WriteLine(string.Join("; ", field.IsInSector(new Vector2(0,0), new Vector2(2,-2), 45.0)));

        Console.WriteLine(string.Join("; ", field.IsInSector(new Vector2(0,0), new Vector2(2,0), 90.0)));

    }

}

​

public static class Extenctions {

    public static IEnumerable<IXYPoint> IsInSector(this IEnumerable<IXYPoint> source, Vector2 pointToCheck, Vector2 direction, double halfAngle)

    {

        if(!source.Any() || pointToCheck == null || halfAngle <= 0)

            return new IXYPoint[0];

        var dirVector = new Vector2() {

            X = direction.X - pointToCheck.X,

            Y = direction.Y - pointToCheck.Y

        };

        Console.WriteLine("DirVector: " + dirVector.ToString());

        var radius = Distance(direction, pointToCheck);

        var result = new List<IXYPoint>();

        foreach(var p in source)

        {

            if(Distance(p, pointToCheck) > radius){

                Console.WriteLine("To far;" + p.ToString());

                continue;

            }

            if(IsPointInSector(dirVector, p, halfAngle))

                result.Add(p);

        }

        return result;

    }

    private static double Distance(IXYPoint from, IXYPoint to)

    {

        return Math.Sqrt(Math.Pow(to.X - from.X,2) + Math.Pow(from.Y-to.Y,2));

    }

    private static bool IsPointInSector(IXYPoint directionVector, IXYPoint pointToCheck, double halfAngle)

    {

        var rq0 = Math.Pow(directionVector.X,2) + Math.Pow(directionVector.Y,2);

        var rq = Math.Pow(pointToCheck.X,2) + Math.Pow(pointToCheck.Y,2);

        return rq0 >= rq && (directionVector.X*pointToCheck.X + directionVector.Y*pointToCheck.Y)/Math.Sqrt(rq0*rq) >= Math.Cos(halfAngle/180.0*Math.PI);

    }

}