using System;

using System.Windows;

public class Program

{

    public static void Main()

    {

        Vector2 x = new Vector2(1,2,5);     

        Vector2 y = new Vector2(10,20,5);

​

        var result = Math.Atan2(x.CrossProduct(y), x.DotProduct(y)); //3d

        var result2 = Math.Atan2(x.CrossProduct2(y), x.DotProduct2(y)); //2d

        Console.WriteLine(result);      

        Console.WriteLine(result2);

    }

}

​

​

public class Vector2

{

    public float x;

    public float y; 

    public float z;

​

    public Vector2(float x, float y, float z)

    {

        this.x = x;

        this.y = y;

        this.z = z;

    }

    public double DotProduct(Vector2 vector)

    {

      return (vector.x * this.x) + (vector.y * this.y) + (vector.z * this.z);

    }

​

    public double CrossProduct(Vector2 vector)

    {       

        double a,b,c;

        a = this.y * vector.z - this.z * vector.y;

        b = this.z * vector.x - this.x * vector.z;

        c = this.x * vector.y - this.y * vector.x;

        return Math.Sqrt(a*a + b*b + c*c);

    }

    public double DotProduct2(Vector2 vector)

    {

      return (vector.x * this.x) + (vector.y * this.y);

    }

​

    public double CrossProduct2(Vector2 vector)

    {

      return (vector.y * this.y) - (vector.y * this.x);

    }

}