C#

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

namespace Apostash_Simeiwn
{
    class Simeio
    {
        private double x;
        private double y;


        public Simeio(double x, double y)
        {
            this.x = x;
            this.y = y;

        }
    }
}

{
    public partial class Form1 : Form
    {


        public Form1()
        {
            InitializeComponent();
        }

        private void bttApostash_Click(object sender, EventArgs e)
        {
            txtApostash.Text += Environment.NewLine + Convert.ToString(GetDistance(double.Parse(txtXa.Text),double.Parse(txtXb.Text),double.Parse(txtYa.Text),double.Parse(txtYb.Text)));
        }
        public static double GetDistance(double txtXa, double txtYa, double txtXb, double txtYb)
        {
            double a = txtXa - txtXb;
            double b = txtYa - txtYb;
            double distance;
            return  distance = Math.Sqrt(a * a + b * b);

        }

        private void bttOO_Click(object sender, EventArgs e)
        {
            if (Math.Sqrt(Math.Pow(double.Parse(txtXa.Text), 2) + Math.Pow(double.Parse(txtYa.Text), 2)) < Math.Sqrt(Math.Pow(double.Parse(txtXb.Text), 2) + Math.Pow(double.Parse(txtYb.Text), 2)))
            {
                txtOO.Text += Environment.NewLine + "Το σημείο Α";
            }
            else
            {
                txtOO.Text += Environment.NewLine + "Το σημείο Β";

            }
        }

        private void bttTermatismos_Click(object sender, EventArgs e)
        {
            Environment.Exit(0);
        }

        private void bttSineuthiaka_Click(object sender, EventArgs e)
        {
            double l1 = (double.Parse(txtYb.Text) - double.Parse(txtYa.Text)) / (double.Parse(txtXb.Text) - double.Parse(txtXa.Text));
            double l2 = (double.Parse(txtYc.Text) - double.Parse(txtYb.Text)) / (double.Parse(txtXc.Text) - double.Parse(txtXb.Text));

            if (l1 == l2)
            {
                txtSineuthiaka.Text += Environment.NewLine + "Τα σημεία είναι συνευθειακά ";
            }
            else
            {
                MessageBox.Show("Δεν είναι συνευθειακά.Εισάγετε νέα");
            }
        }


    }
}

public class OrderDetail
{
    // ?? fields, properties, methods

   // constructor ??
}

public class Order
{
   public List<OrderDetail> OrderDetails;

   // ?? fields, properties, methods

   // constructor ??
}

public class Invoice
{
   public List<Order> Orders;

   public int InvoiceNumber { set; get; }

   public DateTime InvoiceDate { set; get; }

   // constructor ??

}

Sascha Lefèvre wrote:

Is there a prize?

OriginalGriff wrote:

A passing grade for the OP...