Primalitate

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace proiect
{
    public partial class Form1 : Form
    {
        int n, i = 2, ok, invalid = 0, contor;
        public Form1()
        {
            InitializeComponent();
        }
        public static int primfunctie(int a)
        {
            for (int i = 2; i <= Math.Sqrt(a); i++)
                if (a % i == 0) return 0;
            return 1;
        }

        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void label2_Click(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            if ((textBox1.Text) != "")
               {
                n = Convert.ToInt32(textBox1.Text);
                if (n <= 9999 && n >= 0)
                {
                    textBox6.Text = "";
                    textBox6.Text = " Numarul este valid.";
                }
                else
                {
                    textBox6.Text = "";
                    textBox6.Text = " Numarul este invalid.";
                    MessageBox.Show("Introduceti un numar pozitiv de maxim 4 cifre.");
                    invalid = 1;
                }
            }
        }

        private void label3_Click(object sender, EventArgs e)
        {

        }

        private void checkBox2_CheckedChanged(object sender, EventArgs e)
        {

        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (textBox1.Text != "")
            {
                if (invalid == 0)
                {

                    n = Convert.ToInt32(textBox1.Text);
                    if (primfunctie(n) == 0)
                    {
                        textBox4.Text = "";
                        textBox4.Text = " Nu este numar prim.";
                    }
                    else
                    {
                        textBox4.Text = "";
                        textBox4.Text = " Este numar prim.";

                    }

                }
                else
                {
                    textBox4.Text = "";
                    textBox4.Text = " Numar invalid.";
                }
            }

        }

        private void button3_Click(object sender, EventArgs e)
        {
            if (textBox1.Text != "") {
            if (invalid == 0)
            {
                if (checkBox1.CheckState == CheckState.Checked)
                {
                    i = Convert.ToInt32(textBox1.Text);
                    if (i < 3)
                    {
                        textBox2.Text = "";
                        textBox2.Text = " Nu exista.";
                    }
                    else
                    {
                        textBox2.Text = "";
                        ok = 1;
                        while (i > 2 && ok >= 0)
                        {
                            i = i - 1;
                            if (primfunctie(i) == 1 && ok >= 0)
                            {
                                textBox2.Text = textBox2.Text + " " + Convert.ToString(i);
                                ok = ok - 1;
                            }

                        }
                    }
                }
                if (checkBox2.CheckState == CheckState.Checked)
                {
                    i = Convert.ToInt32(textBox1.Text);
                    if (i > 9967)
                    {
                        textBox3.Text = "";
                        textBox3.Text = " Nu exista.";
                    }
                    else
                    {
                        textBox3.Text = "";
                        ok = 1;
                        while (i <= 9999 && ok >= 0)
                        {
                            i = i + 1;
                            if (primfunctie(i) == 1 && ok >= 0)
                            {
                                textBox3.Text = textBox3.Text + " " + Convert.ToString(i);
                                ok = ok - 1;
                            }
                        }

                    }
                }
                if (checkBox1.CheckState == CheckState.Unchecked && checkBox2.CheckState == CheckState.Unchecked)
                    MessageBox.Show("Selectati cel putin o optiune.");

            }
            else
            {
                if (checkBox1.CheckState == CheckState.Checked)
                {
                    textBox2.Text = "";
                    textBox2.Text = " Numar invalid.";
                }
                if (checkBox2.CheckState == CheckState.Checked)
                {
                    textBox3.Text = "";
                    textBox3.Text = " Numar invalid.";
                }
            }
        }
        }

        private void button4_Click(object sender, EventArgs e)
        {
            if (textBox1.Text != "")
            {
                if (invalid == 0)
                {
                    n = Convert.ToInt32(textBox1.Text);
                    textBox5.Text = "";
                    if (n < 2)
                    {
                        textBox5.Text = "";
                        textBox5.Text = " Nu exista.";
                        textBox7.Text = " ";
                        textBox7.Text = " 0 numere.";
                    }
                    else
                    {
                        contor = 0;
                        for (i = 2; i <= n; i++)
                        {
                            if (primfunctie(i) == 1)
                            {
                                textBox5.Text = textBox5.Text + " " + Convert.ToString(i);
                                contor = contor + 1;
                            }

                        }
                        textBox7.Text = "";
                        textBox7.Text = Convert.ToString(contor) + " numere.";
                    }
                }
                else
                {
                    textBox5.Text = "";
                    textBox5.Text = " Numar invalid.";
                }
            }
        }

        private void linkLabel1_LinkClicked(object sender, System.Windows.Forms.LinkLabelLinkClickedEventArgs e)
        {
            this.label1.BackColor = System.Drawing.Color.Transparent;
            try
            {
                VisitLink();
            }
            catch (Exception ex)
            {
                MessageBox.Show("Nu s-a putut deschide link-ul.");
            }
        }

        private void VisitLink()
        {
            linkLabel1.LinkVisited = true;
            System.Diagnostics.Process.Start("http://www.pastebin.com");
        }
        private void button5_Click(object sender, EventArgs e)
        {
            textBox1.Text = "";
            textBox2.Text = "";
            textBox3.Text = "";
            textBox4.Text = "";
            textBox5.Text = "";
            textBox6.Text = "";
            textBox7.Text = "";

        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void checkBox1_CheckedChanged(object sender, EventArgs e)
        {

        }

        private void label4_Click(object sender, EventArgs e)
        {

        }
    }
}