public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        double L1;
        double L2;
        double wynik;
        bool l1_ok;
        bool l2_ok;
        bool dzial_ok;
        bool oblicz_ok;
        private void tb_L1_TextChanged(object sender, EventArgs e)
        {

            l1_ok = double.TryParse(tb_L1.Text, out L1);
            if (l1_ok == true)
            {
                cb_dzial.Visible = true;
            }
            else
            {
                cb_dzial.Text = null;
                cb_dzial.Visible = false;
                tb_L2.Visible = false;
                bt_oblicz.Visible = false;
                tb_wynik.Visible = false;
                tb_wynik.Text = null;
            }
            if (L1 < 0)
            {
                tb_L1.ForeColor = Color.Red;
            }
            else
            {
                tb_L1.ForeColor = DefaultForeColor;
            }

        }

        private void cb_dzial_TextChanged(object sender, EventArgs e)
        {
            if (l1_ok == true & cb_dzial.Text != null)
            {
                tb_L2.Visible = true;
                dzial_ok = true;
            }
            else tb_L2.Visible = false;
        }


        private void tb_L2_TextChanged(object sender, EventArgs e)
        {
            l2_ok = double.TryParse(tb_L2.Text, out L2);
            if (l1_ok == true & dzial_ok ==true & l2_ok==true)
            {
                bt_oblicz.Visible = true;
            }
            else
            {
                bt_oblicz.Visible = false;
            }
            if (L2 < 0)
            {
                tb_L2.ForeColor = Color.Red;
            }
            else
            {
                tb_L2.ForeColor = DefaultForeColor;
            }
        }


       private void bt_oblicz_Click(object sender, EventArgs e)
       {
           bool oblicz_ok = true;
           if (l1_ok == true & dzial_ok == true & l2_ok == true &oblicz_ok==true)
           {
               tb_wynik.Visible = true;
           }
           else
           {
               tb_wynik.Visible = false;
           }


           char dzialanie = char.Parse(cb_dzial.Text);
           switch (dzialanie)
           {
               case '+':
                   { tb_wynik.Text = (L1 + L2).ToString(); }
                   break;
               case '-':
                   { tb_wynik.Text = (L1 - L2).ToString(); }
                   break;
               case '*':
                   { tb_wynik.Text = (L1 * L2).ToString(); }
                   break;
               case '/':
                   { tb_wynik.Text = (L1 / L2).ToString(); }
                   break;
           }

           wynik = double.Parse(tb_wynik.Text);

           if (wynik < 0)
           {
               tb_wynik.ForeColor = Color.Red;
           }
           else
           {
               tb_wynik.ForeColor = DefaultForeColor;
           }
       }

       private void tb_wynik_TextChanged(object sender, EventArgs e)
       {
           if (l1_ok == true & dzial_ok == true & l2_ok == true & oblicz_ok==true)
           {
               l1_ok = false;
           }

       }

       private void tb_wynik_DoubleClick(object sender, EventArgs e)
       {
           bool l1_ok= false;
           bool l2_ok= false;
           bool dzial_ok= false ;
           bool oblicz_ok = false;
           tb_L1.Text = null;
           tb_L2.Text = null;
           cb_dzial.Text = null;
           tb_wynik.Text = null;

       }

       private void Form1_KeyDown(object sender, KeyEventArgs e)
       {
           if (e.KeyCode == Keys.Return)
           {
               if (tb_L1.Focused == true)
               {
                   cb_dzial.Focus();
               }
               if (cb_dzial.Focused == true)
               { tb_L2.Focus(); }
               if (tb_L2.Focused == true)
               { this.bt_oblicz_Click(this,null);
               tb_wynik.Focus();
               }
           }

           if (e.KeyCode==Keys.Escape)
           {
               if (tb_wynik.Focused==true)
               {
                   tb_L2.Focus();
                   //tb_L2.Text = null;
                   //tb_wynik.Visible = false;
                   //bt_oblicz.Visible = false;
                   //l2_ok = false;
               }
               if (tb_L2.Focused == true)
               {
                   cb_dzial.Focus();
                   cb_dzial.Text = null;
                   tb_L2.Visible = false;
                  // dzial_ok = false;
               }
               if (cb_dzial.Focused == true)
               {
                   tb_L1.Focus();
                   tb_L1.Text = null;
                   cb_dzial.Visible = false;
                  // l1_ok = false;
               }
           }

       }








    }