Как рассчитать выбранные пары ключевых значений combobox?

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Все работает отлично, но я не могу вывести правильные вычисления (subtotal, tax, total) для выбранных пар KeyValuePairs в разных comboBox. Спасибо!

Что я уже пробовал:

namespace BillCalculator
{
    public partial class Form1 : Form
    {
        double total = 0;
        double subtotal = 0;
        double tax = 0;

        public Form1()
        {
            InitializeComponent();

            
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            // Create a List to store our KeyValuePairs
            List<KeyValuePair<string, double>> data = new List<KeyValuePair<string, double>>();

            //**BEVERAGE**           
            data.Add(new KeyValuePair<string, double>("Select Item", 0.00));
            data.Add(new KeyValuePair<string, double>("Soda", 1.95));
            data.Add(new KeyValuePair<string, double>("Tea", 1.50));
            data.Add(new KeyValuePair<string, double>("Coffee", 1.25));
            data.Add(new KeyValuePair<string, double>("Mineral Water", 2.95));
            data.Add(new KeyValuePair<string, double>("Juice", 2.50));
            data.Add(new KeyValuePair<string, double>("Milk", 1.50));

            // Clear the combobox
            beverageBox.DataSource = null ;
            beverageBox.Items.Clear();

            // Bind the combobox
            beverageBox.DataSource = new BindingSource(data, null);
            beverageBox.DisplayMember = "Key";
            beverageBox.ValueMember = "Value";


            //**APPETIZER**

            
            {
                List<KeyValuePair<string, double>> data1 = new List<KeyValuePair<string, double>>();

                appetizerBox.BindingContext = new BindingContext();

                data1.Add(new KeyValuePair<string, double>("Select Item", 0.00));
                data1.Add(new KeyValuePair<string, double>("Buffalo Wings", 5.95));
                data1.Add(new KeyValuePair<string, double>("Buffalo Fingers", 6.95));
                data1.Add(new KeyValuePair<string, double>("Potato Skins", 8.95));
                data1.Add(new KeyValuePair<string, double>("Nachos", 8.95));
                data1.Add(new KeyValuePair<string, double>("Mushroom Caps", 10.95));
                data1.Add(new KeyValuePair<string, double>("Shrimp Cocktail", 12.95));
                data1.Add(new KeyValuePair<string, double>("Chips and Sala", 6.96));

                // Clear the combobox
                appetizerBox.DataSource = null;
                appetizerBox.Items.Clear();

                // Bind the combobox
                appetizerBox.DataSource = new BindingSource(data1, null);
                appetizerBox.DisplayMember = "Key";
                appetizerBox.ValueMember = "Value";
            }

            //**MAIN COURSE**
            {
                List<KeyValuePair<string, double>> data2 = new List<KeyValuePair<string, double>>();

                mainCBox.BindingContext = new BindingContext();

                data2.Add(new KeyValuePair<string, double>("Select Item", 0.00));
                data2.Add(new KeyValuePair<string, double>("Chicken Alfredo", 13.95));
                data2.Add(new KeyValuePair<string, double>("Chicken Picatta", 13.95));
                data2.Add(new KeyValuePair<string, double>("Turkey Club", 11.95));
                data2.Add(new KeyValuePair<string, double>("Lobster Pie", 19.95));
                data2.Add(new KeyValuePair<string, double>("Prime Rib", 20.95));
                data2.Add(new KeyValuePair<string, double>("Shrimp Scampi", 18.95));
                data2.Add(new KeyValuePair<string, double>("Turkey Dinner", 13.96));
                data2.Add(new KeyValuePair<string, double>("Stuffed Chicken", 14.96));
                data2.Add(new KeyValuePair<string, double>("Seafood Alfredo", 15.96));
                
                // Clear the combobox
                mainCBox.DataSource = null;
                mainCBox.Items.Clear();

                // Bind the combobox
                mainCBox.DataSource = new BindingSource(data2, null);
                mainCBox.DisplayMember = "Key";
                mainCBox.ValueMember = "Value";
            }

            //**DESSERT**
            {
                List<KeyValuePair<string, double>> data3 = new List<KeyValuePair<string, double>>();

                dessertBox.BindingContext = new BindingContext();

                data3.Add(new KeyValuePair<string, double>("Select Item", 0.00));
                data3.Add(new KeyValuePair<string, double>("Apple Pie", 5.95));
                data3.Add(new KeyValuePair<string, double>("Sundae", 3.95));
                data3.Add(new KeyValuePair<string, double>("Carrot Cake", 5.95));
                data3.Add(new KeyValuePair<string, double>("Mud Pie", 4.95));
                data3.Add(new KeyValuePair<string, double>("Apple Crisp", 5.95));
                
                // Clear the combobox
                dessertBox.DataSource = null;
                dessertBox.Items.Clear();

                // Bind the combobox
                dessertBox.DataSource = new BindingSource(data3, null);
                dessertBox.DisplayMember = "Key";
                dessertBox.ValueMember = "Value";
            }
        }
        private void Appetizer_Click(object sender, EventArgs e)
        {

        }
 
  
        private void beverageBox_SelectedIndexChanged(object sender, EventArgs e)
        {
          // Get the selected item in the combobox
    KeyValuePair<string, double> selectedPair = (KeyValuePair<string, double>)beverageBox.SelectedItem;

            // Show selected information on screen
            lblSelectedKey.Text = selectedPair.ToString();
            total = selectedPair.Value;
        }

        private void appetizerBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            // Get the selected item in the combobox
            KeyValuePair<string, double> selectedPair = (KeyValuePair<string, double>)appetizerBox.SelectedItem;

            // Show selected information on screen
            lblSelectedKey2.Text = selectedPair.ToString();
            total = selectedPair.Value;
        }

        private void mainCBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            // Get the selected item in the combobox
            KeyValuePair<string, double> selectedPair = (KeyValuePair<string, double>)mainCBox.SelectedItem;

            // Show selected information on screen
            lblSelectedKey3.Text = selectedPair.ToString();
            total = selectedPair.Value;
        }

        private void dessertBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            // Get the selected item in the combobox
            KeyValuePair<string, double> selectedPair = (KeyValuePair<string, double>)dessertBox.SelectedItem;

            // Show selected information on screen
            lblSelectedKey4.Text = selectedPair.ToString();
            total = selectedPair.Value;
        }

        private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
           
        }

        private void button1_Click(object sender, EventArgs e)
        {
    

            if (beverageBox.SelectedIndex == 0)
            {
                subtotal = total;
            }
           
            if (appetizerBox.SelectedIndex == 0)
            {
                subtotal = total;
            }

            if (mainCBox.SelectedIndex == 0)
            {
                subtotal = total;                 
            }

            if (dessertBox.SelectedIndex == 0)
            {
                subtotal = total;
            }
           
            textBox1.Text = Convert.ToString(subtotal);
            tax = subtotal * 0.2;
            textBox2.Text = Convert.ToString(tax);
            total = tax + subtotal;
            textBox3.Text = Convert.ToString(total);
        }
    }
 }

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