我如何循环这段代码，以便我可以添加到总成本值

我如何循环这段代码，以便我可以添加到总成本值?

using System.Threading.Tasks;
using System.Windows.Forms;
namespace Workshop_Selector
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void calculateBtn_Click(object sender, EventArgs e)
        {

            // Declaring variables to zero so they can found in a list
            decimal registrationFee = 0;
            decimal days = 0;
            decimal lodgingFee = 0;

            if (workshopListBox.SelectedIndex != -1 && locationListBox.SelectedIndex != -1) //when list items selected are not below zero
            {

                //From workshop list data inputs calculate the variables
                switch (workshopListBox.SelectedIndex)
                {

                    case 0:              // in the first case of "Handling Stress" the workshop lasts for 3 days and costs 1000 euro
                        days = 3;
                        registrationFee = 1000;
                        break; //Case finisher
                    case 1:                 // in the second case of "Time Mgnt" the workshop lasts for 3 days and costs 800 euro
                        days = 3;
                        registrationFee = 800;
                        break;
                    case 2:                 // in the third case of "Supervision skills" the workshop lasts for 3 days and costs 1500 euro
                        days = 3;
                        registrationFee = 1500;
                        break;
                    case 3:                 // in the fourth case of "Negotiation" the workshop lasts for 5 days and costs 1300 euro
                        days = 5;
                        registrationFee = 1300;
                        break;
                    case 4:                 // in the fifth case of "How to Interview" the workshop lasts for 3 days and costs 500 euro
                        days = 1;
                        registrationFee = 500;
                        break;
                }
                //From location list data inputs calculate the variables

                switch (locationListBox.SelectedIndex)
                {

                    case 0:              //In the first case in the location list lodging fee per day in Galway is 150 euro
                        lodgingFee = 150;
                        break;
                    case 1:              //In the second case in the location list lodging fee per day in Dublin is 225 euro
                        lodgingFee = 225;
                        break;
                    case 2:              //In the third case in the location list lodging fee per day in Limerick is 175 euro
                        lodgingFee = 175;
                        break;
                    case 3:              //In the fourth case in the location list lodging fee per day in Cork is 300 euro
                        lodgingFee = 300;
                        break;
                    case 4:              //In the fifth case in the location list lodging fee per day in Athlone is 175 euro
                        lodgingFee = 175;
                        break;
                    case 5:              //In the sixth case in the location list lodging fee per day in Ennis is 150 euro
                        lodgingFee = 150;
                        break;
                }
            } 

            //end of if statements and variables
            // Illustrate results
            costLabel.Text = "Registration: " + registrationFee.ToString("c") +
                " 'nLodging Fee: " + lodgingFee.ToString("c") + " x " +
                days.ToString() + " days = " + (lodgingFee * days).ToString("c");
            totalCostTextBox.Text = (registrationFee + (lodgingFee * days)).ToString("c");

        }
    }
}

    private void totalCostTextBox_TextChanged(object sender, EventArgs e)
    {
        if (totalCostTextBox.Text == "€ 0,00")
            MessageBox.Show("hey!");
    }

       //end of if statements and variables
        // Illustrate results
        costLabel.Text = "Registration: " + registrationFee.ToString("c") +
            " 'nLodging Fee: " + lodgingFee.ToString("c") + " x " +
            days.ToString() + " days = " + (lodgingFee * days).ToString("c");
        decimal totalCost = registrationFee + (lodgingFee * days);
        if (totalCost == 0)
        {
            MessageBox.Show("total cost was zero! Re-Enter data");
        }
        else
        {
            totalCostTextBox.Text = totalCost.ToString("c");
        }