C# GPA Calculator

Solution:

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 GPA_Calculator
{
    public partial class Form1 : Form
    {
        String lettergrd;
        double credit;
        double caltimes = 0;
        double totalcal = 0;
        double totalcredit = 0;
        double finalgpa = 0;
        double A = 4.0;
        double AMINUS = 3.70;
        double BPLUS = 3.33;
        double B = 3.0;
        double BMINUS = 2.70;
        double CPLUS = 2.30;
        double C = 2.0;
        double CMINUS = 1.70;
        double D = 1.00;
        double F = 0.0;
        ComboBox defaultcombo = new ComboBox();
        NumericUpDown defaultnumeric = new NumericUpDown();

        public Form1()
        {
            InitializeComponent();
        }

        private void btncalculateGPA_Click(object sender, EventArgs e)
        {
            lettergrd = "";
            credit = 0;
            caltimes = 0;
            totalcal = 0;
            totalcredit = 0;
            finalgpa = 0;
            defaultcombo = new ComboBox();
            defaultnumeric = new NumericUpDown();
            for (int i = 0; i < 7; i++)
            {

                switch (i)
                {
                    case 0:
                        defaultcombo = cmbCourse1;
                        defaultnumeric = numCourse1;
                        break;
                    case 1:
                        defaultcombo = cmbCourse2;
                        defaultnumeric = numCourse2;
                        break;
                    case 2:
                        defaultcombo = cmbCourse3;
                        defaultnumeric = numCourse3;
                        break;
                    case 3:
                        defaultcombo = cmbCourse4;
                        defaultnumeric = numCourse4;
                        break;
                    case 4:
                        defaultcombo = cmbCourse5;
                        defaultnumeric = numCourse5;
                        break;
                    case 5:
                        defaultcombo = cmbCourse6;
                        defaultnumeric = numCourse6;
                        break;
                    case 6:
                        defaultcombo = cmbCourse7;
                        defaultnumeric = numCourse7;
                        break;
                }

                lettergrd = defaultcombo.Text;
                credit = Double.Parse(defaultnumeric.Value.ToString());


                if (lettergrd != String.Empty && credit != 0)
                {

                    switch (lettergrd)
                    {
                        case "A":
                            caltimes = credit * A;
                            break;
                        case "A-":
                            caltimes = credit * AMINUS;
                            break;
                        case "B+":
                            caltimes = credit * BPLUS;
                            break;
                        case "B":
                            caltimes = credit * B;
                            break;
                        case "B-":
                            caltimes = credit * BMINUS;
                            break;
                        case "C+":
                            caltimes = credit * CPLUS;
                            break;
                        case "C":
                            caltimes = credit * C;
                            break;
                        case "C-":
                            caltimes = credit * CMINUS;
                            break;
                        case "D":
                            caltimes = credit * D;
                            break;
                        case "F":
                            caltimes = credit * F;
                            break;

                    }

                    totalcredit = totalcredit + credit;
                    totalcal = totalcal + caltimes;
                }

            }

            finalgpa = totalcal / totalcredit;

            lblgpa.Text = finalgpa.ToString("#.00");
        }

        private void btnreset_Click(object sender, EventArgs e)
        {
            cmbCourse1.ResetText();
            cmbCourse2.ResetText();
            cmbCourse3.ResetText();
            cmbCourse4.ResetText();
            cmbCourse5.ResetText();
            cmbCourse6.ResetText();
            cmbCourse7.ResetText();
    
            numCourse1.ResetText();
            numCourse2.ResetText();
            numCourse3.ResetText();
            numCourse4.ResetText();
            numCourse5.ResetText();
            numCourse6.ResetText();
            numCourse7.ResetText();
        }
    }
}