C# visual c注册系统错误无效输入

我们被要求使用visual c制作一个注册程序，我提出了代码，但我认为每当我输入单位数时都会出现错误，它总是提示无效输入，即使我输入了9个以上的单位，因为我们的教授说单位不能小于9。这是我做的代码

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 MachineProblem2
{
public partial class frmEnrollment : Form
{
    public frmEnrollment()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        rdbInstallment.Checked = true;
        rdbRegular.Checked = true;
    }

    private void label1_Click(object sender, EventArgs e)
    {

    }

    private void btnCompute_Click(object sender, EventArgs e)
    {
        int units = Convert.ToInt32(txtUnits.Text);
        int tuitionfee = 1250;
        double miscfee = .15;
        int labfee = 750;
        //double scholar = 0;
        //double fullpayment = 0;
        //double installment = .005;
        int totalTuition = 0;
        double totalMiscFee = 0;
        int totalLabFee = 0;
        int totalDiscount = 0;
        int computeModePayment = 0;
        int computeScholarDiscount = 0;
        int scholarDiscount = 80;
        int fullpaymentDiscount = 5;
        double modeOfPayment = 0;
        double status = 0;
        double convertedDiscount = 0;
        double total = 0;
        double installment = 0;

        if (txtUnits.Text == "")
        {
            MessageBox.Show("Input a number", "Input", MessageBoxButtons.OK,
                MessageBoxIcon.Error);

        }


        if (units < 9)
        {
            MessageBox.Show("Error Input!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            txtUnits.Clear();
            txtUnits.Focus();


        }
        else
            totalLabFee = units * labfee;
        txtLabFee.Text = totalLabFee.ToString("C");


        totalTuition = units * tuitionfee;
        txtTuitionFee.Text = totalTuition.ToString("C");

        totalMiscFee = totalTuition * miscfee;
        txtMiscFee.Text = totalMiscFee.ToString("C");

        totalLabFee = units * labfee;
        txtLabFee.Text = totalLabFee.ToString("C");

        if (rdbFull.Checked)
        {
            computeModePayment = fullpaymentDiscount;
        }
        else
            computeModePayment = 0;

        if (rdbScholar.Checked)
        {
            computeScholarDiscount = scholarDiscount;
        }
        else
            computeScholarDiscount = 0;

        totalDiscount = computeModePayment + computeScholarDiscount;
        txtDiscount.Text = totalDiscount.ToString();

        if (rdbFull.Checked)
        {
            modeOfPayment = totalTuition * 0.05;
        }
        else
            modeOfPayment = 0;

        if (rdbScholar.Checked)
        {
            status = totalTuition * .8;
        }
        else
            status = 0;

        convertedDiscount = modeOfPayment + status;
      //  txtConvertedDiscount.Text = convertedDiscount.ToString("C");

        if (rdbInstallment.Checked)
        {
            installment = totalTuition * .005;
            total = (totalTuition + totalLabFee + totalMiscFee - convertedDiscount) + installment;
            txtTotal.Text = total.ToString("C");
        }
        else
            total = totalTuition + totalLabFee + totalMiscFee - convertedDiscount;
        txtTotal.Text = total.ToString("C");

    }

    private void btnClear_Click(object sender, EventArgs e)
    {
        txtUnits.Clear();
        txtTuitionFee.Clear();
        txtMiscFee.Clear();
        txtLabFee.Clear();
        txtDiscount.Clear();
        txtTotal.Clear();
       // txtConvertedDiscount.Clear();
        txtUnits.Focus();

        rdbFull.Checked = false;
        rdbInstallment.Checked = false;
        rdbRegular.Checked = false;
        rdbScholar.Checked = false;
    }

    private void btnExit_Click(object sender, EventArgs e)
    {

        if (MessageBox.Show("Are you sure you want to exit?", "Exit Application",
           MessageBoxButtons.YesNo, MessageBoxIcon.Question) == System.Windows.Forms.DialogResult.Yes)
        {
        }
        Close();
    }

}
}

这是我们教授的指导，希望你能帮助我

*单位不得小于9 *学费按每单元1250英镑计算 *杂项费用按每台1250美元计算 *实验室费用按每台750美元计算 *学者八折优惠 *全额付款可享受5%的折扣 *分期付款收取0.5%的费用

int units = Convert.ToInt32(txtUnits.Text).Trim();