c#中的基本错误捕获

我需要有一个基本的错误捕获，如果用户键入负利率，则产生错误语句。目前，我有这个贷款计算器，产生一个关于负利率的错误，但它是在一个if else语句。我也有一个try catch语句，但它只在用户输入数字以外的东西时产生错误消息。关于如何让try catch语句产生负利率错误的任何方向/提示将是伟大的。由于

namespace Program_3_Car_Calc
{
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
    private void button1_Click(object sender, EventArgs e) // Calculate message box 
    {
        double LoanAmount = 0;
        double Payment = 0;
        double InterestRate = 0;
        double DownPayment = 0;
        int PaymentPeriods = 0;
        // Inputs
        try
        {
            InterestRate = Convert.ToDouble(txtRate.Text);
            PaymentPeriods = Convert.ToInt16(Convert.ToDouble(txtTime.Text) * 12);
            LoanAmount = Convert.ToDouble(txtLoan.Text);
            DownPayment = Convert.ToDouble(txtDown.Text);
            // Calculate the monthly payment 
            if (InterestRate > 0)
            {
                InterestRate = InterestRate / 100;
            }
            else
            {
                MessageBox.Show("Error, Please enter a positive number");
            }
            Payment = ((LoanAmount - DownPayment) * Math.Pow((InterestRate / 12) + 1,
           (PaymentPeriods)) * InterestRate / 12) / (Math.Pow(InterestRate / 12 + 1,
           (PaymentPeriods)) - 1);
           Payment = Math.Round(Payment, 2);
            // Ouptput the results of the monthly payment 
            lblMonthlyPayment.Text = "Your Monthly Payment will be: " + "$" + Payment.ToString   ("N2");
        }
        catch
        {
            MessageBox.Show("You have entered an invalid character");
        }

    }
    private void CloseGoodbye(string message, string title = " ", int amount = 0) //Optional Parameter for amount of time message box appears 
    {
        for (int i = 0; i < amount; i++)
        MessageBox.Show(message, title);
        Application.Exit();
    }
    private void cmdFinished_Click(object sender, EventArgs e) // Finished Message box 
    {
        CloseGoodbye("Goodbye", "Loan Calc", 1);
    }

}