Switching to MVVM

using System;
using System.Collections.ObjectModel;
using System.Windows;

namespace MVVM
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private double principle;
        private double interestRate;
        private int duration;
        private double payment;
        private ObservableCollection<PaymentInfo> payments = new ObservableCollection<PaymentInfo>();

        public MainWindow()
        {
            InitializeComponent();
        }

        private void btnExit_Click(object sender, RoutedEventArgs e)
        {
            Close();
        }

        private void btnCalculate_Click(object sender, RoutedEventArgs e)
        {
            if (txtPrincipleAmount.Text.Length > 0)
            {
                principle = Convert.ToDouble(txtPrincipleAmount.Text);
            }
            else
            {
                MessageBox.Show("Please enter principle amount", "Error", MessageBoxButton.OK, MessageBoxImage.Error);
                return;
            }

            if (txtInterestRate.Text.Length > 0)
            {
                interestRate = Convert.ToDouble(txtInterestRate.Text);
                interestRate /= 100;
                interestRate /= 12;
            }
            else
            {
                MessageBox.Show("Please enter interest", "Error", MessageBoxButton.OK, MessageBoxImage.Error);
                return;
            }

            if (txtDuration.Text.Length > 0)
            {
                duration = Convert.ToInt32(txtDuration.Text);
            }
            else
            {
                MessageBox.Show("Please enter duration", "Error", MessageBoxButton.OK, MessageBoxImage.Error);
                return;
            }

            CalculatePayment();
        }

        // Calculate the complete amortization schedule and fill the data grid control
        private void CalculatePayment()
        {
            int totalpayments = duration * 12;

            Title = "Amortization Schedule for " + 
                Convert.ToString(totalpayments) + " Payments";

            // calculate interest term
            double interestTerm = Math.Pow((1 + interestRate), totalpayments);

            // calculate payment
            payment = (principle * interestRate) / (1 - (1 / interestTerm));

            payments.Clear();

            for (int iIndex = 1; iIndex <= totalpayments; ++iIndex)
            {
                PaymentInfo paymentInfo = new PaymentInfo();
                paymentInfo.PaymentNo = iIndex;
                paymentInfo.Balance = CalculateBalance(iIndex);
                paymentInfo.Payment = payment;
                paymentInfo.Interest = CalculateInterestPart(iIndex);
                paymentInfo.Principle = CalculatePrinciple(iIndex);

                payments.Add(paymentInfo);
            }

            lstAmortization.ItemsSource = payments;
        }

        // Calculate the remaining balance at particular payment
        private double CalculateBalance(int month)
        {
            double interestTerm = Math.Pow((1 + interestRate), month);
            double totalInterest = principle * interestTerm;
            double totalPaid = payment * (interestTerm - 1) / interestRate;
            return totalInterest - totalPaid;
        }

        // Calculate the Interest part of any particular payment
        private double CalculateInterestPart(int month)
        {
            double interestTerm = Math.Pow((1 + interestRate), (month - 1));
            double totalInterest = principle * interestTerm;
            double totalPaid = payment * (interestTerm - 1) / interestRate;
            return (totalInterest - totalPaid) * interestRate;
        }

        // Calculate the principle part of any particular payment
        private double CalculatePrinciple(int month)
        {
            return payment - CalculateInterestPart(month);
        }
    }

    public class PaymentInfo
    {
        public int PaymentNo
        { get; set; }

        public double Payment
        { get; set; }

        public double Principle
        { get; set; }

        public double Interest
        { get; set; }

        public double Balance
        { get; set; }
    }
}