WinFormDiApp/WinFormDi/MainWindow.cs

using DIDemoLib;
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 WinFormDiApp
{
    public partial class MainWindow : Form
    {
        private readonly IMessages _messages;
        private readonly frmPayments _payments;
        private readonly frmReadPayments _readPayments;

        public MainWindow(
            IMessages messages,
            frmPayments payments,
            frmReadPayments readPayments
            )
        {
            InitializeComponent();
            _messages = messages;
            _payments = payments;
            _readPayments = readPayments;
        }

        private void MainWindow_Load(object sender, EventArgs e)
        {
            helloText.Text = _messages.SayHello();
        }

        private void MainWindow_FormClosing(object sender, FormClosingEventArgs e)
        {
            goodbyeText.Text = _messages.SayGoodBye();
            MessageBox.Show("Closing...", "Warning", MessageBoxButtons.OK);
        }

        private void btnCheckPayments_Click(object sender, EventArgs e)
        {
            _payments.ShowDialog();
        }

        private void btnLoadPayments_Click(object sender, EventArgs e)
        {
            _readPayments.ShowDialog();
        }
    }
}