4人で作る簡易レジアプリ｜GitHub Desktopで初めてのチーム開発チュートリアル

namespace TeamRegisterApp
{
    public class Product
    {
        public string Name { get; set; } = "";
        public decimal Price { get; set; }
        public int Quantity { get; set; }
        public decimal Subtotal => Price * Quantity;
    }
}

金額にはdecimalを使い、今回は整数の円だけを扱います。Nameを空文字列で初期化しています。SubtotalはPriceとQuantityから計算するプロパティです。

using System.Collections.Generic;

namespace TeamRegisterApp
{
    public class ProductCatalog
    {
        public List<Product> GetProducts()
        {
            // 担当Bが実装する
            return new List<Product>();
        }
    }
}

using System.Collections.Generic;

namespace TeamRegisterApp
{
    public class CartService
    {
        public bool Add(string name, decimal price, int quantity)
        {
            return false; // 担当Cが実装する
        }
        public List<Product> GetItems()
        {
            return new List<Product>();
        }
        public decimal GetTotal()
        {
            return 0m;
        }
        public bool RemoveAt(int index)
        {
            return false;
        }
        public void Clear()
        {
        }
    }
}

namespace TeamRegisterApp
{
    public class PaymentService
    {
        public bool TryGetChange(decimal total, string cashText,
            out decimal change, out string message)
        {
            change = 0m;
            message = "会計機能は準備中です。";
            return false; // 担当Dが実装する
        }
    }
}

outは、戻り値とは別に結果を呼び出し側へ渡す仕組みです。このメソッドは成功・失敗をboolで返し、お釣りとメッセージをoutで返します。

using System.Collections.Generic;

namespace TeamRegisterApp
{
    public class ProductCatalog
    {
        public List<Product> GetProducts()
        {
            return new List<Product>
            {
                new Product { Name = "コーヒー", Price = 150m, Quantity = 1 },
                new Product { Name = "パン", Price = 180m, Quantity = 1 },
                new Product { Name = "おにぎり", Price = 130m, Quantity = 1 }
            };
        }
    }
}

GetProductsは商品候補の一覧を作って返します。カートに入っている商品とは別です。150mのmはdecimal型の数値であることを表します。

using System.Collections.Generic;

namespace TeamRegisterApp
{
    public class CartService
    {
        private readonly List<Product> items = new List<Product>();

        public bool Add(string name, decimal price, int quantity)
        {
            if (string.IsNullOrWhiteSpace(name) || price <= 0m ||
                price > 100000m || price != decimal.Truncate(price) ||
                quantity < 1 || quantity > 99 || items.Count >= 100)
            {
                return false;
            }

            items.Add(new Product
            {
                Name = name,
                Price = price,
                Quantity = quantity
            });
            return true;
        }

        public List<Product> GetItems()
        {
            return new List<Product>(items);
        }

        public decimal GetTotal()
        {
            decimal total = 0m;
            foreach (Product item in items)
            {
                total += item.Subtotal;
            }
            return total;
        }

        public bool RemoveAt(int index)
        {
            if (index < 0 || index >= items.Count)
            {
                return false;
            }
            items.RemoveAt(index);
            return true;
        }

        public void Clear()
        {
            items.Clear();
        }
    }
}

画面ではなくitemsがカートの本体です。Addでは商品一覧のオブジェクトを使い回さず、購入数量を持つ別のProductを作ります。単価は1〜100,000円の整数、数量は1〜99、最大100行とします。GetItemsはリストの入れ物をコピーして返しますが、Product自体の複製ではありません。この課題の画面側は読み取りだけに使います。

using System.Globalization;

namespace TeamRegisterApp
{
    public class PaymentService
    {
        public bool TryGetChange(decimal total, string cashText,
            out decimal change, out string message)
        {
            change = 0m;
            message = "";

            if (total <= 0m)
            {
                message = "商品を追加してください。";
                return false;
            }

            decimal cash;
            if (!decimal.TryParse(cashText, NumberStyles.None,
                CultureInfo.InvariantCulture, out cash))
            {
                message = "預かり金は半角数字だけで入力してください。";
                return false;
            }

            if (cash < total)
            {
                message = "預かり金が不足しています。";
                return false;
            }

            change = cash - total;
            message = "会計できました。";
            return true;
        }
    }
}

TryParseは入力を数値に変換できたかどうかを返します。NumberStyles.Noneで余分な記号を許可しません。商品がない場合、数字にできない場合、金額が不足する場合を先に確認してから、お釣りを計算します。

using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace TeamRegisterApp
{
    public partial class Form1 : Form
    {
        private readonly ProductCatalog catalog = new ProductCatalog();
        private readonly CartService cart = new CartService();
        private readonly PaymentService payment = new PaymentService();
        private List<Product> products = new List<Product>();

        public Form1()
        {
            InitializeComponent();
            products = catalog.GetProducts();
            foreach (Product product in products)
            {
                productComboBox.Items.Add(product.Name);
            }
            if (products.Count > 0)
            {
                productComboBox.SelectedIndex = 0;
            }
            RefreshCart();
        }

        private void AddButton_Click(object? sender, EventArgs e)
        {
            int index = productComboBox.SelectedIndex;
            if (index < 0 || index >= products.Count)
            {
                MessageBox.Show("商品を選択してください。");
                return;
            }
            Product product = products[index];
            int quantity = (int)quantityNumericUpDown.Value;
            if (!cart.Add(product.Name, product.Price, quantity))
            {
                MessageBox.Show("追加できません。数量や100行の上限を確認してください。");
                return;
            }
            RefreshCart();
        }

        private void RemoveButton_Click(object? sender, EventArgs e)
        {
            if (cartDataGridView.SelectedRows.Count == 0)
            {
                MessageBox.Show("削除する行を選択してください。");
                return;
            }
            int index = cartDataGridView.SelectedRows[0].Index;
            if (cart.RemoveAt(index))
            {
                RefreshCart();
            }
        }

        private void ClearButton_Click(object? sender, EventArgs e)
        {
            cart.Clear();
            RefreshCart();
        }

        private void CheckoutButton_Click(object? sender, EventArgs e)
        {
            decimal change;
            string message;
            if (payment.TryGetChange(cart.GetTotal(), cashTextBox.Text,
                out change, out message))
            {
                changeLabel.Text = $"お釣り：{change:N0}円";
            }
            else
            {
                changeLabel.Text = "お釣り：未計算";
                MessageBox.Show(message);
            }
        }

        private void CashTextBox_TextChanged(object? sender, EventArgs e)
        {
            changeLabel.Text = "お釣り：未計算";
        }

        private void RefreshCart()
        {
            cartDataGridView.Rows.Clear();
            foreach (Product item in cart.GetItems())
            {
                cartDataGridView.Rows.Add(item.Name, item.Price,
                    item.Quantity, item.Subtotal);
            }
            cartDataGridView.ClearSelection();
            totalLabel.Text = $"合計：{cart.GetTotal():N0}円";
            cashTextBox.Clear();
            changeLabel.Text = "お釣り：未計算";
        }
    }
}

RefreshCartはCartServiceから一覧を受け取り、Rows.Clear → Rows.Addで表示を更新します。画面の配置・列の作成はデザイナー、商品の操作と部品の呼び出しはこのコードが担当します。

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;

namespace TeamRegisterApp
{
    public partial class Form1 : Form
    {
        private readonly ProductCatalog catalog = new ProductCatalog();
        private readonly CartService cart = new CartService();
        private readonly PaymentService payment = new PaymentService();
        private List<Product> products = new List<Product>();

        private readonly ComboBox productComboBox = new ComboBox();
        private readonly NumericUpDown quantityNumericUpDown = new NumericUpDown();
        private readonly Button addButton = new Button();
        private readonly DataGridView cartDataGridView = new DataGridView();
        private readonly Button removeButton = new Button();
        private readonly Button clearButton = new Button();
        private readonly Label totalLabel = new Label();
        private readonly TextBox cashTextBox = new TextBox();
        private readonly Button checkoutButton = new Button();
        private readonly Label changeLabel = new Label();

        public Form1()
        {
            InitializeComponent();
            BuildScreen();
            products = catalog.GetProducts();
            foreach (Product product in products)
            {
                productComboBox.Items.Add(product.Name);
            }
            if (products.Count > 0)
            {
                productComboBox.SelectedIndex = 0;
            }
            RefreshCart();
        }

        private void BuildScreen()
        {
            Text = "4人チームの簡易レジ";
            ClientSize = new Size(800, 540);
            FormBorderStyle = FormBorderStyle.FixedSingle;
            MaximizeBox = false;

            productComboBox.SetBounds(20, 20, 230, 30);
            productComboBox.DropDownStyle = ComboBoxStyle.DropDownList;
            quantityNumericUpDown.SetBounds(270, 20, 80, 30);
            quantityNumericUpDown.Minimum = 1;
            quantityNumericUpDown.Maximum = 99;
            quantityNumericUpDown.Value = 1;
            addButton.SetBounds(370, 20, 130, 30);
            addButton.Text = "商品を追加";

            cartDataGridView.SetBounds(20, 70, 760, 300);
            cartDataGridView.AllowUserToAddRows = false;
            cartDataGridView.AllowUserToDeleteRows = false;
            cartDataGridView.ReadOnly = true;
            cartDataGridView.MultiSelect = false;
            cartDataGridView.SelectionMode = DataGridViewSelectionMode.FullRowSelect;
            cartDataGridView.AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.Fill;
            cartDataGridView.RowHeadersVisible = false;
            cartDataGridView.Columns.Add("ProductName", "商品名");
            cartDataGridView.Columns.Add("Price", "単価（円）");
            cartDataGridView.Columns.Add("Quantity", "数量");
            cartDataGridView.Columns.Add("Subtotal", "小計（円）");
            foreach (DataGridViewColumn column in cartDataGridView.Columns)
            {
                column.SortMode = DataGridViewColumnSortMode.NotSortable;
            }
            cartDataGridView.Columns["Price"]!.DefaultCellStyle.Format = "N0";
            cartDataGridView.Columns["Subtotal"]!.DefaultCellStyle.Format = "N0";

            removeButton.SetBounds(20, 390, 130, 30);
            removeButton.Text = "選択行を削除";
            clearButton.SetBounds(170, 390, 140, 30);
            clearButton.Text = "次のお客様／全消去";
            totalLabel.SetBounds(340, 390, 400, 30);
            Label cashLabel = new Label();
            cashLabel.SetBounds(20, 450, 120, 25);
            cashLabel.Text = "預かり金（円）";
            cashTextBox.SetBounds(150, 450, 160, 30);
            checkoutButton.SetBounds(330, 450, 100, 30);
            checkoutButton.Text = "会計";
            changeLabel.SetBounds(460, 450, 320, 50);

            Controls.AddRange(new Control[]
            {
                productComboBox, quantityNumericUpDown, addButton,
                cartDataGridView, removeButton, clearButton, totalLabel,
                cashLabel, cashTextBox, checkoutButton, changeLabel
            });
            addButton.Click += AddButton_Click;
            removeButton.Click += RemoveButton_Click;
            clearButton.Click += ClearButton_Click;
            checkoutButton.Click += CheckoutButton_Click;
            cashTextBox.TextChanged += CashTextBox_TextChanged;
        }

        private void AddButton_Click(object? sender, EventArgs e)
        {
            int index = productComboBox.SelectedIndex;
            if (index < 0 || index >= products.Count)
            {
                MessageBox.Show("商品を選択してください。");
                return;
            }
            Product product = products[index];
            int quantity = (int)quantityNumericUpDown.Value;
            if (!cart.Add(product.Name, product.Price, quantity))
            {
                MessageBox.Show("追加できません。数量や100行の上限を確認してください。");
                return;
            }
            RefreshCart();
        }

        private void RemoveButton_Click(object? sender, EventArgs e)
        {
            if (cartDataGridView.SelectedRows.Count == 0)
            {
                MessageBox.Show("削除する行を選択してください。");
                return;
            }
            int index = cartDataGridView.SelectedRows[0].Index;
            if (cart.RemoveAt(index))
            {
                RefreshCart();
            }
        }

        private void ClearButton_Click(object? sender, EventArgs e)
        {
            cart.Clear();
            RefreshCart();
        }

        private void CheckoutButton_Click(object? sender, EventArgs e)
        {
            decimal change;
            string message;
            if (payment.TryGetChange(cart.GetTotal(), cashTextBox.Text,
                out change, out message))
            {
                changeLabel.Text = $"お釣り：{change:N0}円";
            }
            else
            {
                changeLabel.Text = "お釣り：未計算";
                MessageBox.Show(message);
            }
        }

        private void CashTextBox_TextChanged(object? sender, EventArgs e)
        {
            changeLabel.Text = "お釣り：未計算";
        }

        private void RefreshCart()
        {
            cartDataGridView.Rows.Clear();
            foreach (Product item in cart.GetItems())
            {
                cartDataGridView.Rows.Add(item.Name, item.Price,
                    item.Quantity, item.Subtotal);
            }
            cartDataGridView.ClearSelection();
            totalLabel.Text = $"合計：{cart.GetTotal():N0}円";
            cashTextBox.Clear();
            changeLabel.Text = "お釣り：未計算";
        }
    }
}

BuildScreenで部品の配置・列の作成・イベント登録を行います。本編ではこれらをデザイナーで設定しました。参考例は本編の画面と完全に同じ配置を保証するものではありません。