Implement Order Domain Objects - Cart, CartItem, CartItemOption (#1)

src/AUSG.Eats.Order.Domain/Cart.cs

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+namespace AUSG.Eats.Order.Domain;
+
+public class Cart
+{
+    private readonly List<CartItem> _items = new();
+
+    private long? _userId;
+
+    public Cart(long? userId = null)
+    {
+        _userId = userId;
+    }
+
+    public IEnumerable<CartItem> Items => _items.AsReadOnly();
+
+    public void AddToCart(CartItem newItem)
+    {
+        _items.Add(newItem);
+    }
+
+    public void RemoveFromCart(CartItem itemToRemove)
+    {
+        _items.Remove(itemToRemove);
+    }
+
+    public void AlterCartItem(CartItem itemToChange)
+    {
+        if (!_items.Remove(itemToChange)) // Id 비교이므로 제거된다.
+            throw new ArgumentException("없는 CartItem을 변경하려고 한다.");
+        _items.Add(itemToChange);
+    }
+
+    public override bool Equals(object? obj)
+    {
+        // use null propagation (잘 이해 못하고 있습니다.)
+        // see TC: CSharpTests#compare_with_null_instance_returns_false
+        if (obj is not Cart other)
+            return false;
+        return _userId == other._userId;
+    }
+
+    public override int GetHashCode()
+    {
+        // Non-readonly property referenced in 'GetHashCode()' ??
+        return _userId.GetHashCode();
+    }
+}

src/AUSG.Eats.Order.Domain/CartItem.cs

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+namespace AUSG.Eats.Order.Domain;
+
+public class CartItem
+{
+    private long? _id;
+
+    public CartItem(long? id)
+    {
+        _id = id;
+    }
+
+    public override bool Equals(object? obj)
+    {
+        // use null propagation (잘 이해 못하고 있습니다.)
+        // see TC: CSharpTests#compare_with_null_instance_returns_false
+        if (obj is not CartItem other)
+            return false;
+        return _id == other._id;
+    }
+
+    public override int GetHashCode()
+    {
+        // Non-readonly property referenced in 'GetHashCode()' ??
+        return _id.GetHashCode();
+    }
+}

src/AUSG.Eats.Order.Test/CSharpTests.cs

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+using Xunit;
+
+namespace AUSG.Eats.Order.Test;
+
+public class CSharpTests
+{
+    [Fact(DisplayName = "Integer fields are initialized to 0")]
+    public void int_init_value_is_0_in_class()
+    {
+        var sut = new IntInit();
+
+        Assert.Equal(0, sut.id);
+    }
+
+    [Fact(DisplayName = "Nullable integer fields are initialized to 0")]
+    public void nullable_int_init_value_is_null_in_class()
+    {
+        var sut = new IntInit();
+
+        // Assert.Null은 사용할 수 없다.
+        // Boxing allocation: conversion from 'int?' to 'object' requires boxing of value type
+        // Assert.Null(sut.nullableId);
+
+        // Nullable<int?>의 null 여부를 체크할 수 있는 메소드가 없다.
+        Assert.True(sut.nullableId == null);
+    }
+
+    [Fact(DisplayName = "Comparing with null instance returns false using null propagation")]
+    public void compare_with_null_instance_returns_false()
+    {
+        var NonNullInstance = new EqualsImplEx(1L);
+        var NullInstance = new EqualsImplEx();
+
+        Assert.True(NullInstance.Id == null);
+        Assert.NotEqual(NonNullInstance, NullInstance);
+    }
+
+    private class IntInit
+    {
+        public int id { get; set; }
+        public int? nullableId { get; set; }
+    }
+
+    private class EqualsImplEx
+    {
+        public EqualsImplEx(long? id = null)
+        {
+            Id = id;
+        }
+
+        public long? Id { get; }
+
+        public override bool Equals(object? obj)
+        {
+            // use null propagation
+            if (obj is not EqualsImplEx other)
+                return false;
+            return Id == other.Id;
+        }
+    }
+}

src/AUSG.Eats.Order.Test/OrderTests.cs

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+using AUSG.Eats.Order.Domain;
+using Xunit;
+
+namespace AUSG.Eats.Order.Test;
+
+public class OrderTests
+{
+    private CartItem MakeNewCartItem(long? id = null)
+    {
+        return new CartItem(id);
+    }
+
+    [Fact(DisplayName = "CartItem 객체는 Id로 식별할 수 있다.")]
+    public void CartItem_Can_Be_Identified_By_Id()
+    {
+        // given
+        const long pid = 1L;
+        var cartItem1 = MakeNewCartItem(pid);
+        var cartItem2 = MakeNewCartItem(pid);
+
+        // then
+        Assert.Equal(cartItem1, cartItem2);
+        Assert.Equal(cartItem1.GetHashCode(), cartItem2.GetHashCode());
+    }
+
+    [Fact(DisplayName = "Cart 객체는 UserId로 식별할 수 있다.")]
+    public void Cart_Shares_UserId_and_Can_be_Identified_Using_UserId()
+    {
+        // given
+        var userId = 1L;
+        var cart1 = new Cart(userId);
+        var cart2 = new Cart(userId);
+
+        // then
+        Assert.Equal(cart1, cart2);
+        Assert.Equal(cart1.GetHashCode(), cart2.GetHashCode());
+    }
+
+    [Fact(DisplayName = "Cart 객체는 CartItem을 추가할 수 있다.")]
+    public void Cart_can_Add_CartItem()
+    {
+        // given
+        var cart = new Cart();
+        var cartItem1 = MakeNewCartItem();
+
+        // when
+        cart.AddToCart(cartItem1);
+
+        // then
+        Assert.Equal(cart.Items.ElementAt(0), cartItem1);
+    }
+
+    [Fact(DisplayName = "Cart 객체는 CartItem을 제거할 수 있다.")]
+    public void Cart_can_Remove_CartItem()
+    {
+        // given
+        var cart = new Cart();
+        var cartItem1 = MakeNewCartItem();
+        cart.AddToCart(cartItem1);
+        Assert.Equal(cart.Items.ElementAt(0), cartItem1);
+
+        // when
+        cart.RemoveFromCart(cartItem1);
+
+        // then
+        Assert.Empty(cart.Items);
+    }
+
+    [Fact(DisplayName = "Cart 객체는 CartItem을 변경할 수 있다.")]
+    public void Cart_can_Alter_CartItem()
+    {
+        // given
+        const long cartItemId = 1L;
+        var cart = new Cart();
+        var oldItem = MakeNewCartItem(cartItemId);
+        cart.AddToCart(oldItem);
+
+        // when
+        var newItem = MakeNewCartItem(cartItemId);
+        cart.AlterCartItem(newItem);
+
+        // then
+        var alteredItem = cart.Items.ElementAt(0);
+        Assert.Equal(alteredItem, oldItem);
+    }
+
+    [Fact(DisplayName = "Cart 객체는 CartItem을 변경할 때 해당 Item이 이미 List에 없다면 예외를 발생시킨다.")]
+    public void Cart_throw_ArgumentException_when_Alter_CartItem()
+    {
+        // given
+        var cart = new Cart();
+        const long cartItemIdWhichDoesNotExist = 1L;
+        var oldItem = MakeNewCartItem(cartItemIdWhichDoesNotExist);
+
+        // when
+        void AlterItem()
+        {
+            cart.AlterCartItem(oldItem);
+        }
+
+        // then
+        Assert.Throws<ArgumentException>((Action) AlterItem);
+    }
+}