diff --git a/homework.cs b/homework.cs
index 91a7fc0..46c8370 100644
--- a/homework.cs
+++ b/homework.cs
@@ -1,82 +1,133 @@
-using System;
-using System.Collections.Generic;
-using System.Threading;
-using System.Threading.Tasks;
-
-public class Book
-{
-    public string Title { get; set; }
-    public decimal Price { get; set; }
-    public int Inventory { get; set; }
-}
-
-public class Database
-{
-    private readonly List<Book> _books = new List<Book>();
-    private readonly object _lock = new object();
-
-    public Database()
-    {
-        _books.Add(new Book { Title = "C#入门", Price = 39.9m, Inventory = 10 });
-        _books.Add(new Book { Title = "异步编程", Price = 59.9m, Inventory = 5 });
-    }
-
-    public Task<List<Book>> GetBooksAsync()
-    {
-        return Task.FromResult(_books);
-    }
-
-    public async Task UpdateInventoryAsync(string title, int quantity)
-    {
-        await Task.Delay(100); // 模拟网络延迟
-
-        // TODO: 使用 lock 语句保证线程安全
-        // 提示：在 lock 块中查找书籍并更新库存，若库存不足则输出提示
-    }
-}
-
-public class BookStore
-{
-    private readonly Database _db = new Database();
-
-    // TODO: 实现异步购书方法CheckoutAsync，调用 UpdateInventoryAsync
-    public async Task CheckoutAsync(string bookTitle, int quantity)
-    {
-    }
-
-    public async Task SimulateMultipleUsers()
-    {
-        var books = await _db.GetBooksAsync();
-        Console.WriteLine("当前书店库存：");
-        foreach (var book in books)
-        {
-            Console.WriteLine($"- {book.Title}：{book.Inventory} 本");
-        }
-
-        Console.WriteLine("\n 开始模拟多用户购书...\n");
-
-        // TODO: 使用 Task.WhenAll 模拟多个用户并发购书
-        // 提示：创建多个 Task 调用 CheckoutAsync，并传入不同书名和数量
-        var tasks = new List<Task>
-        {
-            
-        };
-
-
-        Console.WriteLine("\n购买后库存：");
-        books = await _db.GetBooksAsync();
-        foreach (var book in books)
-        {
-            Console.WriteLine($"- {book.Title}：{book.Inventory} 本");
-        }
-    }
-}
-
-public class Program
-{
-    public static async Task Main()
-    {
-        var store = new BookStore();
-        await store.SimulateMultipleUsers();
-    }
-}
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+public class Book
+{
+    public string Title { get; set; }  // { get; set; }：调用时自动读取或设置值
+    public decimal Price { get; set; }
+    public int Inventory { get; set; }  // 库存数量
+}
+
+public class Database
+{
+    private readonly List<Book> _books = new List<Book>();
+    private readonly object _lock = new object();
+
+    public Database()
+    {
+        _books.Add(new Book { Title = "C#入门", Price = 39.9m, Inventory = 10 });
+        _books.Add(new Book { Title = "异步编程", Price = 59.9m, Inventory = 5 });
+    }
+
+    public Task<List<Book>> GetBooksAsync()
+    {
+        return Task.FromResult(_books);
+    }
+
+    public async Task UpdateInventoryAsync(string title, int quantity)
+    {
+        await Task.Delay(100); // 模拟网络延迟
+
+        // TODO: 使用 lock 语句保证线程安全
+        // 提示：在 lock 块中查找书籍并更新库存，若库存不足则输出提示
+        lock (_lock)
+        {
+            bool bookFound = false;
+            foreach (Book book in _books)
+            {
+                if (book.Title == title)
+                {
+                    bookFound = true;
+                    if (book.Inventory + quantity >= 0)  // 允许增减库存
+                    {
+                        book.Inventory += quantity;
+                    }
+                    else
+                    {
+                        throw new InvalidOperationException($"库存不足！书名: 《{book.Title}》，当前库存: {book.Inventory}，尝试修改量: {quantity}");
+                    }
+                    break;  // 找到书后就直接退出循环
+                }
+            }
+            if (!bookFound)  // 一次性处理未找到书情况
+            {
+                throw new KeyNotFoundException($"找不到书籍: 《{title}》");
+            }
+        }
+    }
+}
+
+public class BookStore
+{
+    private readonly Database _db = new Database();
+
+    // TODO: 实现异步购书方法CheckoutAsync，调用 UpdateInventoryAsync
+    public async Task CheckoutAsync(string bookTitle, int quantity)
+    {
+        try
+        {
+            await UpdateInventoryAsync(bookTitle, -quantity);  // 减少库存（quantity应为负数，例如-1表示购买1本）
+            await Task.Delay(100);  // 模拟支付/物流延迟
+            Console.WriteLine($"已成功购买 {quantity} 本《{bookTitle}》");
+        }
+        catch (InvalidOperationException ex)  // 假设UpdateInventoryAsync在库存不足时抛出异常
+        {
+            Console.WriteLine($"购买失败: {ex.Message}");
+        }
+        catch (KeyNotFoundException ex)  // 假设UpdateInventoryAsync在找不到书时抛出异常
+        {
+            Console.WriteLine($"购买失败: {ex.Message}");
+        }
+    }
+
+    public async Task SimulateMultipleUsers()
+    {
+        var books = await _db.GetBooksAsync();
+        Console.WriteLine("当前书店库存：");
+        foreach (var book in books)
+        {
+            Console.WriteLine($"- {book.Title}：{book.Inventory} 本");
+        }
+
+        Console.WriteLine("\n 开始模拟多用户购书...\n");
+
+        // TODO: 使用 Task.WhenAll 模拟多个用户并发购书
+        // 提示：创建多个 Task 调用 CheckoutAsync，并传入不同书名和数量
+        var tasks = new List<Task>
+        {
+            CheckoutAsync("C#入门", 2),
+            CheckoutAsync("C#入门", 3),
+            CheckoutAsync("异步编程", 1),
+            CheckoutAsync("异步编程", 2),
+            CheckoutAsync("异步编程", 3)
+        };
+
+        try
+        {
+            await Task.WhenAll(tasks);  // 显式等待所有任务
+            Console.WriteLine("所有购买完成！");
+        }
+        catch (Exception ex)
+        {
+            Console.WriteLine($"购买失败: {ex.Message}");
+        }
+
+        Console.WriteLine("\n购买后库存：");
+        books = await _db.GetBooksAsync();
+        foreach (var book in books)
+        {
+            Console.WriteLine($"- {book.Title}：{book.Inventory} 本");
+        }
+    }
+}
+
+public class Program
+{
+    public static async Task Main()
+    {
+        var store = new BookStore();
+        await store.SimulateMultipleUsers();
+    }
+}