用c#连接数据库写一个简单的购物系统

好的，以下是一个简单的购物系统的 C# 代码示例：

using System;
using System.Data.SqlClient;

namespace ShoppingSystem
{
    class Program
    {
        static void Main(string[] args)
        {
            string connectionString = "Data Source=ServerName;Initial Catalog=DatabaseName;User ID=UserName;Password=Password";
            SqlConnection connection = new SqlConnection(connectionString);
            connection.Open();

            Console.WriteLine("欢迎使用购物系统！");
            Console.WriteLine();

            while (true)
            {
                Console.WriteLine("请选择操作：");
                Console.WriteLine("1. 查看商品列表");
                Console.WriteLine("2. 添加商品到购物车");
                Console.WriteLine("3. 查看购物车");
                Console.WriteLine("4. 结算");
                Console.WriteLine("5. 退出");
                Console.WriteLine();

                string choice = Console.ReadLine();

                if (choice == "1")
                {
                    Console.WriteLine("商品列表：");
                    Console.WriteLine();

                    SqlCommand command = new SqlCommand("SELECT * FROM Products", connection);
                    SqlDataReader reader = command.ExecuteReader();

                    while (reader.Read())
                    {
                        Console.WriteLine("{0} - {1} - ￥{2}", reader["Id"], reader["Name"], reader["Price"]);
                    }

                    reader.Close();
                    Console.WriteLine();
                }
                else if (choice == "2")
                {
                    Console.WriteLine("请输入要添加的商品编号：");
                    int productId = int.Parse(Console.ReadLine());

                    SqlCommand command = new SqlCommand("SELECT * FROM Products WHERE Id = @Id", connection);
                    command.Parameters.AddWithValue("@Id", productId);
                    SqlDataReader reader = command.ExecuteReader();

                    if (reader.Read())
                    {
                        Console.WriteLine("已添加商品：{0} - ￥{1}", reader["Name"], reader["Price"]);

                        reader.Close();

                        command = new SqlCommand("INSERT INTO ShoppingCart (ProductId) VALUES (@ProductId)", connection);
                        command.Parameters.AddWithValue("@ProductId", productId);
                        command.ExecuteNonQuery();
                    }
                    else
                    {
                        Console.WriteLine("未找到商品！");
                        reader.Close();
                    }

                    Console.WriteLine();
                }
                else if (choice == "3")
                {
                    Console.WriteLine("购物车中的商品：");
                    Console.WriteLine();

                    SqlCommand command = new SqlCommand("SELECT Products.Name, Products.Price FROM ShoppingCart INNER JOIN Products ON ShoppingCart.ProductId = Products.Id", connection);
                    SqlDataReader reader = command.ExecuteReader();

                    while (reader.Read())
                    {
                        Console.WriteLine("{0} - ￥{1}", reader["Name"], reader["Price"]);
                    }

                    reader.Close();
                    Console.WriteLine();
                }
                else if (choice == "4")
                {
                    Console.WriteLine("结算：");

                    SqlCommand command = new SqlCommand("SELECT SUM(Products.Price) AS TotalPrice FROM ShoppingCart INNER JOIN Products ON ShoppingCart.ProductId = Products.Id", connection);
                    SqlDataReader reader = command.ExecuteReader();

                    if (reader.Read())
                    {
                        Console.WriteLine("总价：￥{0}", reader["TotalPrice"]);
                    }

                    reader.Close();

                    command = new SqlCommand("DELETE FROM ShoppingCart", connection);
                    command.ExecuteNonQuery();

                    Console.WriteLine("谢谢惠顾！");
                    Console.WriteLine();
                }
                else if (choice == "5")
                {
                    break;
                }
                else
                {
                    Console.WriteLine("无效的选择！");
                    Console.WriteLine();
                }
            }

            connection.Close();
        }
    }
}

注意，这个代码示例中使用了 SQL Server 数据库，并且需要将 `ServerName`、`DatabaseName`、`UserName` 和 `Password` 替换成实际的值。同时，还需要在数据库中创建 `Products` 和 `ShoppingCart` 表，具体的表结构如下：

CREATE TABLE Products (
    Id INT PRIMARY KEY,
    Name VARCHAR(50),
    Price DECIMAL(18, 2)
);

CREATE TABLE ShoppingCart (
    Id INT PRIMARY KEY IDENTITY,
    ProductId INT FOREIGN KEY REFERENCES Products(Id)
);

这个购物系统的逻辑很简单，就是可以查看商品列表、添加商品到购物车、查看购物车和结算。当用户选择结算时，程序会计算购物车中的商品总价，并清空购物车。