c#窗体应用编写一个外卖系统

编写一个外卖系统需要以下几个步骤：

1. 设计数据库：需要设计一个合理的数据库结构用于存储外卖系统的数据，如订单、用户、菜品等信息。

2. 开发用户界面：使用C#的窗体应用开发界面，包括登录页面、主页面、购物车页面、订单页面等。

3. 编写业务逻辑：实现用户注册、登录、查询菜品、添加购物车、提交订单等功能的业务逻辑。

4. 数据库连接：使用ADO.NET连接数据库，对数据进行增删改查操作，确保数据的一致性和完整性。

以下是一个简单的外卖系统的代码示例：

//登陆界面
private void btnLogin_Click(object sender, EventArgs e)
{
    string userName = txtUserName.Text.Trim();
    string password = txtPassword.Text.Trim();
    if (string.IsNullOrEmpty(userName) || string.IsNullOrEmpty(password))
    {
        MessageBox.Show("用户名和密码不能为空！");
        return;
    }

    //验证用户名和密码
    if (CheckUser(userName, password))
    {
        //跳转到主界面
        MainForm mainForm = new MainForm();
        mainForm.Show();
        this.Hide();
    }
    else
    {
        MessageBox.Show("用户名或密码错误！");
    }
}

//验证用户名和密码
private bool CheckUser(string userName, string password)
{
    //连接数据库
    string connectionString = "Data Source=.;Initial Catalog=Takeout;Integrated Security=True";
    SqlConnection connection = new SqlConnection(connectionString);
    connection.Open();

    //查询用户信息
    SqlCommand command = new SqlCommand($"SELECT COUNT(*) FROM Users WHERE UserName='{userName}' AND Password='{password}'", connection);
    int count = (int)command.ExecuteScalar();

    connection.Close();
    return count > 0;
}

//查询菜品
private void btnSearch_Click(object sender, EventArgs e)
{
    string keyword = txtKeyword.Text.Trim();
    if (string.IsNullOrEmpty(keyword))
    {
        MessageBox.Show("请输入关键字！");
        return;
    }

    //连接数据库
    string connectionString = "Data Source=.;Initial Catalog=Takeout;Integrated Security=True";
    SqlConnection connection = new SqlConnection(connectionString);
    connection.Open();

    //查询菜品信息
    SqlCommand command = new SqlCommand($"SELECT * FROM Dishes WHERE Name LIKE '%{keyword}%'", connection);
    SqlDataReader reader = command.ExecuteReader();
    DataTable dt = new DataTable();
    dt.Load(reader);
    dataGridView1.DataSource = dt;

    connection.Close();
}

//添加购物车
private void btnAddToCart_Click(object sender, EventArgs e)
{
    if (dataGridView1.SelectedRows.Count <= 0)
    {
        MessageBox.Show("请先选择要添加的菜品！");
        return;
    }

    //获取选中的菜品信息
    int id = (int)dataGridView1.SelectedRows[0].Cells["ID"].Value;
    string name = dataGridView1.SelectedRows[0].Cells["Name"].Value.ToString();
    decimal price = (decimal)dataGridView1.SelectedRows[0].Cells["Price"].Value;

    //添加到购物车
    DataRow row = dtCart.NewRow();
    row["ID"] = id;
    row["Name"] = name;
    row["Price"] = price;
    row["Quantity"] = 1;
    dtCart.Rows.Add(row);

    //计算总价
    decimal totalPrice = 0;
    foreach (DataRow dr in dtCart.Rows)
    {
        totalPrice += (decimal)dr["Price"] * (int)dr["Quantity"];
    }
    lblTotalPrice.Text = $"总价：{totalPrice:C}";

    MessageBox.Show("添加成功！");
}

//提交订单
private void btnSubmitOrder_Click(object sender, EventArgs e)
{
    if (dtCart.Rows.Count <= 0)
    {
        MessageBox.Show("购物车为空！");
        return;
    }

    //连接数据库
    string connectionString = "Data Source=.;Initial Catalog=Takeout;Integrated Security=True";
    SqlConnection connection = new SqlConnection(connectionString);
    connection.Open();

    //插入订单信息
    SqlCommand command = new SqlCommand("INSERT INTO Orders(UserName, TotalPrice) VALUES(@UserName, @TotalPrice); SELECT @@IDENTITY;", connection);
    command.Parameters.AddWithValue("@UserName", lblUserName.Text);
    command.Parameters.AddWithValue("@TotalPrice", lblTotalPrice.Text.Substring(3));
    int orderId = Convert.ToInt32(command.ExecuteScalar());

    //插入订单明细信息
    foreach (DataRow dr in dtCart.Rows)
    {
        command = new SqlCommand("INSERT INTO OrderDetails(OrderID, DishID, DishName, Price, Quantity) VALUES(@OrderID, @DishID, @DishName, @Price, @Quantity)", connection);
        command.Parameters.AddWithValue("@OrderID", orderId);
        command.Parameters.AddWithValue("@DishID", dr["ID"]);
        command.Parameters.AddWithValue("@DishName", dr["Name"]);
        command.Parameters.AddWithValue("@Price", dr["Price"]);
        command.Parameters.AddWithValue("@Quantity", dr["Quantity"]);
        command.ExecuteNonQuery();
    }

    connection.Close();
    MessageBox.Show("订单提交成功！");

    //清空购物车
    dtCart.Rows.Clear();
    lblTotalPrice.Text = "总价：￥0.00";
}