C# 条件可选 联表查询 代码

在C#中，如果你想要执行条件可选的联表查询（也称为JOIN操作），特别是在LINQ（Language Integrated Query）上下文中，你可以使用`Join`或`GroupJoin`方法。假设你有两个实体类，比如`Customer`和`Order`，并且你想找出每个客户是否有订单，你可以这样做：

using System;
using System.Collections.Generic;
using System.Linq;

public class Customer
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Order
{
    public int Id { get; set; }
    public int CustomerId { get; set; }
    public DateTime Date { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        // 假设这是两个实体的示例数据
        List<Customer> customers = new List<Customer>
        {
            new Customer { Id = 1, Name = "Alice" },
            new Customer { Id = 2, Name = "Bob" },
            new Customer { Id = 3, Name = "Charlie" }
        };

        List<Order> orders = new List<Order>
        {
            new Order { Id = 1, CustomerId = 1, Date = DateTime.Now },
            new Order { Id = 2, CustomerId = 2, Date = DateTime.Now.AddDays(-1) },
        };

        // 使用 LINQ 进行联表查询
        var result = from customer in customers
                     join order in orders on customer.Id equals order.CustomerId into customerOrders
                     from orderInCustomer in customerOrders.DefaultIfEmpty() // 条件可选
                     select new
                     {
                         Customer = customer,
                         HasOrder = orderInCustomer != null,
                         LatestOrderDate = orderInCustomer?.Date // 如果有订单，则获取日期
                     };

        foreach (var item in result)
        {
            Console.WriteLine($"Customer: {item.Customer.Name}, Has Order: {item.HasOrder}, Latest Order Date: {item.LatestOrderDate}");
        }

        // 或者使用扩展方法版本
        result = customers.Join(orders,
            c => c.Id,
            o => o.CustomerId,
            (c, o) => new
            {
                Customer = c,
                HasOrder = o != null,
                LatestOrderDate = o.Date ?? default(DateTime)
            });

        // ...继续处理结果
    }
}

在这个例子中，`DefaultIfEmpty()` 方法确保即使某个客户没有订单，也会返回一个空的 `Order` 对象，表示该条件为假。`??` 运算符用于安全地提供默认值。