在C# LINQ中获取具有相同外键的多列数据

public class Order
{
    public int OrderID { get; set; }
    // other properties
}

public class OrderItem
{
    public int OrderID { get; set; }
    // other properties
}

List<Order> orders = new List<Order>();
List<OrderItem> orderItems = new List<OrderItem>();

var result = orders.GroupJoin(orderItems,
                order => order.OrderID,
                orderItem => orderItem.OrderID,
                (order, items) => new
                {
                    Order = order,
                    OrderItems = items
                })
                .SelectMany(x => x.OrderItems.DefaultIfEmpty(),
                (order, item) => new
                {
                    OrderID = order.Order.OrderID,
                    // other columns from Order and OrderItem
                });