Linq 內部加入組，然后選擇組，為什么有空組？

Question

我正在嘗試獲取包含各自訂單的客戶列表，但如果客戶沒有任何訂單，則應將其排除在該列表之外。

        var customerOrders = (from customer in customers
            join order in orders on customer.CustomerId equals order.CustomerId into
                orderGroup
            select new
            {
                Customer.CustomerId,
                Orders = orderGroup.ToList()
            }).ToList();

在 customerOrders 列表中，我收到沒有訂單或訂單列表為空的客戶。

我怎樣才能只得到有訂單的客戶？

Answer 1

使用where orderGroup.Any() ：

var customerOrders =
(
    from customer in customers
    join order in orders on customer.CustomerId equals order.CustomerId into orderGroup
    where orderGroup.Any()
    select new
    {
        customer.CustomerId,
        Orders = orderGroup.ToList()
    }
).ToList();

或者group order by customer.CustomerId into g ：

var customerOrders =
(
    from customer in customers
    join order in orders on customer.CustomerId equals order.CustomerId
    group order by customer.CustomerId into g
    select new
    {
        CustomerId = g.k,
        Orders = g.ToList()
    }
).ToList();