Linq SelectMany查询

Question

我有以下查询:

 DateTime cutoffDate = new DateTime(1997, 1, 1); 

 var orders = 
     from c in customers 
     where c.Region == "WA" 
     from o in c.Orders 
     where o.OrderDate >= cutoffDate 
     select new { c.CustomerID, o.OrderID };

怎么能用Linq Lambda写的呢？BTW,这被称为SelectMany查询吗？

这也可以通过连接来完成,如上所示,这样做的优点和缺点是什么.

Answer 1

是的,这是一个SelectMany.您使用SelectMany"展平"嵌套或分层集合(在这种情况下,订单嵌套在客户下)到一个简单的单层集合中.

customers.Where(c => c.Region == "WA")
    .SelectMany(c => c.Orders)
    .Where(o => o.Orderdate >= cutoffDate)
    .Select(x => new { x.OrderID, x.Customer.CustomerID });

如果订单是客户的财产,则无需使用联接.