LINQ左外连接语法差异

Question

有这样做左外至少有两种方法在LINQ

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

class Order 
{ 
    public int ID { get; set; } 
    public string Product { get; set; } 
} 

    static void Main() 
    { 
     // Example customers. 
     var customers = new Customer[] 
     { 
      new Customer{ID = 5, Name = "Sam"}, 
      new Customer{ID = 6, Name = "Dave"}, 
      new Customer{ID = 7, Name = "Julia"}, 
      new Customer{ID = 8, Name = "Sue"}, 
      new Customer{ID = 9, Name = "Joe"} 
     }; 

     // Example orders. 
     var orders = new Order[] 
     { 
      new Order{ID = 5, Product = "Book"}, 
      new Order{ID = 6, Product = "Game"}, 
      new Order{ID = 7, Product = "Computer"}, 
      new Order{ID = 8, Product = "Shirt"}, 
      new Order{ID = 8, Product = "TShirt"} 
     }; 


     // First Way 
     var query = from c in customers 
       join o in orders on c.ID equals o.ID into co 
       from x in co.DefaultIfEmpty() 
       where x != null && x.Product == "Shirt" 
       select new {c, x}; 

     // Second Way 
     var query2 = from c in customers 
        from o in orders 
        where c.ID == o.ID && o.Product == "Shirt" 
        select new {c, o}; 
    } 

JOIN我发现（使用“到”）有很多的第一种方式的例子，那就是这样我曾经做过我的左外连接。最近我找到了第二条路，看起来更简单。我测试了它们并且它们都产生了相同的结果。现在我的问题是否存在任何隐藏的差异，表现，还是仅仅是语法糖？

非常感谢。

Answer 1

第一种方式，应使用，如果你不使用！= NULL检查

左外连接所有匹配右表，如果不存在匹配的项目选择一切从左表在右表中，结果仍然返回，但右表中的值为空（linq将其转换为集合项类型的默认值）

这两个查询都有效地执行内连接，因此它们将会相同。

要为什么地球上您使用左外连接，得到不同的结果

// First Way 
var query = from c in customers 
join o in orders on c.ID equals o.ID into co 
from x in co.DefaultIfEmpty() 
where c.Id>5 
select new {c, x}; 

// Second Way 
var query2 = from c in customers 
from o in orders 
where c.ID == o.ID && c.ID > 5 
select new {c, o};