元组与匿名类型与 Expando 对象。 （关于 LINQ 查询）

Question

我是一个终于开始理解匿名类型的初学者。

（参见旧帖子 匿名 linq 查询选择的返回类型是什么？发送回此数据的最佳方式是什么？）

因此，在 LINQ 查询中，您可以在 linq 中形成所需的返回值类型查询对吗？看来做到这一点的方法是匿名类型，对吧？

有人可以向我解释一下我是否以及何时可以使用 Tuple/Expando 对象来代替吗？他们看起来都很相似？

Answer 1

元组和 Expando 对象通常不在 LINQ 中使用。它们与匿名类型有很大不同。

匿名类型通常用于“塑造”LINQ 查询；例如，您可以定义一个具有 string Name 属性和 int Age 属性的类型。

元组是仅充当“对”或“三元组”类型结构的类型。例如，可以定义 Tuple，但属性名称命名为 Item1 和 Item2，而不是 姓名和年龄。元组通常不用于塑造 LINQ 查询，因为这些属性名称使代码不太清晰。

ExpandoObject 完全不同。它允许您在运行时向现有对象添加属性。

Answer 2

您没有说明问题的上下文，因此我将同时回答 LinqToObjects 和 LinqToSql。

在 LinqToObjects 中，假设您有一个 List;来源。

  //Straight projection.
  //no new instances are created when query is evaluated.
IEnumerable<Customer> result =
  from c in source where c.Name == "Bob"
  select c;

  //Different Type projection
  //a new instance of CustomerName is created
  // for each element in the result when the query is evaluated.
IEnumerable<CustomerName> result =
  from c in source where c.Name == "Bob"
  select new CustomerName() {Name = c.Name};

  //Anonymous Type Projection    
  //a new instance of an anonymous type is created
  // for each element in the result when the query is evaluated.
  //You don't have access to the type's name
  // since the compiler names the type,
  // so you must use var to talk about the type of the result.
var result =
  from c in source where c.Name == "Bob"
  select new {Name = "Bob"};

  //Tuple Projection (same as Different Type Projection)
  //a new instance of Tuple is created
  // for each element in the result when the query is evaluated.
IEnumerable<Tuple<string, int>> result = 
  from c in source where c.Name == "Bob"
  select new Tuple<string, int>(){First = c.Name, Second = c.Id};

---

在 LinqToSql 中，假设您有一个 IQueryabledb.Customers

  //Straight projection
  //when the query is resolved
  // DataContext.Translate<Customer> is called
  // which converts the private dbReader into new Customer instances.
IQueryable<Customer> result =
  from c in db.Customers where c.Name == "Bob"
  select c;

  //Different Type Projection
  //when the query is resolved
  // DataContext.Translate<CustomerName> is called
  // which converts the private dbReader into new CustomerName instances.
  // 0 Customer instances are created.
IQueryable<Customer> result =
  from c in db.Customers where c.Name == "Bob"
  select new CustomerName() {Name = c.Name};

  //Different Type Projection with a twist
  //when the query is resolved
  // DataContext.Translate<CustomerGroup> is called
  // which converts the private dbReader into new CustomerGroup instances.
  // 0 Customer instances are created.
  //the anonymous type is used in the query translation
  // yet no instances of the anonymous type are created.
IQueryable<Customer> result =
  from c in db.Customers
  group c by new {c.Name, TheCount = c.Orders.Count()} into g
  select new CustomerGroup()
  {
    Name = g.Key.Name,
    OrderCount = g.Key.TheCount,
    NumberInGroup = g.Count()
  };

好的，现在就足够了。