元组与匿名类型与Expando对象。(关于LINQ查询)

您没有说明您的问题的上下文,所以我将同时回答LinqToObjects和LinqToSQL。

在LinqtoObjects中,假设您有一个 List<Customer> source .

  //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 IQueryable<Customer> db.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()
  };

好吧,现在就够了。

tuples和expando对象通常不在linq中使用。它们都与匿名类型截然不同。

匿名类型通常用于“塑造”LINQ查询;例如,您可以定义具有 string Name 属性与AN int Age 财产。

元组是仅充当“对”或“三重集”类型结构的类型。例如A Tuple<string, int> 可以定义,但属性的名称是命名的 Item1 和 Item2 不是 Name 和 Age . 元组通常不用于形成LINQ查询,因为这些属性名使代码不那么清晰。

Expandoobject完全不同。它允许您在运行时向现有对象添加属性。