元組對比匿名類型與Expando對象。 （關於LINQ查詢）

Question

我是一個初學者，最終開始瞭解匿名類型。

（見舊後What is the return type for a anonymous linq query select? What is the best way to send this data back?）所以在LINQ

查詢您構成LINQ查詢權在你想返回值的類型？看來這樣做的方式是匿名類型的權利？

有人可以向我解釋是否以及何時可以使用Tuple/Expando對象嗎？他們都似乎很simliar？

Answer 1

你沒有說明你的問題的上下文，所以我會回答LinqToObjects和LinqToSql。

在LinqToObjects中，假設您有一個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，假設你有一個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，那現在夠了。

Answer 2

元組和擴展對象通常不在LINQ中使用。它們與匿名類型完全不同。

匿名類型通常用於「塑造」LINQ查詢;例如，您可以定義一個具有string Name屬性和int Age屬性的類型。

元組是僅充當「對」或「三元」類結構的類型。例如，可以定義Tuple<string, int>，但屬性的名稱分別命名爲Item1和Item2，而不是Name和Age。元組通常不用於形成LINQ查詢，因爲這些屬性名稱使得代碼不太清晰。

ExpandoObject是完全不同的。它允許您在運行時將屬性添加到現有對象。