IEnumerable的＆LT; T＆GT;和IQueryable的＆LT; T＆GT;澄清？ [英] IEnumerable<T> and IQueryable<T> clarification?

问题描述

阅读这个的问题后， 我需要澄清一些事情

after reading this question , i need to clear up some things

IQueryable<Customer> custs = from c in db.Customers
where c.City == "<City>"
select c;

IEnumerable<Customer> custs = from c in db.Customers
where c.City == "<City>"
select c;

问题：

1）它是确定的说法是：在第一个查询中SQLServer是运行在整个操作过程，包括where子句和返回的仅相关行 - 而第二个呢 SELECT * ...返回所有行到C＃和 的过滤器？

1) Is it ok to say that : in the first query the sqlServer is running the whole operation including where clause and returning ONLY the relevant rows - while the second one does SELECT * ... and returns all rows into c# and THEN filters ?

2）什么样，如果我有一个的集合的仅仅是 - 在内存中。 （ VAR lstMyPerson =新的名单，其中，MyPerson＆GT;（））

2) what about if i have a collection merely - in memory. ( var lstMyPerson = new List<MyPerson>() )

    IQueryable<MyPerson> lst = from c in lstMyPerson 
    where c.City == "<City>"
    select c;

VS

    IEnumerable<MyPerson> custs = from c in lstMyPerson 
    where c.City == "<City>"
    select c;

这将是执行中的差异呢？

what will be the difference in execution now ?

推荐答案

1：不，那是不正确

既然你只的存储的结果到的IEnumerable＆LT;客户＆GT; ，但仍然有完全相同的前pression的产生的结果，他们都将在服务器上执行，只返回相关行。

Since you're only storing the result into an IEnumerable<Customer>, but still have the exact same expression that produces the result, they will both execute on the server and return only the relevant rows.

您将获得在行为的差异这一点：

You would get the difference in behavior with this:

IEnumerable<Customer> custs = from c in (IEnumerable<Customer>)db.Customers
    where c. City == "<City>"
    select c;

在这种情况下会迫使 db.Customers 收集用作的IEnumerable＆LT; T＆GT; ，其中当枚举将获取整个集合。

In this case you are forcing the db.Customers collection to be used as IEnumerable<T>, which when enumerated will fetch the entire collection.

需要注意的是这样的：

IEnumerable<Customer> x = from c in db.Customers
                          where c.City == "<City>"
                          select c;

是不一样的，因为这

is not the same as this:

IEnumerable<Customer> x = from c in db.Customers
                          select c;
IEnumerable<Customer> y = x.Where(c => c.City == "<City>");

在第一种情况下，在其中，子句将是SQL的组成部分，在第二个它不会。这就是为什么链接的问题/答案涉及的不同，而你的code没有。

In the first case, the where clause will be a part of the SQL, in the second it won't. That is why the linked question/answer involves a difference, whereas your code does not.

另外请注意，的只有的你写不会，其实在服务器上执行任何东西，因为它们会有效地只存储一个延迟集合的语句。如果你继续前进，列举这些集合，在这一点上的相关位将在服务器上执行。

Also note that only the statements you have written will not in fact execute anything at all on the server, since they will effectively only store a lazy collection. If you go ahead and enumerate those collections, at that point the relevant bits will be executed on the server.

2：名单，其中，T＆GT; 不执行或对扩展方法的IQueryable＆LT; T＆GT; ，也不会对LINQ运营商参与返回任何兼容的IQueryable＆LT; T＆GT;

2: List<T> does not implement or have extension methods for IQueryable<T>, nor will the LINQ operators involved return anything compatible with IQueryable<T>

在这种情况下，第一个将无法编译。

In this case, the first will not compile.

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