何时真正执行LINQ查询? [英] When is the LINQ query actually get executed?

问题描述

假设我们有以下LINQ查询:

Suppose we have the following LINQ query:

var query =
    from    c in Customers
    where   c.Country == "Italy"
    orderby c.Name
    select  new { c.Name, c.City };

编译器将像这样转换它:

Compiler will convert it like this:

IEnumerable<Customer> query =
        Customers
        .Where( c => c.Country == "Italy" );
        .OrderBy( c => c.Name )
        .Select( c => new { c.Name, c.City } );

然后我会像这样使用查询:

Then I coud use the query like this:

foreach ( var name_city_pair in query ) ...

问题是:

• 似乎已经查询了由查询指定的数据当我使用foreach循环时.那么，什么时候执行此查询操作?当我定义 IEnumerable< Customer> 的LINQ查询对象时是?

• It seems the data specified by the query is already queried out when I use the foreach loop. So when is this query action take place? Is it when I define the LINQ query object of IEnumerable<Customer> ?

如果数据量太大，是否有任何后期查询机制?(我不确定用正确的词来形容这一点，但是我希望你能得到我.)

If the data number is too large, is there any late query mechanism? (I am not sure about the right word to describe this, but I hope you get me.)

推荐答案

LINQ在可能的情况下使用延迟执行.在您的示例中，仅当您遍历结果时才执行查询.

LINQ uses deferred execution where possible. In your example the query is only executed when you iterate over the results.

以 To 开头的方法(例如 ToList )使查询立即执行.另外，某些返回单个值的方法(例如 Count )会使查询立即执行.

The methods that start with To (such as ToList) cause the query to be executed immediately. Also some methods that return a single value such as Count cause the query to be executed immediately.

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