问题描述
返回 IQueryable 与返回 IEnumerable 之间有什么区别，什么时候应该优先于另一个?
What is the difference between returning IQueryable<T> vs. IEnumerable<T>, when should one be preferred over the other?
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;
两者都将被推迟执行，何时应该优先于另一个?
Will both be deferred execution and when should one be preferred over the other?
推荐答案
是的，两者都会给你 延迟执行.
Yes, both will give you deferred execution.
区别在于IQueryable 是允许 LINQ-to-SQL(实际上是 LINQ.-to-anything)工作的接口.因此，如果您进一步优化对 IQueryable，如果可能，该查询将在数据库中执行.
The difference is that IQueryable<T> is the interface that allows LINQ-to-SQL (LINQ.-to-anything really) to work. So if you further refine your query on an IQueryable<T>, that query will be executed in the database, if possible.
对于 IEnumerable 情况下，它将是 LINQ-to-object，这意味着所有与原始查询匹配的对象都必须从数据库加载到内存中.
For the IEnumerable<T> case, it will be LINQ-to-object, meaning that all objects matching the original query will have to be loaded into memory from the database.
在代码中:
IQueryable<Customer> custs = ...;
// Later on...
var goldCustomers = custs.Where(c => c.IsGold);
该代码将执行 SQL 以仅选择黄金客户.另一方面，以下代码将执行数据库中的原始查询，然后过滤掉内存中的非金牌客户:
That code will execute SQL to only select gold customers. The following code, on the other hand, will execute the original query in the database, then filtering out the non-gold customers in the memory:
IEnumerable<Customer> custs = ...;
// Later on...
var goldCustomers = custs.Where(c => c.IsGold);
这是一个非常重要的区别，正在处理 IQueryable<T> 在很多情况下可以避免从数据库中返回太多行.另一个主要示例是进行分页:如果您使用 Take 和 跳过 href="https://msdn.microsoft.com/en-us/library/system.linq.iqueryable.aspx" rel="noreferrer">IQueryable，你只会得到请求的行数；在 IEnumerable 上执行此操作将导致您的所有行都加载到内存中.
This is quite an important difference, and working on IQueryable<T> can in many cases save you from returning too many rows from the database. Another prime example is doing paging: If you use Take and Skip on IQueryable, you will only get the number of rows requested; doing that on an IEnumerable<T> will cause all of your rows to be loaded in memory.
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