常见的LINQ /标准查询操作错误/错误的步骤？ [英] Common Linq / Standard Query Operator mistakes/mis-steps?

问题描述

对于程序员不来自函数式编程背景，是否有一个失误，以避免？

For programmers that do not come from a functional programming background, are there an mistakes to avoid?

推荐答案

人们往往会犯的最大错误是误解了懒惰和评价规则LINQ查询：

The biggest mistake people tend to make is to misunderstand the laziness and evaluation rules for a LINQ query:

查询是懒惰：他们不执行，直到你遍历他们：

Queries are lazy: they are not executed until you iterate over them:

// This does nothing! No query executed!
var matches = results.Where(i => i.Foo == 42);

// Iterating them will actually do the query.
foreach (var match in matches) { ... }

此外，结果不被缓存。他们在每次迭代他们的时间计算：

Also, results are not cached. They are computed each time you iterate over them:

var matches = results.Where(i => i.ExpensiveOperation() == true);

// This will perform ExpensiveOperation on each element.
foreach (var match in matches) { ... }

// This will perform ExpensiveOperation on each element again!
foreach (var match in matches) { ... }

底线：什么时候你的查询被执行

Bottom line: know when your queries get executed.

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