LINQ& amp;IEnumerable& lt; String& gt;重新评估 [英] LINQ & IEnumerable<String> Re-evaluation
问题描述
早上好
我编写了以下LINQ查询
I wrote the following LINQ query
public static Func<String, IEnumerable<String>> ListaCorreccoes = (StrPalavra) =>
{
return (from StrTmp in ListaPossiveisCorreccoes(StrPalavra)
from StrTmp2 in ListaPossiveisCorreccoes(StrTmp)
where PalavraConhecida(StrTmp2)
select StrTmp2).Distinct();
};
令我惊讶的是，它比使用两个 foreach cicle快得多.使用 ListaTmp = ListaCorreccoes("comutador"); 运行此函数后，其中 ListaTmp 的类型为 IEnumerable< String> ，我需要打印 ListaTmp 的基数使用
which, to my surprise, is much faster than using two foreach cicles. After running this function using ListaTmp = ListaCorreccoes("comutador");, where ListaTmp is of type IEnumerable<String>, I need to print ListaTmp's cardinality using
Console.Write(">> Prima qualquer tecla para listar todas as " + ListaTmp.Count() + " correcções...");
并使用
foreach (String StrTmp in ListaTmp)
Console.WriteLine(StrTmp);
但是，代码的最后一行和倒数第二行都导致ListaTmp，因此查询被重新评估 ，这非常奇怪，因为该变量被赋值为查询.为什么这段代码会有如此奇怪的行为?
However, both the last and prior-to-last lines of code cause ListaTmp and thus the query to be re-evaluated, which is so very strange since the variable is being assigned the result of the query. Why has this code such a strange behaviour?
非常感谢您.
推荐答案
这是因为LINQ使用延迟执行.请参阅 Charlie Calvert的有关LINQ和Deferred执行的文章.
That's because LINQ uses deferred execution. See Charlie Calvert's article on LINQ and Deferred Execution.
尝试以下方法:
var ListaTmp = ListaCorreccoes("comutador").ToList();
这一次枚举并将结果存储在 List< string> 中.
This enumerates once and stores the result in a List<string>.
您可能会发现Jon Skeet的文章很有用:
You might find Jon Skeet's articles useful:
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