LINQ & IEnumerable<String> Re-evaluation

问题

Good morning,

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();            

    };

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...");

and to print the contents of ListaTmp, using

foreach (String StrTmp in ListaTmp)
            Console.WriteLine(StrTmp);

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?

Thank you very much.

回答1:

That's because LINQ uses deferred execution. See Charlie Calvert's article on LINQ and Deferred Execution.

Try this instead:

var ListaTmp = ListaCorreccoes("comutador").ToList();

This enumerates once and stores the result in a List<string>.

You might find Jon Skeet's articles useful:

• Iterators, iterator blocks and data pipelines
• Iterator block implementation details: auto-generated state machines

回答2:

This is because ListaTmp is not a result set, but a query. Specifically for the Count, you're actually creating a new query (you expend the Linq expression) which you then execute.

The real type of ListaTmp is not IEnumerable, but IQueryable, which is a Linq query.

来源：https://stackoverflow.com/questions/3976113/linq-ienumerablestring-re-evaluation