帮助F#:"收集被修改"

Question

我对F#很新,我遇到了F#中的"Collection was modified"问题.我知道当我们在同时修改(添加/删除)它时迭代Collection时,这个问题很常见.stackoverflow中的先前线程也指向此.

但就我而言,我正在开发两套不同的产品:我有两个系列:

• originalCollection原始集合,我想从中删除东西
• colToRemove包含我要删除的对象的集合

以下是代码:

   Seq.iter ( fun input -> ignore <| originalCollection.Remove(input)) colToRemove

我收到以下运行时错误:+ $ exception {System.InvalidOperationException:Collection已被修改; 枚举操作可能无法执行. System.ChrowHelper.ThrowInvalidOperationException(ExceptionResource资源)位于System.Collections.Generic.List 1.Enumerator.MoveNextRare() at System.Collections.Generic.List1.Enumerator.MoveNext()at Microsoft.FSharp.Collections.IEnumerator.next@174 [T](FSharpFunc 2 f, IEnumerator1 e,FSharpRef 1 started, Unit unitVar0) at Microsoft.FSharp.Collections.IEnumerator.filter@169.System-Collections-IEnumerator-MoveNext() at Microsoft.FSharp.Collections.SeqModule.Iterate[T](FSharpFunc2 action,IEnumerable `1来源)

这是代码块:

        match newCollection with
        | Some(newCollection) ->

            // compare newCollection to originalCollection.
            // If there are things that exist in the originalCollection that are not in the newCollection, we want to remove them
            let colToRemove = Seq.filter (fun input -> Seq.exists (fun i -> i.id = input.id) newCollection) originalCollection
            Seq.iter ( fun input -> ignore <| originalCollection.Remove(input)) colToRemove

        | None -> ()

谢谢!

注意:在此处处理单线程环境,因此不存在可能导致此异常的多线程问题.

Answer 1

这里的问题是,colToRemove它不是一个独立的集合,而是一个集合的投影originalCollection.因此,更改会originalCollection更改迭代期间不允许的投影.上述代码的C#等价物如下

var colToRemove = originalCollection
  .Where(input -> newCollection.Any(i -> i.id == input.id));
foreach (var in input in colToRemove) {
  originalCollection.Remove(input);
}

您可以colToRemove通过该List.ofSeq方法进行独立收集来解决此问题.

 let colToRemove = 
   originalCollection
   |> Seq.filter (fun input -> Seq.exists (fun i -> i.id = input.id) newCollection) originalCollection
   |> List.ofSeq