Question

我一直在外地得到这个例外。我不明白如何在迭代时修改集合。我在方法的开头将所有内容复制到局部变量。

  public void Flush() {
            var tempEntities = attachedEntities.Select(item => item).ToList();
            attachedEntities.Clear();

            var tempEntitiesToDelete = entitiesToDelete.Select(item => item).ToList();
            entitiesToDelete.Clear();

            foreach (var attachedEntity in tempEntities) {
                var isTransient = (bool)GetPrivateField(attachedEntity.GetType(), attachedEntity, "isTransient");
                if (isTransient)
                    db.Insert(attachedEntity);
                else
                    db.Update(attachedEntity);
            }

            foreach (var entity in tempEntitiesToDelete)
                db.Delete(entity);
        }

堆栈跟踪

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
  at System.Collections.Generic.List`1 Enumerator[Compass.Mobile.Core.DataAccess.IEntity].VerifyState () [0x00000] in <filename unknown>:0 
  at System.Collections.Generic.List`1 Enumerator[Compass.Mobile.Core.DataAccess.IEntity].MoveNext () [0x00000] in <filename unknown>:0 
  at System.Linq.Enumerable <CreateSelectIterator>c__Iterator1D`2[Compass.Mobile.Core.DataAccess.IEntity,Compass.Mobile.Core.DataAccess.IEntity].MoveNext () [0x00000] in <filename unknown>:0 
  at System.Collections.Generic.List`1[Compass.Mobile.Core.DataAccess.IEntity].AddEnumerable (IEnumerable`1 enumerable) [0x00000] in <filename unknown>:0 
  at System.Collections.Generic.List`1[Compass.Mobile.Core.DataAccess.IEntity]..ctor (IEnumerable`1 collection) [0x00000] in <filename unknown>:0 
  at System.Linq.Enumerable.ToList[TSource] (IEnumerable`1 source) [0x00000] in <filename unknown>:0 
  at Compass.Mobile.Core.DataAccess.Session.Flush () [0x00000] in <filename unknown>:0 
  at Compass.Mobile.Core.DataAccess.Session.Commit () [0x00000] in <filename unknown>:0 
  at Compass.Mobile.Core.Bootstrap.CommandBus.Flush () [0x00000] in <filename unknown>:0

Answer 1

替换它：

foreach (var entity in tempEntitiesToDelete)
     db.Delete(entity);

使用：

for (var i = tempEntitiesToDelete.Count - 1; i >= 0; i--)
   db.Delete(tempEntitiesToDelete[i]);

当我尝试在循环播放时删除时出现此问题;它试图修改项目列表。因此，向后循环修复它对我来说。

Answer 2

根据堆栈跟踪判断，foreach（或枚举tempEntities这是一个普通列表）内部没有失败，但在ToList次调用之内Select迭代器的基础列表在继续下一个项目之前检查其状态。

堆栈跟踪中的这一行让我相信：

at System.Linq.Enumerable <CreateSelectIterator>c__Iterator1D`2[Compass.Mobile.Core.DataAccess.IEntity,Compass.Mobile.Core.DataAccess.IEntity].MoveNext () [0x00000] in <filename unknown>:0

您的foreach循环变量只是一个列表，因此它不会通过Select。这让我相信attachedEntities或entitiesToDelete在您Select之上发生变化时会发生变化：

/* Failing here... */
var tempEntities = attachedEntities.Select(item => item).ToList();
attachedEntities.Clear();

/* ...or here*/
var tempEntitiesToDelete = entitiesToDelete.Select(item => item).ToList();
entitiesToDelete.Clear();

/* ...but not here! */
foreach (var attachedEntity in tempEntities) {
   // ...
}

很可能是并发问题。

顺便说一下，你真的不需要Select (item => item)，它只是一个身份投射。

错误：收集已修改;枚举操作可能无法执行

2 个答案: