在每个循环中删除一个对象 [英] Remove an object when in a for each loop

问题描述

我正在制作一个生存游戏，并试图删除一个对象，当它离开屏幕。这里是代码：

pre $ Public Sub tmrEnemyMove_Tick（sender As Object，e As EventArgs）Handle tmrEnemyMove.Tick
Dim koopaAnimation作为布尔

对于每个敌人作为敌人在lstEnemy
enemy.enemy.Left = enemy.enemy.Left - 20

如果enemy.enemy.Tag = koopa然后
enemy.enemy.Image = Image.FromFile（Application.StartupPath +\Graphics\koopa+ Trim（Str（koopaPosition））+.png）
如果koopaAnimation =假然后
如果koopaPosition = 0那么
koopaPosition = 1
否则
koopaPosition = 0
如果
结束如果
koopaAnimation = True
结束如果

如果picMario.Left< enemy.enemy.Left AndAlso enemy.enemy.Left< picMario.Right或picMario.Left< enemy.enemy.Right AndAlso enemy.enemy.Right< picMario.Right然后
如果picMario.Top< enemy.enemy.Top AndAlso enemy.enemy.Top< picMario.Bottom或picMario.Top< enemy.enemy.Bottom AndAlso enemy.enemy.Bottom< picMario.Bottom Then
'MsgBox（Collision）
End If
End If

If enemy.enemy.Left< 0 Then
lstEnemy.Remove（enemy）
Me.Controls.Remove（enemy.enemy）
End If
Next
End Sub

$ b

我得到的错误是：
在mscorlib.dll $ b $中发生未处理的类型为System.InvalidOperationException的异常b更多信息：收集已修改;枚举操作可能不会执行。

如果有人能帮上忙的话，谢谢。

在枚举过程中，不能从集合中删除对象。根本不能修改集合。这将导致一个错误（集合被修改;枚举操作可能不会执行）。但是，您可以将要删除/移除的对象添加到另一个集合中：



  Dim removeEnemies =新列表（敌人） 
对于每个敌人作为敌人在lstEnemy 
'...'
如果enemy.enemy.Left< 0然后
 removeEnemies.Add（enemy.enemy）
结束如果
下一个
 
对于每个敌人在移除敌人
 lstEnemy.Remove（敌人）
 Me.Controls.Remove（enemy.enemy）
下一个
  

这些方法会导致一个列表来改变它的版本（在枚举过程中被检查）：
$ b $ ul


添加

清除

插入

InsertRange

删除

RemoveRange

RemoveAt

反向

/ li>

另一个选项是使用 For-Loop 并向后循环：

  For I作为Int32 = lstEnemy.Count  -  1到0步骤-1 
 Dim enemy = lstEnemy（i）
'...'
如果enemy.enemy.Left< 0 Then 
 lstEnemy.Remove（enemy）
 Me.Controls.Remove（enemy.enemy）
 End If 
 Next 
  

这不会引起这个错误，但它不可读。你需要从 list.Count - 1 到 0 去，因为你想删除将会改变 Count 属性和在项目被移除之前可用的索引现在导致 ArgumentOutOfRangeException 。

最后但并非最不重要的，你可以使用 List.RemoveAll ：

  lstEnemy。 RemoveAll（函数（敌人）enemy.enemy.Left< 0）
  

I'm making a survival game and trying to remove an object when it goes off the screen. Here is the code:

Public Sub tmrEnemyMove_Tick(sender As Object, e As EventArgs) Handles tmrEnemyMove.Tick
    Dim koopaAnimation As Boolean

    For Each enemy As enemy In lstEnemy
        enemy.enemy.Left = enemy.enemy.Left - 20

        If enemy.enemy.Tag = "koopa" Then
            enemy.enemy.Image = Image.FromFile(Application.StartupPath + "\Graphics\koopa" + Trim(Str(koopaPosition)) + ".png")
            If koopaAnimation = False Then
                If koopaPosition = 0 Then
                    koopaPosition = 1
                Else
                    koopaPosition = 0
                End If
            End If
            koopaAnimation = True
        End If

        If picMario.Left < enemy.enemy.Left AndAlso enemy.enemy.Left < picMario.Right Or picMario.Left < enemy.enemy.Right AndAlso enemy.enemy.Right < picMario.Right Then
            If picMario.Top < enemy.enemy.Top AndAlso enemy.enemy.Top < picMario.Bottom Or picMario.Top < enemy.enemy.Bottom AndAlso enemy.enemy.Bottom < picMario.Bottom Then
                'MsgBox("Collision")
            End If
        End If

        If enemy.enemy.Left < 0 Then
            lstEnemy.Remove(enemy)
            Me.Controls.Remove(enemy.enemy)
        End If
    Next
End Sub

The error I get is: An unhandled exception of type 'System.InvalidOperationException' occurred in mscorlib.dll Additional information: Collection was modified; enumeration operation may not execute.

If anyone could help that would be great, thanks.

解决方案

You cannot delete an object from a collection during enumeration. You cannot modify the collection at all. That will cause an error(Collection was modified; enumeration operation may not execute). But you could add the objects that you want to delete/remove to another collection:

Dim removeEnemies = New List(Of enemy)
For Each enemy As enemy In lstEnemy
    ' ... '
    If enemy.enemy.Left < 0 Then
        removeEnemies.Add(enemy.enemy)
    End If
Next

For Each enemy In removeEnemies
    lstEnemy.Remove(enemy)
    Me.Controls.Remove(enemy.enemy)
Next

These methods will cause a list to change it's version(which is checked during enumeration):

• Add
• Clear
• Insert
• InsertRange
• Remove
• RemoveRange
• RemoveAt
• Reverse
• [the Indexer setter]
• Sort

Another option is to use a For-Loop and loop it backwards:

 For i As Int32 = lstEnemy.Count - 1 To 0 Step -1
    Dim enemy = lstEnemy(i)
    ' ... '
    If enemy.enemy.Left < 0 Then
        lstEnemy.Remove(enemy)
        Me.Controls.Remove(enemy.enemy)
    End If
Next

This will not raise that error but it's not as readable. You need to go from list.Count - 1 To 0 because you want to remove items which would change the Count property and an index that was available before the item was removed causes now an ArgumentOutOfRangeException.

Last but not least, you can use List.RemoveAll:

lstEnemy.RemoveAll(Function(enemy) enemy.enemy.Left < 0)

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