vb.net 在 for each 循环中删除对象

Remove an object when in a for each loop

提问by Tyler H

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

我正在制作一个生存游戏并试图在它离开屏幕时移除一个物体。这是代码：

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.

我得到的错误是： mscorlib.dll 中发生类型为“System.InvalidOperationException”的未处理异常附加信息：集合已修改；枚举操作可能无法执行。

If anyone could help that would be great, thanks.

如果有人可以提供帮助，那就太好了，谢谢。

回答by Tim Schmelter

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:

您不能在枚举期间从集合中删除对象。您根本无法修改集合。这将导致错误（集合已修改；枚举操作可能无法执行）。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 = 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

All of the following methods will cause a list to change its version (which is checked during enumeration):

以下所有方法都会导致列表更改其版本（在枚举期间检查）：

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

• 添加
• 清除
• 插入
• 插入范围
• 消除
• 删除范围
• 删除在
• 逆转
• [索引器设置器]
• 种类

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

另一种选择是使用 aFor-Loop并将其向后循环：

 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 - 1To 0because you want to remove items which would change the Countproperty and an index that was available before the item was removed causes now an ArgumentOutOfRangeException.

这不会引发该错误，但它的可读性较差。您需要从list.Count - 1To 开始，0因为您想要删除会更改Count属性的项目，并且在删除项目之前可用的索引现在会导致ArgumentOutOfRangeException.

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

最后但并非最不重要的是，您可以使用 List.RemoveAll：

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

回答by Adrian

.NET really doesn't like it when you change a collection when you're in the middle of enumerating its contents. You might try chAnging your foreachloop to a forloop if you're planning on removing elements from the collection like this.

当您在枚举其内容的过程中更改集合时，.NET 真的不喜欢它。如果您打算像这样从集合中删除元素，则可以尝试将foreach循环更改为for循环。

回答by Fernando Mota

One example using entity framework (ElementAt(i)):

使用实体框架 (ElementAt(i)) 的一个示例：

for (int i = 0; i < db.Itens.Count(); i++)
{
    Item item = db.Itens.ElementAt(i);
    if (item.Id == 0) // put a condition
    {
        db.Itens.Remove(item);
        i--;
    }
}