关于C#：为什么ControlCollection不引发InvalidOperationException？

Why does ControlCollection NOT throw InvalidOperationException?

在处理控件跳过迭代的foreach循环出现此问题之后，它向我发出了一个错误，即允许在更改的集合上进行迭代：

例如，以下内容：

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List<Control> items = new List<Control>
{
new TextBox {Text ="A", Top = 10},
new TextBox {Text ="B", Top = 20},
new TextBox {Text ="C", Top = 30},
new TextBox {Text ="D", Top = 40},
};

foreach (var item in items)
{
items.Remove(item);
}

投掷

InvalidOperationException: Collection was modified; enumeration operation may not execute.

但是，在.NET表单中，您可以执行以下操作：

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this.Controls.Add(new TextBox {Text ="A", Top = 10});
this.Controls.Add(new TextBox {Text ="B", Top = 30});
this.Controls.Add(new TextBox {Text ="C", Top = 50});
this.Controls.Add(new TextBox {Text ="D", Top = 70});

foreach (Control control in this.Controls)
{
control.Dispose();
}

它跳过元素，因为迭代器在不断变化的集合上运行，而不引发异常

缺陷？如果底层集合更改，那么是否需要迭代器来抛出InvalidOperationException？

所以我的问题是，为什么在一个不断变化的ControlCollection上的迭代不抛出invalidOperationException？

附录：

IEnumerator的文件中说：

The enumerator does not have exclusive access to the collection; therefore, enumerating through a collection is intrinsically not a thread-safe procedure. Even when a collection is synchronized, other threads can still modify the collection, which causes the enumerator to throw an exception.

相关讨论

答案可以在EDOCX1的参考源中找到。

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private class ControlCollectionEnumerator : IEnumerator {
private ControlCollection controls;
private int current;
private int originalCount;

public ControlCollectionEnumerator(ControlCollection controls) {
this.controls = controls;
this.originalCount = controls.Count;
current = -1;
}

public bool MoveNext() {
// VSWhidbey 448276
// We have to use Controls.Count here because someone could have deleted
// an item from the array.
//
// this can happen if someone does:
// foreach (Control c in Controls) { c.Dispose(); }
//
// We also dont want to iterate past the original size of the collection
//
// this can happen if someone does
// foreach (Control c in Controls) { c.Controls.Add(new Label()); }

if (current < controls.Count - 1 && current < originalCount - 1) {
current++;
return true;
}
else {
return false;
}
}

public void Reset() {
current = -1;
}

public object Current {
get {
if (current == -1) {
return null;
}
else {
return controls[current];
}
}
}
}

特别注意MoveNext()中明确提出这一点的评论。

在我看来，这是一个错误的"修复"，因为它通过引入一个微妙的错误来掩盖一个明显的错误(如操作说明，元素被悄悄地跳过)。