c# - 为什么 ControlCollection 不抛出 InvalidOperationException？

Question

跟随这个问题Foreach loop for disposing controls skipping iterations让我烦恼的是允许对不断变化的集合进行迭代:

例如，以下内容:

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);
}

抛出

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

但是在 .Net 表单中，您可以:

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();
}

它会跳过元素，因为迭代器会遍历不断变化的集合，不会抛出异常

错误？如果底层集合发生变化，迭代器不需要抛出 InvalidOperationException 吗？

所以我的问题是为什么对不断变化的 ControlCollection 进行迭代不会抛出 InvalidOperationException？

附录:

documentation for IEnumerator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.

Answer 1

这个问题的答案可以在 the Reference Source for ControlCollectionEnumerator 中找到。

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];
            }
        }
    }
}

请特别注意 MoveNext() 中的注释，它明确解决了这个问题。

IMO 这是一个被误导的“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(元素被默默地跳过，如 OP 所述)。