Question

好的，让我们从这个非常简单的按钮点击方法开始

    private void button1_Click(object sender, EventArgs e)
    {
        int counter = 1;
        List<int> items = new int[] { 1, 2, 3 }.ToList();
        List<int>.Enumerator enm = items.GetEnumerator();

        // 1
        if (!enm.MoveNext())
            throw new Exception("Unexpected end of list");
        if (enm.Current != counter)
            throw new Exception(String.Format("Expect {0} but actual {1}", counter, enm.Current));
        counter++;

        // 2
        if (!enm.MoveNext()) 
            throw new Exception("Unexpected end of list");
        if (enm.Current != counter) 
            throw new Exception(String.Format("Expect {0} but actual {1}", counter, enm.Current));
        counter++;

        //3
        if (!enm.MoveNext())
            throw new Exception("Unexpected end of list");
        if (enm.Current != counter)
            throw new Exception(String.Format("Expect {0} but actual {1}", counter, enm.Current));
        counter++;

        if (enm.MoveNext()) 
            throw new Exception("Unexpected continuation of list");
    }

这个方法什么也不做，因为每个断言都优雅地传递。事情很好，直到我相信我应该引入一种方法来消除冗余

    static void AssertNext(ref int counter, List<int>.Enumerator e)
    {
        if (!e.MoveNext()) 
            throw new Exception("Unexpected end of list");
        if (e.Current != counter) 
            throw new Exception(String.Format("Expect {0} but actual {1}", counter, e.Current));
        counter++;
    }

    private void button2_Click(object sender, EventArgs e)
    {
        var counter = 1;
        var items = new int[] { 1, 2, 3 }.ToList();
        var enm = items.GetEnumerator();
        AssertNext(ref counter, enm);
        AssertNext(ref counter, enm);
        AssertNext(ref counter, enm);
        if (enm.MoveNext()) throw new Exception("Unexpected continuation of list");
    }

尽管如此，这种重构很简单（至少对我来说）。它打破了程序！在第二次调用AssertNext时，似乎枚举器已经重置为起始点并导致断言失败。

我无法理解发生了什么。我真的觉得这个谜题初学者。

我想念的是什么？

Answer 1

我想象它与List.Enumerator是一个结构有关。你将它传递给一个方法，操纵它，然后返回。原始实例可能不会发生操作。

Answer 2

List<T>.Enumerator是一个值类型，意味着它被复制到方法的本地范围，被更改，然后在离开方法时被销毁。尝试通过引用传递它。

枚举器作为参数

2 个答案: