"SelectedIndexChanged"；在"Items.Clear()"之后不触发在列表框中 [英] "SelectedIndexChanged" not firing after "Items.Clear()" in ListBox

问题描述

对于一个列表框(选择"模式设置为一个")，我希望跟踪是否有选定项或没有选定项.为此，我为SelectedIndexChanged预订了一个方法，并检查SelectedIndex是否为-1.但是，我注意到，即使SelectedIndex更改为-1(如果尚未为-1)，该事件在调用Items.Clear()之后也不会触发.

For a ListBox (With Selection mode set to One), I wish to track whether there's a selected item or none selected. To do so, I subscribed a method to SelectedIndexChanged and checked if the SelectedIndex is -1 or not. However, I noticed that the event doesn't fire after calling Items.Clear(), even though SelectedIndex changes to -1 (if it wasn't already -1).

为什么不开火? 我知道我可以通过在清除列表之前将-1分配给SelectedIndex来解决此问题.但是有更好的方法吗?

Why doesn't it fire? I know I can work around this by assigning -1 to SelectedIndex before clearing the list. But is there a better way?

这是复制此代码的简单代码:

Here's a simple code to replicate this:

using System;
using System.Windows.Forms;

namespace ns
{
    class Program
    {
        static ListBox lst = new ListBox();

        public static void Main()
        {
            lst.SelectedIndexChanged += new EventHandler(lst_SelectedIndexChanged);

            lst.Items.Add(1);

            Console.WriteLine("Setting selected index to 0...");
            lst.SelectedIndex = 0; //event fire here

            Console.WriteLine("(Selected Index == {0})", lst.SelectedIndex);

            Console.WriteLine("Clearing  all items...");
            lst.Items.Clear(); //event *should* fire here?!

            //proof that the selected index has changed
            Console.WriteLine("(Selected Index == {0})", lst.SelectedIndex);
        }

        static void lst_SelectedIndexChanged(object sender, EventArgs e)
        {
            Console.WriteLine("[!] Selected Index Changed:{0}", lst.SelectedIndex);
        }
    }
}

修改: 我正在考虑通过制作从ListBox继承的类或通过制作用户控件来制作自定义列表.但是我不确定该如何处理. 关于使用继承/userControl隐藏/覆盖clear方法的任何想法吗? 是否还需要隐藏/覆盖其他方法，还是有办法避免这种情况?

I am considering making a custom list by making a class that inherits from ListBox, or by making a user control. However I'm not sure how to approach this. Any ideas on hiding/overriding the clear method using either inheritance/userControl? Would it require hiding/overriding other methods as well or is there a way to avoid this?

推荐答案

查看Reflector中的代码，Items上的Clear()方法仅重置.Net对象的内部对象列表(并且不会像您注意到的那样触发OnSelectedIndexChanged).

Looking at the code in Reflector, the Clear() method on Items just resets the .Net object's internal object list (and does not, as you noticed, fire OnSelectedIndexChanged).

SelectedIndex属性返回-1，因为该属性的getter中的逻辑指示如果内部列表中没有任何项，则应返回-1.

The SelectedIndex property returns -1 because the logic in the property's getter dictates that -1 should be returned if there are no items in the internal list.

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