C#下标问题,求解惑,下标是非负数,我赋值-1不会报错

C#下标索引问题,求解惑
为什么看代码注释

//下一首
 private void btnnext_Click(object sender, EventArgs e)
        {
            count = 0;

            SoundPlayer spPlay = new SoundPlayer();
            //list.count 总共是23首歌曲
            if (listBox1.SelectedIndex == list.Count - 1)//23//这里时下一首个,最后一首歌的时候,把索引赋值为-1
            {
                listBox1.SelectedIndex = -1;//为什么这下标赋值 -1 不会报错?下标不是非负数吗?
            }
            try
            {
                spPlay.SoundLocation = list[listBox1.SelectedIndex + 1];
                string name = Path.GetFileName(list[listBox1.SelectedIndex + 1]);
                label2.Text = name;
                spPlay.Play();
                count++;
                listBox1.SelectedIndex = listBox1.SelectedIndex + 1;
            }
            catch
            {

            }
        }
        /// <summary>
        /// 播放上一首
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btnLast_Click(object sender, EventArgs e)
        {
            count = 0;
            //这里时上一首,我之前赋值listBox1.SelectedIndex == list.count;  会报错,原因超出下标.list.count总共是23个
            //超过索引会 报错,为什么-1不报错呢?
            SoundPlayer spPlay = new SoundPlayer();
            if (listBox1.SelectedIndex == 0)   
            {
                spPlay.SoundLocation = list[listBox1.SelectedIndex + list.Count - 1];
                string name = Path.GetFileName(list[listBox1.SelectedIndex + list.Count - 1]);
                label2.Text = name;
                spPlay.Play();
                count++;
                listBox1.SelectedIndex = list.Count - 1;
            }
            else
            {
                try
                {
                    spPlay.SoundLocation = list[listBox1.SelectedIndex - 1];
                    string name = Path.GetFileName(list[listBox1.SelectedIndex - 1]);
                    label2.Text = name;
                    spPlay.Play();
                    count++;
                    listBox1.SelectedIndex = listBox1.SelectedIndex - 1;
                }
                catch
                {

                }

            }

        }

listBox1.SelectedIndex是listBox控件封装后的暴露出来的属性，赋值为-1是控件定义成无选择。
list[n]的n是list的下标，list的下标从0 开始的，没有负值

下标设置为-1的时候就表示没有选择任何项，同理返回-1也表示没有选择任何项