为什么listbox在删除选中项的时候SelectedIndex会变成-1？

为什么listbox在选中一项的时候SelectedIndex是正确的，但是删除的时候就自己变成了-1？

-1 说明没有选择任何一项

       //
        // 摘要:
        //     Gets or sets the zero-based index of the currently selected item in a System.Windows.Forms.ListBox.
        //
        // 返回结果:
        //     A zero-based index of the currently selected item. A value of negative one (-1)
        //     is returned if no item is selected.
        //
        // 异常:
        //   T:System.ArgumentOutOfRangeException:
        //     The assigned value is less than -1 or greater than or equal to the item count.
        //
        //   T:System.ArgumentException:
        //     The System.Windows.Forms.ListBox.SelectionMode property is set to None.