代码示例
int正常传值时不会像引用类型一样传地址，但是直接传SelectedIndex会出现类似传地址的效果，解决方案是使用一个中间变量。
SelectedIndexOld = SelectedIndex ;
SelectedIndex = -1;
SelectedIndexOld=-1
int s = SelectedIndex; SelectedIndex = -1; SelectedIndexOld = s;
SelectedIndexOld!=-1
SelectedIndexOld=-1
int s = SelectedIndex; SelectedIndex = -1; SelectedIndexOld = s;
SelectedIndexOld!=-1