dataGridView 的 AllowUserToAddRows 问题

private void dataGridView1_KeyDown(object sender, KeyEventArgs e)
        {
            if(e.KeyCode==Keys.Delete)
            {
                DialogResult dr = MessageBox.Show("确定要删除本行吗", "提示");
                int selectindex=dataGridView1.CurrentRow.Index;
                int autoindex=dataGridView1.RowCount;
                int boolindex=autoindex-selectindex;
                if(dr==DialogResult.OK)
                {
                    switch(boolindex)
                    {
                        case 1:
                            dataGridView1.AllowUserToAddRows = false;
                            break;
                        default :
                            MessageBox.Show(boolindex.ToString());
                            break;
                    }
                }
            }
        }

本人这样写以后满足 条件的时候 AllowUserToAddRows  为false ,但是主界面还是有多出的一行,但是假如本人不做判断直接在if以后把他的属性改为FALSE,再主界面就没有多处的一行,这是什么原因呢