权限无效,无法更改 Pen,为什么?

[luciferx]

一下是部分函数:
private void draw_function(int line)
{
int i,in_pen;
Pen pen;
if (line >= 0)//如不是需要加粗的线条
{
in_pen = colour[line];
pen = pens[in_pen];
}
else//如是需要加粗的线条
{
line = -1 - line;
in_pen = colour[line];
pen = pens[in_pen];
pen.Width = 3;//错误的地方！！！！！！！权限无效,无法更改 Pen,权限无效,无法更改 Pen
}
PointF[] point = new PointF[Maxlong];
interval_x = offset_x / 600;
interval_y = offset_y / 400;
change_point(ref point[0], x[line, 0], y[line, 0]);
try
{
for (i = 1; i <= limit[line]; i++)
{
change_point(ref point[i], x[line, i], y[line, i]);
g.DrawLine(pen, point[i - 1], point[i]);
}
}
catch (Exception){ }
pictureBox.Image = map;

}
为什么啊？？？？？

[dqw120]

等待高手解答..