如何改变groupbox的边框线颜色？

imports system.componentmodel

public class groupboxcontrol
    inherits panel

    private lbl as label
    private mbordercolor as color = color.blue

    <browsable（true)> _
        public overrides property text（) as string
        get
            return lbl.text
        end get
        set（byval value as string)
            lbl.text = value
            me.invalidate（)
        end set
    end property

    public property bordercolor（) as color
        get
            return mbordercolor
        end get
        set（byval value as color)
            mbordercolor = value
            me.invalidate（)
        end set
    end property

    private sub groupboxcontrol_controladded（byval sender as object, byval e as system.windows.forms.controleventargs) handles me.controladded
        if e.control.left < 5 then e.control.left = 5
        if e.control.top < 15 then e.control.top = 15
        if e.control.right > me.width - 10 then e.control.left = me.width - 10 - e.control.width
        if e.control.bottom > me.height - 10 then e.control.top = me.height - 10 - e.control.height
    end sub

    private sub groupboxcontrol_paint（byval sender as object, byval e as system.windows.forms.painteventargs) handles me.paint
        e.graphics.drawrectangle（new pen（mbordercolor), new rectangle（2, 8, me.width - 5, me.height - 12))
    end sub

    public sub new（)
        lbl = new label
        lbl.autosize = true
        lbl.text = "groupbox"
        me.controls.add（lbl)
    end sub

    private sub groupboxcontrol_resize（byval sender as object, byval e as system.eventargs) handles me.resize
        lbl.location = new point（10, 1)
    end sub

end class

public class form1

    dim withevents testgroupbox as new groupbox

    private sub form1_load（byval sender as system.object, byval e as system.eventargs) handles mybase.load
        testgroupbox.text = "testgroupbox1"
        testgroupbox.visible = true
        me.controls.add（testgroupbox)
    end sub

    private sub testgroupbox_paint（byval sender as object, byval e as system.windows.forms.painteventargs) handles testgroupbox.paint
        e.graphics.clear（testgroupbox.backcolor)
        e.graphics.drawstring（testgroupbox.text, testgroupbox.font, brushes.red, 10, 1)
        e.graphics.drawline（pens.blue, 1, 7, 8, 7)
        e.graphics.drawline（pens.blue, 90, 7, testgroupbox.width - 2, 7)
        e.graphics.drawline（pens.blue, 1, 7, 1, testgroupbox.height - 2)
        e.graphics.drawline（pens.blue, 1, testgroupbox.height - 2, testgroupbox.width - 2, testgroupbox.height - 2)
        e.graphics.drawline（pens.blue, testgroupbox.width - 2, 7, testgroupbox.width - 2, testgroupbox.height - 2)
    end sub

end class