我在类下新建一个线程，但是在这个线程下不能执行PictureBox1.Refresh() 总是报错 ，build没有错，是运行时出错
请问是怎么回事啊
Public Class Form1
Dim th As Thread
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

       ' CameraSTART()
        th = New Thread(AddressOf ddd)
        th.Start()
    End Sub
    Public Sub ddd()
        PictureBox1.Refresh()
          PictureBox1.CreateGraphics.DrawLine(Pens.Blue, 10, 400 , 20, 50)
    End Sub

End Class     

是不是要用委托 ，可是怎么用呢