C#线程中使用directdraw出现错误

下面的代码是我在线程中调用的，编译可以通过，运行时出现COM强制转换错误
但如果把load_map只做为一个函数在主程序中调用，就可以顺利进行。


public struct map_value
 {
      public int xx;
      public int yy；
  }
      
  public struct map_image
   {
       public DirectDrawSurface7 map;         
       public int x;
       public int y;
       public int x_num;
       public int y_num;
       public int sign;
     }
  public map_image[,] image_buff = new map_image[400,400];

// load_map 在线程中启动
  public void load_map(object vmap)
        {
            string str;
            Bitmap bm;
            IntPtr hbitmap;
            IntPtr hDCSrc;
            IntPtr hDCDst;
            map_value pmap = (map_value)vmap;

            str = "d:\\map\\" + level_buff[num_level].level_num.ToString() + "\\" + pmap.xx.ToString() + "," + pmap.yy.ToString() + ".jpg ";
            image_buff[pmap.xx, pmap.yy].map = DirectDraw.CreateSurface(ref Surface3);
/*    运行时的错误提示是：
未处理的“System.InvalidCastException”类型的异常出现在 map.exe 中。

其他信息: 无法将类型为“System.__ComObject”的 COM 对象强制转换为接口类型“DxVBLib.DirectDraw7”。此操作失败的原因是对 IID 为“{9F76FDE7-8E92-11D1-8808-00C04FC2C602}”的接口的 COM 组件调用 QueryInterface 因以下错误而失败: 不支持此接口 (异常来自 HRESULT:0x80004002 (E_NOINTERFACE))。
*/

            bm = new Bitmap(str);
            hbitmap = bm.GetHbitmap();
            hDCSrc = CreateCompatibleDC((IntPtr)null);
            SelectObject(hDCSrc, hbitmap);
            hDCDst = (IntPtr)image_buff[pmap.xx, pmap.yy].map.GetDC();
            BitBlt(hDCDst, 0, 0, map_w, map_h, hDCSrc, 0, 0, SRCCOPY);
            image_buff[pmap.xx, pmap.yy].map.ReleaseDC((int)hDCDst);
            DeleteDC(hDCSrc);
            DeleteDC(hDCDst);
            DeleteObject(hbitmap);
            bm.Dispose();
  
       }

//主程序中创建线程的部分
for (xx = least_x; xx - j_x < w + level_buff[num_level].least_num_x; xx++)
            {
                for (yy = least_y; yy - j_y < h + level_buff[num_level].least_num_y; yy++)
                {
                 
                    vmap.xx = xx;
                    vmap.yy = yy;
                    Thread thr = new Thread(new ParameterizedThreadStart(load_map));
                    thr.Start((object)vmap);

                //不启动线程只做为函数调用可以顺利执行
                    //load_map((object)vmap);
                     if (sign == 0)
                    {
                        image_buff[xx, yy].x = (xx - level_buff[num_level].least_num_x - j_x) * 256;
                        image_buff[xx, yy].y = (yy - level_buff[num_level].least_num_y - j_y) * 256;
                    }
                    else
                    {
                        image_buff[xx, yy].x = (xx - level_buff[num_level].least_num_x - j_x) * 256 + image_buff[least_x, least_y].x;
                        image_buff[xx, yy].y = (yy - level_buff[num_level].least_num_y - j_y) * 256 + image_buff[least_x, least_y].y;

                       
                    }
                    image_buff[xx, yy].x_num = xx;
                    image_buff[xx, yy].y_num = yy;

              }
    }