typedef struct tag_Frequency{
                  int   frequency;
                  float level;
               }Frequencytypedef struct tag_Frame{
                  char  *frame;   
                  int   size;
               }Frame;
typedef struct tag_CallbackParam{
                  int   ParamType;        /* Type of parameter */
                  union{
                     char        *string;
                     Frame       frame;
                     float       value;
                     Frequency   frequency;
                  }ParamVal;
            }  Callback_Param; 怎样把它定义为VB的类型啊?