SDK编程。为什么SetScrollPos无法响SB_THUMBTRACK、SB_THUMBPOSITION？请各位朋友帮帮小弟我

SDK编程。为什么SetScrollPos无法响SB_THUMBTRACK、SB_THUMBPOSITION？请各位朋友帮帮我！
#include   <windows.h>

LRESULT   CALLBACK   WinSunProc(HWND   hwnd,UINT   uMsg,WPARAM   wParam,LPARAM   lParam);

int   WINAPI   WinMain(
    HINSTANCE   hInstance,            
    HINSTANCE   hPrevInstance,    
    LPSTR   lpCmdLine,                    
    int   nCmdShow                            
)
{
WNDCLASS   wndclass;
wndclass.cbClsExtra=0;
wndclass.cbWndExtra=0;
wndclass.hbrBackground=(HBRUSH)GetStockObject(SYSTEM_FONT);    
wndclass.hCursor=LoadCursor(NULL,IDC_ARROW);
wndclass.hIcon=NULL;
wndclass.hInstance=hInstance;
wndclass.lpfnWndProc=WinSunProc;
wndclass.lpszClassName= "Windows ";
wndclass.lpszMenuName=NULL;
wndclass.style=CS_HREDRAW   |   CS_VREDRAW;

RegisterClass(&wndclass);

HWND   hwnd;
hwnd=CreateWindow( "Windows ", "WindowsApp ",WS_OVERLAPPEDWINDOW,
0,0,CW_USEDEFAULT,CW_USEDEFAULT,NULL,NULL,hInstance,NULL);

ShowWindow(hwnd,SW_SHOWNORMAL);
UpdateWindow(hwnd);

MSG   msg;
while(GetMessage(&msg,NULL,0,0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return   0;
}
//
HWND   hScrlV,hScrlH;
HINSTANCE   hinstance;
int   iMax,iMin,iHPos,iVPos;
//
LRESULT   CALLBACK   WinSunProc(
    HWND   hwnd,  
    UINT   uMsg,  
    WPARAM   wParam,  
    LPARAM   lParam  
)
{
switch(uMsg)
{
case   WM_CREATE:
//建立水平滚动条
hScrlH=CreateWindow( "SCROLLBAR ",NULL,
WS_CHILD|WS_VISIBLE|SBS_HORZ,
70,40,200,20,hwnd,(HMENU)0,hinstance,0);
//建立垂直滚动条
hScrlV=CreateWindow( "SCROLLBAR ",NULL,
WS_CHILD|WS_VISIBLE|SBS_VERT,
20,10,20,150,hwnd,(HMENU)0,hinstance,0);
//设置大小
SetScrollRange(hScrlV,SB_CTL,0,100,TRUE);//设置垂直
SetScrollRange(hScrlH,SB_CTL,0,100,TRUE);//设置水平
//初始位置
SetScrollPos(hScrlV,SB_CTL,iVPos,TRUE);//设置垂直
SetScrollPos(hScrlH,SB_CTL,iHPos,TRUE);//设置水平
break;
case   WM_HSCROLL:
if(HIWORD(wParam)==0)   //如果是水平滚动条
{
switch(LOWORD(wParam))//标识着动作
{
case   SB_LINELEFT:
iHPos--;
break;
case   SB_LINERIGHT:
iHPos++;
break;
case   SB_PAGELEFT:
iHPos-=10;
break;
case   SB_PAGERIGHT:
iHPos+=10;
break;
case   SB_THUMBTRACK:
iHPos=HIWORD(wParam);
break;
case   SB_THUMBPOSITION:
iHPos=HIWORD(wParam);
break;
default:   break;
}
SetScrollPos(hScrlH,SB_CTL,iHPos,TRUE);
}
break;
case   WM_VSCROLL:
if(HIWORD(wParam)==0)//设为同一类的
{
switch(LOWORD(wParam))
{
case   SB_LINEUP:
iVPos--;
break;
case   SB_LINEDOWN:
iVPos++;
break;
case   SB_PAGEUP:
iVPos-=5;
break;
case   SB_PAGEDOWN:
iVPos+=5;
break;
default:   break;
}
SetScrollPos(hScrlV,SB_CTL,iVPos,TRUE);