Child window combobox behaviour

//#include "stdafx.h"
#include "TestCombo.h"
#include "resource.h"
#include <string.h>
#include <stdio.h>
#include <tchar.h>
#include <windows.h>

#define MAX_LOADSTRING 100
#define WM_TAB (WM_USER) 
#define WM_ESC (WM_USER + 1) 
#define WM_ENTER (WM_USER + 2) 

WNDCLASSEX  wcex;
WNDPROC     lpfnParProc;
HACCEL      hAccelTable;
TCHAR       szClass[MAX_LOADSTRING];
TCHAR       szTitle[MAX_LOADSTRING];       
TCHAR       szWindowClass[MAX_LOADSTRING]; 
MSG         msg;  
HWND        hMain;
HWND        hListtitel;
HWND        hEdittitel; 
HDC         hdc;
HINSTANCE   hInst;
RECT        rt;
PAINTSTRUCT ps;
POINT       pt; 
int         wmId;
int         wmEvent;
int         lmId;
int         lmEvent;

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
LRESULT CALLBACK ChldProc(HWND, UINT, WPARAM, LPARAM);

int APIENTRY _tWinMain (HINSTANCE hInstance,
                        HINSTANCE hPrevInstance,
                        LPTSTR    lpCmdLine,
                        int       nCmdShow)
{
	hInst = hInstance;
  hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_TESTCOMBO));
  LoadString(hInstance, IDC_TESTCOMBO, szWindowClass, MAX_LOADSTRING);
  wcex.cbSize        = sizeof(WNDCLASSEX);
  wcex.style         = CS_HREDRAW | CS_VREDRAW;
  wcex.lpfnWndProc   = WndProc;
  wcex.cbClsExtra    = 0;
  wcex.cbWndExtra    = 0;
  wcex.hInstance     = hInstance;
  wcex.hIcon         = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_TESTCOMBO));
  wcex.hCursor       = LoadCursor(NULL, IDC_ARROW);
  wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
  wcex.lpszMenuName  = MAKEINTRESOURCE(IDC_TESTCOMBO);
  wcex.lpszClassName = szWindowClass;
  wcex.hIconSm       = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));
  if (!RegisterClassEx(&wcex))
  { return FALSE; }
  
  hMain  = CreateWindow (szWindowClass,"Test Combobox",
               WS_TILED | WS_CAPTION | WS_MINIMIZEBOX | WS_SYSMENU,
               100,100,905,667,NULL,NULL,hInstance,NULL);  
               
  hListtitel = CreateWindow("COMBOBOX","",
               WS_CHILD | WS_VISIBLE | CBS_DROPDOWN | WS_VSCROLL | CBS_HASSTRINGS, 
               200, 170, 540, 400,hMain,NULL,hInst,NULL);  
               
  hEdittitel = FindWindowEx(hListtitel,NULL,"EDIT",NULL);
  lpfnParProc = (WNDPROC) SetWindowLong(hEdittitel,GWL_WNDPROC, (DWORD) ChldProc);
  
  InvalidateRect(hMain,NULL,TRUE); 
  ShowWindow(hMain, SW_SHOWNORMAL);
  UpdateWindow(hMain);
  SetFocus(hListtitel);
  
  while (GetMessage(&msg, NULL, 0, 0))
  {
    if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
    {
      TranslateMessage(&msg);
      DispatchMessage(&msg); 
    }
  }
  return (int) msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
  switch (message)
  {
    case WM_PAINT:
      hdc = BeginPaint(hwnd, &ps);
      EndPaint(hwnd, &ps);
      break;
    case WM_DESTROY:
      PostQuitMessage(0);
      break;        
    case WM_TAB: 
                                       // Do something
      break; 
    case WM_ESC: 
                                       // Do something 
      break;
    case WM_ENTER: 
                                       // Do something
      break; 
    case WM_COMMAND:
      wmEvent = HIWORD(wParam);
      if ((hListtitel != 0) && (lParam == (LPARAM)hListtitel))
      {
        switch (wmEvent)
			  {
       	  case CBN_EDITUPDATE: 
		                                	  // Do something
            break; 
       	  case CBN_KILLFOCUS:  
		    	                              // Do something
            break; 
          case CBN_SELCHANGE:  
		    	                              // Do something
            break; 
          }	
        break; 
      }
    default:
    	return DefWindowProc(hwnd, message, wParam, lParam);
  }
  return 0;
}

LRESULT CALLBACK ChldProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
  switch (msg)
  {        
  	case WM_KEYDOWN: 
      switch (wParam) 
      { 
        case VK_TAB: 
          SendMessage(hMain, WM_TAB, 0, 0); 
          return 0; 
        case VK_ESCAPE:  
          SendMessage(hMain, WM_ESC, 0, 0); 
          return 0; 
        case VK_RETURN: 
          SendMessage(hMain, WM_ENTER, 0, 0); 
          return 0; 
       } 
       break; 
  } 
  return CallWindowProc(lpfnParProc, hwnd, msg, wParam, lParam); 

  hListtitel = CreateWindow("COMBOBOX","",
               WS_CHILD | WS_VISIBLE | CBS_DROPDOWN | WS_VSCROLL | CBS_HASSTRINGS, 
               200, 170, 540, 400,hWnd,NULL,hInst,NULL);          
  pt.x = 1; 
  pt.y = 1; 
// get the handle to the edit part of the combobox
  hEdittitel = ChildWindowFromPoint(hListtitel, pt); 
// set pointer to the desired procedure
  lpfnParProc = (WNDPROC) SetWindowLong(hEdittitel,GWL_WNDPROC, (DWORD) ChldProc); 

• Add a global var to hold a pointer to the original WindowProcedure for the edit box
• Add a prototype for the SubclassProcedure
• Add code to replace the windowProc for the edit-box
• Add the subclass procedure

#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include "resource.h"
#include <stdio.h>

HINSTANCE hInst;
HWND dlgHwnd, comboHwnd, comboEditHwnd;
WNDPROC lpfnEditWndProc;

LRESULT CALLBACK SubClassProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam);


BOOL CALLBACK DialogProc(HWND hwndDlg, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch(uMsg)
    {
        case WM_INITDIALOG:
            dlgHwnd = hwndDlg;
            comboHwnd = GetDlgItem(hwndDlg, IDC_COMBO1);


            comboEditHwnd = FindWindowEx(comboHwnd, NULL, "Edit", NULL);
            lpfnEditWndProc = (WNDPROC) SetWindowLong(comboEditHwnd, GWL_WNDPROC, (DWORD) SubClassProc);
            return TRUE;

        case WM_CLOSE:
            EndDialog(hwndDlg, 0);
            return TRUE;

        case WM_COMMAND:
            switch(LOWORD(wParam))
            {
                /*
                 * TODO: Add more control ID's, when needed.
                 */
                case IDC_BTN_QUIT:
                    EndDialog(hwndDlg, 0);
                    return TRUE;

                case IDC_BTN_TEST:
                    MessageBox(hwndDlg, "You clicked \"Test\" button!", "Information", MB_ICONINFORMATION);
                    return TRUE;
            }
    }

    return FALSE;
}


/********************************************************

    FUNCTION:   SubClassProc

    PURPOSE:    Pass WM_KEYDOWN, WM_KEYUP, WM_CHAR messages to the parent of the comboBox that
                owns this Edit window.

    NOTE:       dlgHwnd is a global var, initialized when the dialog is initialized

*********************************************************/
LRESULT CALLBACK SubClassProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch (msg)
    {
        case WM_KEYDOWN:
        case WM_KEYUP:
        case WM_CHAR:
                PostMessage(dlgHwnd, msg, wParam, lParam);
        break;
    }
    //  Call the original window procedure for default processing.
     return CallWindowProc(lpfnEditWndProc, hwnd, msg, wParam, lParam);
}


int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)
{
    hInst = hInstance;

    // The user interface is a modal dialog box
    return DialogBox(hInstance, MAKEINTRESOURCE(DLG_MAIN), NULL, (DLGPROC)DialogProc);
}