Get info from call back function

Thank you, it works.

In my DLL I have the following function:

NewWindowHookCallBack(pCode: Integer; pHandle: WPARAM; pWindow: LPARAM): LRESULT; stdcall;

From what I have read, I should be able to get the Class and the Window title from pWindow: LPARAM.  That would save me from using additional APIs to find this information.

LPARAM is a long pointer to a structure. I have been trying to access values from this structure but with no success. Would you have any example of that ?

Thank you.

Hmmm. Could you give me the code where you try to use LPARAM?
I guess (from what you've written) that you try to use LPARAM as a pointer in the main application after you got the message from the dll, right? If yes: That won't work...   :-(
Some theory:
Each win32 process has it's own memory/address context and can address the full 32bit pointer range from $0 to $FFFFFFFF. That means: The same pointer that points to some informations in process A, DOES NOT point to the same informations if you're using it in process B. In process B it points to a completely different memory location. Perhaps you'll even get an Access Violation.
So if you want to transport memory blocks from one process to another, you'll have to use the WM_COPYDATA message.

Regards, Madshi.

I am trying to access the LPARAM info from the DLL itself. I need to look at the class first to see if it is a #32770 dialog in which case I don't allow it to open. Secondly, because I want to allow some #32770 (that's the class of the dialogs), I will either look at the Window title (which I really don't like in case my application is used with other versions of Windows) or the size and position.

Here is my function (which you wrote most of):

function NewWindowHookCallBack(pCode: Integer; pHandle: WPARAM; pWindow: LPARAM): LRESULT; stdcall;
var
  vWindowClass: array[0..80] of char;
begin
  Result:=0;

  if pCode = HCBT_CREATEWND then
  begin
    GetClassName(pHandle, vWindowClass, sizeof(vWindowClass) - 1);
    if IsWindow(pHandle) and (StrComp(vWindowClass, '#32770') = 0) then
    begin
      PostMessage(FindWindow('TFormMainMenu', nil), WM_User + 1, 555, 777);
      Result:=1;
    end;
  end;
end;


I found the description of the structure pWindow points to in Win32.Hlp:

typedef struct tagCBT_CREATEWND { // cbtcw  
    LPCREATESTRUCT lpcs;
    HWND           hwndInsertAfter;
} CBT_CREATEWND;


AND lpcs:

typedef struct tagCREATESTRUCT { // cs  
    LPVOID    lpCreateParams;
    HINSTANCE hInstance;
    HMENU     hMenu;
    HWND      hwndParent;
    int       cy;
    int       cx;
    int       y;
    int       x;
    LONG      style;
    LPCTSTR   lpszName;
    LPCTSTR   lpszClass;
    DWORD     dwExStyle;
} CREATESTRUCT;


I was trying something like classename := pWindow^.lpcs.lpszClass
but no luck... I guess I need a refresher on pointers...

Thank you.

Hmm. "pWindow" is a LPARAM type, that is nothing but a simple integer. So if you write "pWindow^", Delphi doesn't know what to do with that, you see that?
So you'll have to write "classename := PCBTCreateWnd(pWindow)^.lpcs^.lpszClass". Now Delphi knows what to do.

Regards, Madshi.

Hi Madshi, what you gave me compiles and works except that for some components (TToolbar being one of them) it gives me a Access Violation. I am just trying it with a Thread Hook within my program (not a dll) for now and it looks as follows:

function NewWindowHookCallBack(pCode: integer; pMsg: WPARAM; pWindow: LPARAM): LRESULT; stdcall;
begin
  Result:=0;
  if pCode = HCBT_CREATEWND then
  begin
    if (StrComp(PCBTCreateWnd(pWindow)^.lpcs^.lpszClass, '#32770') = 0) then
      Result := 1;
  end;
  CallNextHookEx(whNewWindowThread, pCode, pMsg, pWindow);
end;


Any idea what causes that. My application has several form (5) that all get created. The one with the tollbar causes the problem. When I remove the toolbar it's fine.

Thank you.

I now know that the TComboBox component causes the problem. I created an empty form with the hook - no problem. As soon as I add a combobox, an access violation occurs.

Hmm. That's strange. I've no idea why it happens. Please check whether pWindow is <>nil and whether lpcs is <>nil. According to the documentation, it shouldn't, but who knows...
Perhaps you should use your original solution with GetClassName. Was something wrong with that one?

Regards, Madshi.