Dynamic resizing of ActiveX controls hosted by ATL Composite Control

August 19, 2009

Did you cheer when you found out that ATL 3.0 would support ActiveX control hosting? I did. This functionality was at the top of my wish list for ATL. ATL 3.0 has finally arrived. What now? In one news group, I saw the following question: “How do I resize ActiveX controls hosted by Composite Control?” The answer to this question is very simple, but the reasoning behind it is not. Therefore, I decided to write this article to share my knowledge of the inner workings of ATL’s hosting of ActiveX controls.

How does ATL implement the hosting of ActiveX controls

If you will look in ATL code, you will notice that code for CComCompositeControl takes less then two hundred lines. How is it possible to fit all hosting support into two hundred lines of code? Well, the answer is quite simple – CComCompositeControl does not have support for ActiveX hosting. The real hero behind the scenes is CAxHostWindow.

CAxHostWindow implements necessary interfaces in order to support the hosting of common ActiveX controls (including windowless) and Microsoft Web Browser control. In case you do not need to host a Web Browser control, you can define _ATL_NO_DOCHOSTUIHANDLER in your project and save about 4K in release dll. CAxHostWindow derives from CWindowImpl, and this means it needs a window to operate. CAxHostWindow can host only one control at a time, and as a result, one CAxHostWindow is required for each hosted control. Without going into all the aspects of implementation (look into atlhost.h for more details), I would like to concentrate here on the way CAxHostWindow handles WM_SIZE and WM_PAINT.

When CAxHostWindow receives WM_SIZE, it checks if the hosted control supports IOleObject and sets its new extent. In addition, if the hosted control is a windowless control, then SetObjectRects() will be called with the new control size equal to its window size. An important observation is that a control hosted by CAxHostWindow always occupies the full size of the window. When handling WM_PAINT, CAxHostWindow exhibits interesting behavior only for windowless objects. If it is drawing a windowless control, it creates an off screen device context, paints it with background color (which is by default COLOR_BTNFACE, if parent window is a dialog, or otherwise COLOR_WINDOW), and calls Draw method of IViewObject. After that, CAxHostWindow does BitBlt from off screen context to window context in order to prevent flickering while the control draws itself. Some controls employ a similar flicker prevention technique inside their drawing code, which results in a slower drawing of the control. Consequently, if you are creating a control that you will only use with Composite Control, you can omit the flicker prevention code.

You do not need to create CAxHostWindow directly. ATL implements a helper function AtlAxCreateControl(), which will create an instance of CAxHostWindow and will make all necessary initializations for you. AtlAxCreateControl() has the following four parameters: (1) the name of the control to be hosted; (2) the window handle to be used by CAxHostWindow; (3) IStream, which can be used to initialize control if it supports IPersistStream; and (4) pointer to IUnknown, for IUnknown of created CAxHostWindow.

As you can see below, any application can support the hosting of ActiveX controls with ease, using support provided by ATL.

#include "stdafx.h"
#include "resource.h"

CComModule _Module;


WinMain(HINSTANCE hInst, HINSTANCE hPrevInst, LPSTR lpCmdLine, int nCmdShow)
    _Module.Init(ObjectMap, hInst);

    //Register window class here

    HWND hWnd = CreateWindow( TEXT("ClassName"), TEXT("Title"),
                300, 300, 0L, 0L, hInst, 0L );

    CComPtr spUnk;
    HRESULT hRet = AtlAxCreateControl(T2COLE("Some.Control.1"), hWnd, NULL, &spUnk);
        return -1;

    //Show window and go to GetMessage loop

    return msg.wParam;

Composite Control

Now that we know how hosting works in ATL, we can speculate on how Composite Control works. One possible scenario could go like this: When Composite Control is created, it parses its dialog template; for every ActiveX control description it finds, it creates a child window and then passes it and the control’s name to AtlAxCreateControl(). However, the developers of ATL found a simpler and more elegant solution. In order to understand their solution, we should first look at the helper window class provided by ATL. ATL implements a function AtlAxWinInit(), which registers window class “AtlAxWin”. When the window of this class is created, the handler for WM_CREATE uses the window’s caption as the control’s name and creates the control hosted in this window. It also converts CreateParams into IStream and passes it to the control. The handler for WM_DESTROY releases the host control interface to guarantee its destruction. Now, we can create the control hosted in the window with even less code.

CreateWindow( TEXT("AtlAxWin"), TEXT("Some.control.1"),
    WS_CHILD | WS_VISIBLE, x, y, cx, cy, hWndParent, NULL, hInst, NULL);

Now, we’re ready to investigate how Composite Control creates hosted controls. Composite Control indirectly derives from CAxDialgImpl. When CaxDialogImpl is created, it calls the helper function AtlAxCreateDialog(). This function initializes “AtlAxWin” by calling AtlAxWinInit() and then makes a call to a function mysteriously named _DialogSplitHelper::SplitDialogTemplate(). What this particular function does is essentially parse through the dialog template and finds all CONTROL entries with class names starting with “{“. When Visual Studio saves the dialog resource containing ActiveX controls, it saves information about each control in a form that look like a windows control, but instead of a window class, it writes the CLSID of the ActiveX control.

IDD_DIALOG DIALOGEX 0, 0, 286, 177
FONT 8, "MS Sans Serif"
    CONTROL    "",IDC_CONTROL,"{3343C18D-A997-11D2-B348-00A0244AE119}",
    CONTROL    "Progress1",IDC_PROGRESS1,"msctls_progress32",WS_BORDER,

For each ActiveX control found, SplitDialogTemplate() replaces it with record that looks like the following below:

CONTROL     "{3343C18D-A997-11D2-B348-00A0244AE119}",IDC_CONTROL,"AtlAxWin",
            WS_BORDER, 3,3,133,171

As you can see this record look exactly like the record of a normal windows control, and it can be created by a standard windows function like ::CreateDialogIndirectParam() or ::DialogBoxIndirectParam().


Finally, we get to the bottom of our investigation, and I can review it in short. Composite Control does not have any hosting capabilities, and relies completely on support provided by CAxDialogImpl. CAxDialogImpl uses the ability of “AtlAxWin” to make ActiveX controls look like windows controls. CAxHostWindow provides support for the hosting of ActiveX controls, but it requires a window to do so.

Now, we can go back to the question, “How do I resize ActiveX controls hosted by Composite Control?” We know now that the answer is simple - “In the same way you would resize any windows control in dialog, because Composite Control doesn’t know the difference!” Let’s illustrate it with the following code sample:

LRESULT OnSize(UINT, WPARAM wParam, LPARAM lParam, BOOL& bHandled)
    int cx = LOWORD(lParam);
    int cy = HIWORD(lParam);
    HWND h = GetDlgItem(IDC_CONTROL);
    ::SetWindowPos(h, NULL, 3, 3, cx - 6, cy - 27, SWP_NOZORDER);

    h = GetDlgItem(IDC_PROGRESS1);
    ::SetWindowPos(h, NULL, 3, cy - 21, cx - 6, 18, SWP_NOZORDER);

The implementation of hosting for ActiveX controls in ATL is simple and lightweight, but it obviously has limitations. All advantages of windowless controls are effectively eliminated by one control per window restriction. Methods OnPosRectChange() and RequestNewObjectLayout() are not implemented. This means you cannot implement resizing per the control’s request.

ATL’s “AtlAxWin” shows us a very interesting possibility. We can create a window class that will wrap around some specific ActiveX control, include mapping controls methods into messages, and make this control accessible for non-COM environments! The creation process of a wrapper window class is even possible to automate. This is a very interesting opportunity, indeed!

February 1, 1999