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ActiveX Controls on the Internet 

ActiveX controls are the updated version of the OLE control specification. Controls are a primary architecture for developing programmable software components that can be used in a variety of different containers, including COM-aware Web browsers on the Internet. Any ActiveX control can be an Internet control and can add its functionality to an Active document or be part of a Web page. Controls on a Web page can communicate with each other using scripting.

ActiveX controls are not limited to the Internet. An ActiveX control can also be used in any container, as long as the control supports the interfaces required by that container.

ActiveX controls have several advantages, including:

  • Fewer required interfaces than previous OLE controls.

  • The ability to be windowless and always in-place active.

In order to be an ActiveX control, a control must:

  • Support the IUnknown interface.

  • Be a COM object.

  • Export DLLRegisterServer and DLLUnRegisterServer.

  • Support additional interfaces as needed for functionality.

Making Your Existing Controls Internet-Friendly

Designing a control that will work well in an Internet environment requires consideration for the relatively low transmission rates on the Internet. You can use your existing controls; however, there are steps you should take to make your code size smaller and to make your control properties download asynchronously.

To improve performance of your controls, follow these tips on efficiency considerations:

  • Implement the techniques described in the article ActiveX Controls: Optimization.

  • Consider how a control is instantiated.

  • Be asynchronous; don't hold up other programs.

  • Download data in small blocks.

    When downloading large streams such as bitmaps or video data, access a control's data asynchronously in cooperation with the container. Retrieve the data in an incremental or progressive fashion, working cooperatively with other controls that may also be retrieving data. Code can also be downloading asynchronously.

  • Download code and properties in the background.

  • Become user-interface active as quickly as possible.

  • Consider how persistent data is stored, both properties and large data BLOBs (such as a bitmap image or video data).

    Controls with significant amounts of persistent data, such as large bitmaps or AVI files, require careful attention to downloading method. A document or page can become visible as soon as possible, and allow the user to interact with the page while controls retrieve data in the background.

  • Write efficient routines to keep code size and run time down.

    Small button and label controls, with only a few bytes of persistent data, are suitable for use in the Internet environment and work well inside browsers.

  • Consider progress is communicated to the container.

    Notify the container of progress in the asynchronous download, including when the user can start to interact with a page, and when the download is complete. The container can display progress (such as percent complete) to the user.

  • Consider how controls are registered on the client computer.

Creating a New ActiveX Control

When creating a new control using the Application Wizard, you can choose to enable support for asynchronous monikers as well as other optimizations. To add support to download control properties asynchronously, follow these steps:

To create your project using the MFC ActiveX Control Wizard

  1. Click New on the File menu.

  2. Select MFC ActiveX Control Wizard from the Visual C++ projects and name your project.

  3. On the Control Settings page, select Loads properties asynchronously. Selecting this option sets up the ready state property and the ready state changed event for you.

    You can also select other optimizations, such as Windowless activation, which is described in ActiveX Controls: Optimization.

  4. Choose Finish to create the project.

To create a class derived from CDataPathProperty

  1. Create a class derived from CDataPathProperty.

  2. In each of your source files that includes the header file for your control, add the header file for this class before it.

  3. In this class, override OnDataAvailable. This function is called whenever data is available for display. As data becomes available, you can handle it any way you choose, for example by progressively rendering it.

    The code excerpt below is a simple example of progressively displaying data in an edit control. Note the use of flag BSCF_FIRSTDATANOTIFICATION to clear the edit control.

    void CMyDataPathProperty::OnDataAvailable(DWORD dwSize, DWORD bscfFlag)
        CListCtrl list_ctrl;
        CEdit* edit=list_ctrl.GetEditControl();
        if (bscfFlag & BSCF_FIRSTDATANOTIFICATION && edit->m_hWnd)
            edit->SetSel(0, -1);
        if (!dwSize)
    CString string;
    LPTSTR str=string.GetBuffer(dwSize);
    UINT nBytesRead=Read(str, dwSize);
    if (!nBytesRead)
        edit->SetSel(-1, -1);

    Note that you must include AFXCMN.H to use the CListCtrl class.

  4. When your control's overall state changes (for example, from loading to initialized or user interactive), call COleControl::InternalSetReadyState. If your control has only one data path property, you can add code on BSCF_LASTDATANOTIFICATION to notify the container that your download is complete. For example:

  5. Override OnProgress. In OnProgress, you are passed a number showing the maximum range and a number showing how far along the current download is. You can use these numbers to display status such as percent complete to the user.

The next procedure adds a property to the control to use the class just derived.

To add a property

  1. In Class View, right-click the interface underneath the library node and select Add, then Add Property. This will start the Add Property Wizard.

  2. In the Add Property Wizard, select the Set/Get Methods radio button, type the Property Name, for example, EditControlText, and select BSTR as the Property type.

  3. Click Finish.

  4. Declare a member variable of your CDataPathProperty-derived class to your ActiveX control class.

    CMyDataPathProperty EditControlText;
  5. Implement the Get/Set methods. For Get, return the string. For Set, load the property and call SetModifiedFlag.

    BSTR CDataPathCtrl::GetDataPath() 
        CString strResult;
        strResult = EditControlText.GetPath();
        return strResult.AllocSysString();
    void CDataPathCtrl::SetDataPath(LPCTSTR lpszNewValue) 
        Load(lpszNewValue, EditControlText);
  6. In DoPropExchange, add the following line:

    PX_DataPath(pPX, _T("DataPath"), EditControlText);
  7. Override ResetData to notify the property to reset its control by adding this line:


Deciding Whether to Derive from CDataPathProperty or CCachedDataPathProperty

The previous example describes steps for deriving your control's property from CDataPathProperty. This is a good choice if you are downloading real-time data that frequently changes, and for which you do not need to keep all the data, but only the current value. An example is a stock ticker control.

You can also derive from CCachedDataPathProperty. In this case, the downloaded data is cached in a memory file. This is a good choice if you need to keep all the downloaded data — for example, a control that progressively renders a bitmap. In this case, the class has a member variable containing your data:

    CMemFile m_Cache;

In your ActiveX control class, you can use this memory mapped file in OnDraw to display the data. In your ActiveX control CCachedDataPathProperty-derived class, override the member function OnDataAvailable and invalidate the control, after calling the base class implementation.

COleControl OLEctrl;
void CMyCachedDataPathProperty::OnDataAvailable(DWORD dwSize, DWORD bscfFlag)
    CCachedDataPathProperty:OnDataAvailable(dwSize, bscfFlag);

Downloading Data Asynchronously Using ActiveX Controls

Downloading data over a network should be done asynchronously. The advantage of doing so is that if a large amount of data is transferred or if the connection is slow, the download process will not block other processes on the client.

Asynchronous monikers provide a way to download data asynchronously over a network. A Read operation on an Asynchronous moniker returns immediately, even if the operation has not been completed.

For example, if only 10 bytes are available and Read is called asynchronously on a 1K file, Read does not block, but returns with the currently available 10 bytes.

You implement asynchronous monikers using the CAsyncMonikerFile class. However, ActiveX controls can use the CDataPathProperty class, which is derived from CAsyncMonikerFile, to help implement asynchronous control properties.

The ASYNDOWN sample demonstrates how to set up an asynchronous loop using timers to read the data. ASYNDOWN is described in detail in the Knowledge Base article "HOWTO: AsyncDown Demonstrates Asynchronous Data Download" (Q177244) and is available for download from the Microsoft Download Center. (For more information about downloading files from the Microsoft Download Center, see the article "How to Obtain Microsoft Support Files from Online Services" (Q119591) in the Microsoft Knowledge Base.) You can find Knowledge Base articles on the MSDN Library CD-ROM or at

The basic technique used in ASYNDOWN is to set a timer in CDataPathProperty::OnDataAvailable to indicate when data is available. When the timer message is received, the application reads in 128-byte blocks of data and fills an edit control. If data is not available when the timer message is handled, the timer is turned off. OnDataAvailable turns on the timer if more data arrives later.

Displaying a Control on a Web Page

Here is an example of an object tag and attributes for inserting a control on a Web page.

<PARAM NAME="BackColor" value="#ffffff">
<PARAM NAME="ForeColor" value="#0000ff">
<PARAM NAME="url" VALUE="/ie/controls/chart/mychart.txt">

Updating an Existing OLE Control to Use New ActiveX Control Features

If your OLE control was created with a version of Visual C++ prior to 4.2, there are steps you can take to improve its performance and enhance its functionality. For a detailed discussion of these changes, see ActiveX Controls: Optimization.

If you are adding asynchronous property support to an existing control, you will need to add the ready state property and the ReadyStateChange event yourself. In the constructor for your control, add:


You will update the ready state as your code is downloaded by calling COleControl::InternalSetReadyState. One place you could call InternalSetReadyState is from the OnProgress override of CDataPathProperty-derived class.

Then, follow the steps in Creating a New ActiveX Control.

See Also