Visual Basic by Remote Control

Visual Basic continues to establish itself as a popular development tool for Microsoft Windows, and, predictably, developers continue to push the envelope of what Visual Basic and its associated Control Development Kit (CDK) can do.

Recently, I needed to create a Visual Basic custom control that would mimic the standard Data control, but for a non-SQL, non-ODBC database. The control would have to communicate with the database via an API set, and communicate with other Visual Basic controls in order to build queries and display query results. I wanted all communication with the other controls to be handled automatically at the C level, so that the people adding my control to their Visual Basic forms would not have to do any Basic programming.

Communication with the database API was straightforward, but communication with the other controls turned out to be unexpectedly challenging. I finally got my control to work the way I wanted, but only after learning a lot about some unusual and incompletely documented Visual Basic features. I ended up writing a collection of functions to simplify access to control properties; I used these functions to write a diagnostic VBX that acts like a Spy program for Visual Basic controls, displaying a list of properties and their data types, flags, and values.

In this article, I'll describe how to access control properties at run time, and present my collection of access functions. I'll also describe my diagnostic control, CTRLINFO.VBX, which is available electronically; see "Availability," page 3.

To make my custom control perform queries by example, I wanted it to poll all TextBox fields on the form, using their DataField property for column name and their Text property for column value. After the query result returned, I wanted my control to replace the Text property with the value for that column in the query result. This process required calling VBGetControlProperty() to get the Text and DataField properties to build the query, and then calling VBSetControlProperty() for the Text property to display the result.

The problem is that VBGetControlProperty() and VBSetControlProperty() require that you know the index of the property being accessed. Property indexes are created as #defines in a control's header file, and generally are not documented or otherwise made available to the outside world. (In particular, the index for a standard property is not the corresponding "IPROP_STD_" constant defined in VBAPI.H.) And even if you do happen to know a property index for a foreign control, the control's designers are free to change the index from one version of the control to the next.

The only reliable way to determine a property index for a foreign control is through direct access to the control's property array.

The last section showed how to obtain entries in a control's property array. The entry for a custom property is the address of a PROPINFO structure, which contains all the information you need about the property, including its name, data type, and flags. However, the entry for a standard property is just a constant defined in VBAPI.H, which your application has to interpret somehow. We need a simple method for obtaining a true PROPINFO structure for any property, either custom or standard; this would allow an application to deal uniformly with all properties.

void ProcessPropertyArray (HCTL hctl)
{
        MODEL FAR     *lpModel;
        PPROPINFO FAR *lppPropInfo; // far ptr to array of near pointers
        LPPROPINFO     lpPropInfo;
        lpModel = VBGetControlModel(hctl);
        lppPropInfo = (PPROPINFO FAR*) MAKELONG(
                              lpModel->npproplist, (_segment)lpModel);
        for (; *lppPropInfo; lppPropInfo++)
        { 
               if (*lppPropInfo >= PPROPINFO_STD_LAST)
               {
                     // process *lppPropInfo as a fake address
                     // for a standard property defined in VBAPI.H
                }
                else
                {
                     lpPropInfo = (LPPROPINFO) MAKELONG(
                                       *lppPropInfo,                                                       (_segment)lpModel);
                     // process lpPropInfo as a real address of a
                     // PROPINFO structure
                }
        }
}

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