VB ver 3.0 CDK TN002.TXT: Custom Control Version Management

- Professional Edition of Microsoft Visual Basic for Windows, version 3.0

SUMMARY

The following article contains the complete contents of the TN002.TXT file installed in the CDK directory of the Professional Edition of Visual Basic version 3.0 for Windows.

MORE INFORMATION

TN002.TXT

Microsoft Visual Basic version 3.00 for Windows Microsoft Corporation Technical Notes

TN002.TXT: Custom Control Version Management

This note describes how to use the version management functionality for custom controls.

Introduction

There are cases, however, when a Visual Basic application created with a new version of a custom control runs on a system with an older version of the custom control. Depending on the features and implementation of the custom control, the application may not work correctly -- or worse, may cause VB.EXE or VBRUNx00.DLL to crash.

The following sections describe Visual Basic version 3.0's custom control version management system, which can help avoid potential application failure.

MODEL Structure

This is the definition of the MODEL structure used in Visual Basic version 3.0. Note the addition of the last field (USHORT usCtlVersion).

typedef struct tagMODEL

    {
    USHORT      usVersion;           // VB version used by control
    FLONG       fl;                  // Bitfield structure
    PCTLPROC    pctlproc;            // The control procedure
    FSHORT      fsClassStyle;        // Window class style
    FLONG       flWndStyle;          // Default window style
    USHORT      cbCtlExtra;          // Number of bytes allocated
                                     // for control structure
    USHORT      idBmpPalette;        // BITMAP ID for tool palette
    PSTR        npszDefCtlName;      // Default control name prefix
    PSTR        npszClassName;       // Visual Basic class name
    PSTR        npszParentClassName; // Parent window class, if subclassed
    NPPROPLIST  npproplist;          // Property list
    NPEVENTLIST npeventlist;         // Event list
    BYTE        nDefProp;            // Index of default property
    BYTE        nDefEvent;           // Index of default event
    BYTE        nValueProp;          // Index of control value property
    USHORT      usCtlVersion;        // Identifies current version of the
                                     // custom control. The values 1 and
                                     // 2 are reserved for custom controls
                                     // created with VB1.0 and VB2.0

Control Version

A custom control developer who wants to take advantage of the version management feature in Visual Basic version 3.0 needs to provide a valid version number in the usCtlVersion field. This value must be an unsigned integer (USHORT), and it should be changed any time the custom control changes its backward compatibility with previous versions.

Although any nonzero value is valid, the values 1 and 2 should not be used. Because Visual Basic versions 1.0 and 2.0 do not support version management, Visual Basic automatically assigns values 1 and 2 to any custom control that has the usVersion field in the control's MODEL structure set to VB100_VERSION and VB200_VERSION, respectively.

In order to take advantage of version management, you must set the usVersion field of the control's MODEL structure to VB300_VERSION or greater, or use VB_VERSION defined in a Visual Basic version 3.0 VBAPI.H file. This allows Visual Basic to determine whether the usCtlVersion field is defined in the MODEL structure of a given custom control.

If the custom control contains a value of 0 in the usCtlVersion field, no version control checks are made on this custom control.

How the System Works

When you create a Visual Basic executable (.EXE) file that uses a custom control, Visual Basic retrieves the control version number (usCtlVersion) for that control and stores it in a special section of the .EXE file.

When you execute the application, the Visual Basic run-time support file (VBRUN300.DLL or greater) checks the control version number recorded in the .EXE file against the version number found in the custom control when it is loaded into the system. If the version found in the .EXE file is greater than the version of the control loaded into the system, Visual Basic displays a notification that the particular custom control (.VBX file) is out of date and will not load, consequently forcing the application to terminate.