PRB: VB Doesn't Generate Compile Error for Misspelled Methods

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0
• Microsoft Visual C++, 32-bit Editions, versions 5.0, 6.0

SYMPTOMS

When using a COM component from Visual Basic, you may not receive a compile- time error if you accidentally misspell a property or method name. Instead, Visual Basic will generate the following error at run-time:

Run-time error '438':
Object does not support this property or method.

CAUSE

This situation occurs when the object being referenced supports a dual or dispinterface, and therefore can be late-bound. If the component designer did not mark the interface as being "nonextensible," then Visual Basic cannot determine if the misspelled method/property name is misspelled, or if it is a new method or property added to the interface at a later time. Consequently, Visual Basic does not raise an error at compile-time, but rather checks at run-time whether the component supports a method/property by that name. The run-time error occurs if the object does not have the named method/property.

The COM component creator can indicate whether or not an interface can be extended at a later time. If the interface is marked as "nonextensible," then Visual Basic will know at compile-time whether the object supports the named method/property.

STATUS

This behavior is by design.

MORE INFORMATION

If you use Visual C++ to write your COM components, the default IDL files generated by the IDE do not contain the "nonextensible" keyword. COM components created in Visual Basic always contain this keyword.

If you are building a COM component in Visual C++, and would like to make your dual or dispinterfaces nonextensible, simply add the "nonextensible" keyword to the interface attributes. For example:

    importlib("StdOle2.tlb");

    [
      odl,
      uuid(ACF8C871-1A73-11D2-AC0A-00600832A1F6),
      version(1.0),
      dual,
      nonextensible,     // This is the line you need to add
      oleautomation
    ]
    interface IMyInterface : IDispatch {
        [id(0x68030000), propget]
        HRESULT __stdcall Name([out, retval] long* );
        [id(0x68030000), propput]
        HRESULT __stdcall Name([in] long );
        [id(0x60030001)]
        HRESULT __stdcall PopMsg();
    }; 

Additional query words: kbDSupport kbDSD kbVBp kbVBp500 kbVBp600 kbCtrlCreate kbActiveX kbCOMt
kbVC500 kbVC600

Keywords : kbGrpVB
Version : WINDOWS:5.0,6.0; WINNT:5.0,6.0
Platform : WINDOWS winnt
Issue type : kbprb