COM & .NET Component Services

Object Constructor String

COM+ allows you to pass a construction parameter to new instances of your component. This instance activation service has nothing to do with application scalability, JITA, or object pooling, and is nothing more than a neat service.

On your component’s Properties page, there is a properties group named “Object construction” on the Activation tab. Once you enable this service (by checking the “Enable object construction” checkbox), you can specify a string in free form. Every instance of your component has access to this one string (you cannot specify a string per instance). Because calls to CoCreateInstance( ) or CreateObject( ) do not accept initialization parameters, you have to work to gain access to the constructor string.

The first thing you need to do (besides enable the service) is have your component implement an interface called IObjectConstruct , defined as:

interface IObjectConstruct : IUnknown
{
   HRESULT Construct([in]IDispatch* pConstructionObj);
};

If you enable object construction but do not implement the interface, all client attempts to create a new instance of your component will fail, showing the error code E_NOINTERFACE . They will fail because COM+ will refuse to hand over to the client an object that could not be initialized properly. IObjectConstruct has only one method, Construct( ) , which COM+ uses to pass in a pointer to another interface called IObjectConstructString , defined as:

interface IObjectConstructString : IDispatch { ...