Using the COM Object Wizard
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The COM object wizard performs the following tasks:
- Creates a new unit.
- Defines a new class that descends from TCOMObject and sets up the class factory constructor. For more information on the base class, see Code generated by wizards.
- Adds a type library to your project and adds your object and its interface to the type library.
- Opens the type library in the Type Library Editor
Before you create a COM object, create or open the project for the application containing functionality that you want to implement. The project can be either an application or ActiveX library, depending on your needs.
To bring up the COM object wizard, see Using COM Wizards.
In the wizard, you must specify the following:
- CoClass name: This is the name of the object as it appears to clients. The class created to implement your object has this name with a 'T' prepended. If you do not choose to implement an existing interface, the wizard gives your CoClass a default interface that has this name with an 'I' prepended.
- Instancing: Unless you are creating an in-process server, you need to indicate how COM launches the application that houses your COM object. If your application implements more than one COM object, you should specify the same instancing for all of them.
- Threading Model: Typically, client requests to your object enter on different threads of execution. You can specify how COM serializes these threads when it calls your object. Your choice of threading model determines how the object is registered. You are responsible for providing any threading support implied by the model you choose. For information on how to provide thread support to your application, see Writing Multi-threaded Applications
- Interface: The wizard gives your object a default interface that descends from IUnknown. By default, the wizard gives your interface the same name as the CoClass, preceded by "I" to indicate interface. After exiting the wizard, you can then use the Type Library editor to add properties and methods to this interface. However, you can also select a pre-defined interface for your object to implement. Click the [...] button in the COM object wizard to bring up the Interface Selection wizard, where you can select any dual or custom interface defined in a type library registered on your system. The interface you select becomes the default interface for your new CoClass. The wizard adds all the methods on this interface to the generated implementation class, so that you only need to fill in the bodies of the methods in the implementation unit. Note that if you select an existing interface, the interface is not added to your project's type library. This means that when deploying your object, you must also deploy the type library that defines the interface.
- Include Type Library: You can choose whether you want to include a type library for your object. This is recommended for two reasons: it lets you use the Type Library editor to define interfaces, thereby updating much of the implementation, and it gives clients an easy way to obtain information about your object and its interfaces. If you are implementing an existing interface, Delphi requires your project to use a type library. This is the only way to provide access to the original interface declaration.
- Mark interface Oleautomation: If you have opted to create a type library and are willing to confine yourself to Automation-compatible types, you can let COM handle the marshaling for you when you are not generating an in-process server. By marking your object's interface as OleAutomation in the type library, you enable COM to set up the proxies and stubs for you and handles passing parameters across process boundaries. You can only specify whether your interface is Automation-compatible if you are generating a new interface. If you select an existing interface, its attributes are already specified in its type library. If your object's interface is not marked as OleAutomation, you must either create an in-process server or write your own marshaling code.
You can optionally add a description of your COM object. This description appears in the type library for your object if you create one.