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# Building a COM Component

The EiffelCOM wizard allows for the development of both in-process (DLLs) and out-of-process (EXEs) COM components in Eiffel.

## Using the Generated Code

If the project consists of adding a COM interface to an existing Eiffel project then the wizard will generate (and optionally compile) everything required to implement the COM component. No extra work is required.

If the project uses a COM definition file (either an IDL file or a type library), the EiffelCOM Wizard generates empty features in the classes corresponding to the component's coclasses. The implementation can then be added into these empty features.

Warning: Depending on the project settings, the EiffelCOM Wizard may overwrite the files containing the classes corresponding to the component's colasses if the project is re-generated thereby replacing the implemented features with empty ones. Make sure the destination folder is different from the folder where the classes have been implemented prior to re-generating the project.

Unlike the code generated to access an existing component, the code generated to implement a new COM component will differ for an in-process or an out-of-process component. The difference lies in the component activation code in the class ECOM_<Name_of_system>_REGISTRATION. If the component is in-process then this class includes the four functions that need to be exported from an in-process COM component (DllRegisterServer, DllUnregisterServer, DllGetClassObject, and DllCanUnloadNow). If the component is out-of-process then the registration class includes a feature initializing the component and its graphical user interface.

The architecture of the generated code is similar to the one used by the code generated to access an existing component: classes corresponding to the component's coclasses inherit from classes corresponding to the component's interfaces and implement all the deferred features. There is however a major difference: the classes corresponding to the component's coclasses should not be inherited from. Instead their implementation should be completed. Putting the implementation in a heir class would cause a runtime failure since the EiffelCOM generated code will instantiate the class corresponding to the component's coclass when being called rather than any descendant that would actually contain the implementation. The classes corresponding to the component's coclasses are all suffixed with _COCLASS_IMP.

## Component's GUI

In the case of an out-of-process server, the component might need a Graphical User Interface. There are two different scenarios in which the component can be activated: either its user launched it explicitly (e.g. by double clicking the executable icon) or it was launched by the COM runtime to satisfy a client request. In some cases it might be required for the component to have a different behavior in these two cases (for example the GUI could only be necessary when the component was launched explicitly by the user but should be hidden when launched by the COM runtime). The generated registration class for an out-of-process server includes the feature:

    main_window: WEL_FRAME_WINDOW

This feature is a once function that can return the class corresponding to the component window. By default, this window is displayed only if COM does not start the component. When COM loads an out-of-process component, it appends the command line parameter "-embedding" to the executable. The generated registration class looks for this option and if it is part of the process argument list then it sets the default window appearance to hidden. Of course this default implementation may be modified if needed.

## Exceptions

The COM standard specifies that each COM routine should return a status code to the client. The status code is encoded in a HRESULT value. Such behavior goes against the Command Query Separation Principle in Eiffel. The EiffelCOM generated system should use exceptions instead of returning HRESULT values to the client. By default the EiffelCOM runtime will always return S_OK meaning that the routine succeeded. To notify the client of a problem (e.g. invalid argument value), the EiffelCOM component should raise the corresponding exception from class ECOM_EXCEPTION using the feature trigger from the same class. The EiffelCOM runtime will catch the exception and send the corresponding HRESULT back to the client. Here is what the Eiffel code for a COM component should look like:

    indexing
    	description: "Eiffel coclass server example"
    class
    	 ECOM_SERVER_COCLASS_IMP

    inherit
    	ECOM_SERVER_COCLASS -- Generated by the wizard

    	ECOM_EXCEPTION
    		export
    			{NONE} all
    		end

    feature -- Basic Operations

    	coclass_feature (an_argument: ARGUMENT_TYPE) is
    			 -- Example of a coclass feature
    		do
    			if not is_valid (an_argument) then
    				trigger (E_invalidargument)
    			else
    				-- Normal processing
    			end
    		end

    feature {NONE} -- Implementation

    	is_valid (an_argument: ARGUMENT_TYPE): BOOLEAN is
    			-- Is an_argument a valid argument?
    		do
    			-- Test of validity of an_argument
    		end

    end -- class ECOM_SERVER_COCLASS_IMP

This class inherits from the generated Eiffel coclass and from ECOM_EXCEPTION. It redefines the feature coclass_feature from the generated coclass. This feature is part of the interface functions that can be called by clients of the component. Its implementation uses the featuretrigger from ECOM_EXCEPTION to raise exceptions in case the feature cannot be executed normally (invalid argument). The EiffelCOM runtime catches the exception and maps it into an HRESULT that is sent back to the client.

See Also: How the EiffelCOM Wizard Works, Generated Files, Class Hierarchy, Adding a COM Interface to an Eiffel Project, Accessing a COM Component, Command Line Options