Implementation

The intimidating part of COM is making sure all these details get handled correctly.  Among the details involved include: 1) making sure the client code can locate the COM object to use, 2) ensuring that the client code and the COM object can share information, and 3) making sure that resources used to create the COM object are managed correctly. These items have associated with them legions of new acronyms, data types, and methods. The most important thing to do while learning these new acronyms, data types, and methods is to keep yourself from running from the room screaming. We recommend the books listed below which will help you get up and running with COM.

Fortunately, MFC and ATL have created Wizards, macros, templates, and other tools that automate most of the particularly inscrutable aspects of implementing a COM server or COM client. We have written two companion articles that describe the steps to convert an existing class in a monolithic C++ application into a COM server. Because we needed to access the COM server from an Active Server Page web site and from within Excel using VBA, we needed to provide an Automation interface rather than the more general custom interfaces. Clients such as VBA and VBScript only support automation interfaces and not custom interfaces. Automation uses the IDispatch interface and can only pass automation data such as VARIANTs and not, for example, abstract data types.  However, if you are trying to drive the server from VBA or VBScript, being limited to the basic data types (through VARIANT) is probably not a problem. (The server code itself is, free to use whatever data types it needs to use - the data type limitation is only on what can be passed back and forth to the client application.)

Example

The attached zip file contains the following:
1. Source code to build a COM server: ComSample.zip.
2. Source code to build the View application: DlgView.zip.
3. Source Code for the Document DLL: DocEngine2.zip.
4. COMDEMO.xls.  An Excel 97 spreadsheet file.
5. All the EXEs and DLLs if you simply want to run the code.  Please remember to register the ComSample.DLL.

The View

The Document

The COM Server

Conclusion

EndNotes

[1] These fields are for illustrative purposes only. The DBViewer can work with any relational database that supports SQL. The only significant setup required is to create a table that acts like a data dictionary.

[2] The SQLs must be created dynamically because they generally include criteria such as "get me the last two months of data," or "get me the top ten percent of these values." This means that it is not possible to simple store the actual SQL statement and retrieve it for later use.

[3] Although this process sounds similar to calling an exported function in a DLL, the COM solution is more robust and allows DLL calls to be correctly made even if the DLL was generated by another vendor's compiler or even in a completely different language. Even calling non-COM DLLs generated in C++ but from different vendors is problematic because different vendors implement the features of C++ differently (e.g., RTTI, exception handling). Chapter 1 of Essential COM by Don Box (Addison-Wesley. 1998) provides an excellent description of these issues.

References

Sub Test1()
	Set dll = CreateObject("ComSample.DocEngineInterface")
	Dim str As Variant
	Dim blah As String
	str = dll.HelloWorld()
	blah = CStr(str)
	Worksheets("Sheet1").Range("E7").Select
	ActiveCell.Value = CStr(str)
End Sub