COM is Microsoft's architecture for building software from binary components that expose the functionality with a well-defined interface. Analysis Services understands the COM interface and has the ability to interface with any COM DLLs so that functions within the DLL can be accessed through MDX. In order to create a function that will perform a customized operation for your business, you need to create a DLL using any of the languages that allow you to create a COM DLL, such as Microsoft Visual Basic or Microsoft Visual C++. In this chapter you learn how to create a COM UDF using Visual Basic 6.0.