To use the objects of a program that supports Automation (a server), you have to reference the program's type library. A program's type library (also called object library) is a file that contains a description of the program's object model. After you've referenced the type library of an automation server (also called a component), you can access the objects of the server as though they were internal C# objects.
To create a reference to a type library, first display the Add Reference dialog box by choosing Add Reference from the Project menu (do this now). A number of types of components support automation. Of course, .NET is the latest technology, but in the case of Excel, we're interested in the COM components. COM stands for Component Object Model, and it's been the technology for working with objects within windows for many years. Microsoft's .NET platform is designed to replace COM. This isn't going to happen overnight, however; literally thousands of objects are built on COM technology. In fact, all the Microsoft Office products up to and including Office XP are based on COM.
Click the COM tab now to display the available COM components (programs that have a type library) on your computer. Scroll the list and locate the Microsoft Excel X Object Library (where X is the version of Excel installed on your computer). Double-click the Excel item to add it to the list of selected components at the bottom of the Add Reference dialog box (see Figure 20.1).
Figure 20.1. To use an object library, you need to reference it first.
If you don't see an entry for Microsoft Excel, you probably don't have Excel installed on your computer; therefore, this code won't work.
Click OK now to add the reference to your project. C# doesn't work directly with COM components. Instead, it interacts through a wrapper, a set of code and objects that works as an intermediary between C# and a COM component. Chances are that your machine doesn't have an existing wrapper for Excel; if this is the case, C# automatically creates the wrapper and references the component.
