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Figure 18-5. The COM tab of the Add Reference dialog box will show you all registered COM libraries on your machine Once you select a COM library, the IDE will respond by generating a brand new assembly which contains .NET descriptions of COM metadata. Formally speaking, these are termed interoperability assemblies (or simply, interop assemblies). Interop assemblies do not contain any implementation code, except for a small amount that helps translate COM events to .NET events. However, these interop assemblies are very useful in that they shield your .NET codebase from the complex underbelly of COM internals. In your C# code, you can directly program against the interop assembly, allowing the CLR (and if you use the dynamic keyword, the DLR) to automatically map .NET data types into COM types, and vice versa. Behind the scenes, data is marshaled between the .NET and COM applications using a Runtime Callable Wrapper (RCW), which is basically a dynamically generated proxy. This RCW proxy will marshal and transform .NET data types into COM types, adjust the COM object s reference counter, and map any COM return values into .NET equivalents. Figure 18-6 shows the big picture of .NET to COM interoperability.

The Role of Primary Interop Assemblies (PIAs)

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Many COM libraries created by COM library vendors (such as the Microsoft COM libraries that allow access to the object model of Microsoft Office products) provide an official interoperability assembly termed a primary interop assembly or PIA. PIAs are optimized interop assemblies, which clean up (and possibly extend) the code typically generated when referencing a COM library using the Add Reference dialog box. PIAs are typically listed in the .NET tab of the Add Reference dialog box, just like the core .NET libraries. In fact, if you reference a COM library from the COM tab of the Add Reference dialog box, Visual Studio will not generate a new interoperability library as it would normally do, but use the provided PIA instead. Figure 18-7 shows the PIA of the Microsoft Office Excel object model, which you will be using in the next example.

Before the release of .NET 4.0, when a C# application made use of a COM library (PIA or not), you needed to ensure the client machine has a copy of the interop assembly on their computer. Not only will this increase the size of your application installer package, but the install script must check that the PIA assemblies are indeed present, and if not, install a copy to the GAC. However, under .NET 4.0, you can now elect to embed the interoperability data directly within your compiled .NET application. When you do so, you are no longer required to ship a copy of the interoperability assembly along with your .NET application, as the necessary interoperability metadata is hardcoded in the .NET program. By default, when you select a COM library (PIA or not) using the Add References dialog, the IDE will automatically set the Embed Interop Types property of the library to True. You can see this setting first hand by selecting a referenced interop library in the References folder of the Solution Explorer, and then investigate the Properties window (see Figure 18-8).

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Figure 18-8. .NET 4.0 allows you to embed the parts of an interop assemblies you make use of into your .NET assembly The C# compiler will only include the parts of the interop library you are actually making use of. Thus, if the real interop library has .NET descriptions of hundreds of COM objects, you will only bring in the definitions of the subset you are really making use of in your C# code. Beyond reducing the size of the application you need to ship to the client, you also have an easier installation path, as you don t need to install any missing PIAs on the target machine.