Chapter 2. The Type System

Chapter 1 provided a high-level overview of the issues involved in building distributed systems. It introduced a solution to these issues, the .NET Framework, and used a simple "Hello World" example to highlight the language interoperability offered by the .NET Framework. But, as is so often the case, the devil lies in the details. Chapters 2 through 4 describe in more depth the three CLR subsystems: the type system (described in this chapter) and the metadata and execution systems (described in Chapters 3 and 4, respectively).

As noted in Chapter 1, the facilities provided by the type, metadata, and execution systems are not new. However, the CLR does provide functionality in addition to the services provided by other architectures, such as COM/DCOM, CORBA, and Java. For example:

• The type system supports many programming styles and languages, allowing types defined in one language to be first-class citizens in other languages.

• The metadata system supports an extensibility mechanism, called custom attributes, that allows developers to extend the metadata annotations.

• The execution system ensures security and supports versioning on types in the CLR.

Using the .NET Framework, developers can both define and share types. Defining and sharing new types in a single language is not particularly challenging; allowing a newly defined type to be used in other languages is much more problematic . This chapter offers a sufficiently detailed understanding of the CLR type system so that developers can appreciate how it achieves type interoperability.