Microsoft's .NET platform comprises many different specifications and initiatives. The core functionality of the .NET platform has been given over to the European Computer Manufacturer's Association (ECMA) and is undergoing the standardization process. At the time of this writing, the C# language specification has just passed ECMA's process and is heading for ISO standardization.
You can find the standard documents for the C# programming language and the Common Language Infrastructure (CLI) at www.ecma.ch. An interesting element of the CLI specification is the standard naming convention for variables and methods.
From the programmer's standpoint, the core feature of the .NET platform is its managed environment, which allows the execution of intermediate code compiled by many different programming languages (provided they conform to a common definition of the base data types). This environment embeds many features ranging from sophisticated memory management up to integrated security, to name just two. On top of this managed environment, Microsoft has built a large class library, covering diverse areas of development (Windows-based forms, web development, web services development, XML processing, database access, and many more).
This is only a short overview. To get more into the details we need to learn the precise terms used in the .NET Platform, most of which are indicated by of three-letter acronyms, introduced in the following subsections.
The CLI is the main pillar of the .NET platform. It was submitted to the ECMA in December 2001. The CLI comprises many different specifications:
Common Type System (CTS) The CTS lays the groundwork for integrating disparate programming languages. It specifies how types are declared and used, and language compilers must conform to this specification in order to take advantage of the .NET platform's cross-language integration.
Extensible Metadata The CLI specifies that every unit of deployment (an assembly) must be a self-describing entity. Therefore, an assembly must carry with it data that fully identifies the assembly (its name, version, and optional culture and public key), data that describes the types defined within the assembly, data listing other files referenced, and any special security permissions that are required. Furthermore, the metadata system is extensible, so an assembly might also contain user-defined descriptive elements, known as custom attributes.
The term culture is used by Microsoft as an extension of the term's language (the language used for the user messages of a library) and locale (the dates and number formatting settings for a country). The idea is to cover anything peculiar to a country, or portions of a country.
Common Intermediate Language (CIL) CIL is the programming language of a virtual execution environment with an abstract microprocessor. Compilers targeting the .NET platform do not generate native CPU instructions, but instead generate instructions in the intermediate language of the abstract processor. In this respect, the CLI is similar to Java byte code.
P/Invoke Programs that execute in the .NET runtime each play in their own private sandbox. Unlike the traditional Win32 programming model, a large and complicated layer of software exists between them and the operating system. But the .NET runtime does not completely replace the Win32 API, so there must be a way to bridge between the two worlds. This bridge is called Platform Invocation Service (in short, P/Invoke or PInvoke).
Framework Class Library (FCL) The .NET Framework Class Library (FCL), or .NET Framework for short (or even .NET Fx), is a class hierarchy with a design similar to that of Borland's VCL. Features also available in the VCL are actually quite similar, although the .NET Framework has a much larger set of classes (covering many more areas of programming). The architectural difference between the .NET Framework and the VCL is that the .NET Framework is not only callable from other languages, it can be directly extended by other languages. This means that, as a Delphi programmer, you have the ability to directly inherit from classes in the .NET Framework, just as you would inherit from any other Delphi class. Moreover, the CLI gives you the ability to extend a class written in any language that has a compiler that targets the .NET runtime. The factorable part of the FCL means that parts of the class hierarchy can be factored out; for example, to create a stripped-down version for use on a handheld device.
Extended Portable Executable (PE) File Format Microsoft is using its standard Portable Executable (PE) file format (the file format used by standard Win32 executable files) to support the metadata requirements of the CLI. The advantage of using this format is that the operating system can load a .NET application the same way it loads a native Win32 application. Common loading is about where the similarity ends though, since all managed code is in a special section. The framework modifies the loader so when it finds it is dealing with a .NET entity, it passes control over to the CLR that then works out how to call the managed entry points.
The CLI is a specification, and the CLR is Microsoft's implementation of that specification. Not surprisingly, the CLR is a superset of the specification. To a programmer, the CLR is an all-encompassing run-time library that unifies the vast array of services provided by the Windows operating system and presents them to you in an object-oriented framework.
On a larger scale, the CLR is responsible for every aspect of the .NET universe: loading the executable image, verifying its identity and type safety, compiling CIL code into native CPU instructions, and managing the application during its lifetime. CIL code that is intended to be run in the CLR is called managed code, whereas all other code (such as Intel executable code produced by Delphi 7) is unmanaged.
Closely related to the Common Type System, the CLS is a subset of that specification that defines the rules that govern how types created in different programming languages can interoperate. Not all languages are created equal, and some languages have features that can't be found elsewhere. The CLS tries to find a happy medium, specifying those items that a large set of languages can implement. Microsoft tried to make the CLS as small as possible, yet still accommodate a large set of languages.
Some features of Delphi are not CLS compliant. This does not mean the code cannot be executed by the CLR and work on the .NET platform; it only means you are using a language feature that can't be supported by other languages, so that particular part of your code cannot be used by other .NET applications if they are written in languages other than Delphi.
If you have experience with Java, you might find some of these items familiar. The .NET platform shares many concepts with the Java platform, in particular the intermediate language and virtual execution system, although with the core difference of Java being pcode interpreted by default, whereas .NET is invariably JIT compiled. There are also relevant analogies in the class libraries. In most cases, however, don't take corresponding Java concepts for granted, because differences in the implementation details might significantly affect how you use an apparently similar feature.
The various specifications of the CLI give a glimmer of hope about cross-platform development. But, you probably won't hear the "write once, run anywhere," mantra from Microsoft. This is due to the fact that they consider the user interface to be a key part of an application, and a normal PC screen compared to, say, a mobile phone screen has inherently different capabilities. There are two major ongoing efforts to implement the CLI on other operating systems. The Rotor project (officially called Microsoft Shared Source CLI or MS SSCLI) is written by the Microsoft Research team. Rotor consists of a large donation of software, under Microsoft's Shared Source license, and the necessary tools to build it on the FreeBSD, Win2, and Mac OS X 10.2 operating systems.
The second well-known CLI implementation on another operating system is the Mono Project, which supports Win32 and Linux. Mono is building what is essentially a clean-room implementation of the CLI on Linux and is released with a much more open license. Borland has a wait-and-see approach about Mono; there has been no word about whether Delphi will be a player in that arena.
The Rotor project seems to be aimed toward educators and people who are just curious about how the CLR is implemented, because its license is very open to academia but prohibits any commercial use. The Mono project, however, may give Microsoft some serious competition. The most serious stumbling block to true portability will probably continue to be the graphical user interface. The Rotor project does not include any GUI elements, while Mono has started developing WinForms using the WINE library. There is also a GTK# project associated with Mono, which is a binding of the GTK+ toolkit to the C# language.
Today's computing environment is a nebulous mass of diverse possibilities. Handheld devices have complex operating systems of their own, and Microsoft has a keen interest in this arena. Microsoft is working on a version of the .NET platform called the .NET Compact Framework, which is intended for handheld devices. The .NET platform could also serve as a springboard for tomorrow's technologies. Sixty-four bit processors are just around the corner, and .NET will no doubt be running there.
Does this mean you can run your managed application on Linux, 64-bit Windows, and your PDA? No. It is not reasonable to expect a user interface geared for a 1600×1200 pixel display on a 21-inch monitor to port to a handheld device. So, regarding .NET as a cross-platform tool, you will gain some advantage, but you should not expect your application to be a straight port especially if you are porting to another operating system or a handheld device.
In the following sections, we will examine some components of the CLI in more detail.
Microsoft coined the term assembly to denote a "unit of deployment" in the .NET runtime environment. In manufacturing, an assembly is a group of separate but related parts that are put together to form a single functional unit. Sometimes an assembly consists of only one part, but it might consist of many. So it is with assemblies in .NET.
Typically, an assembly consists of only one file—either a single executable or a dynamic link library. An assembly could consist of a group of related DLLs, or a DLL and associated resources like images or strings. Going back to the manufacturing analogy, each assembly, whether it consists of one part or multiple parts, contains a packing slip called a manifest. A manifest describes the contents of something, and an assembly manifest contains the metadata for an assembly. The manifest is the implementation of the extensible metadata system described in the CLI.
As you look through the installation directory for the Delphi for .NET Preview compiler, you will find a units folder. In this folder are a number of files with the extensions .dcua and .dcuil. These are not PE files, so they cannot be examined with ILDASM.
A .dcua file catalogs the namespaces and types in a .NET assembly (which, keep in mind, can consist of multiple executable files). A .dcuil file corresponds to a namespace. The .dcuil file contains all the compiler symbols for a namespace, as well as references to the .dcua files that contribute to that namespace (an assembly can contribute types to more than one namespace).
A namespace is a hierarchical containment mechanism that organizes classes and other types. We will discuss namespaces in more detail in Chapter 25, "Delphi for .NET Preview: The Language and the RTL."
An application is built against a certain set of assemblies; the application is said to reference these assemblies. When the set of assemblies changes, the compiler must rebuild any .dcuil files that contain references to .dcua files that are no longer in the set. Likewise, if a change is made to an assembly, the compiler must rebuild the corresponding .dcua and .dcuil files for that assembly. A .dcuil file is roughly equivalent to a Delphi .dcu file, but a .dcua file does not have a corresponding file type in the Delphi for Win32 universe.
Different project types produce different types of .NET assemblies: A Delphi program produces an executable assembly; a library project produces a DLL assembly.