Changes

The problem with programming is that computers think exclusively in numbers (the numbers 0 and 1 to be precise) known as ''machine code'' while humans communicate using words. In the very early days programmers actually entered machine code directly into computers to program them. This, as you can imagine, was a laborious and error prone process. The next evolution was to associate brief human readable commands with the corresponding machine code. For example, a programmer could enter the command ''MOV'' to transfer a value from one microprocessor register to another. These commands would then be translated into machine code by a piece of software called an ''assembler'', thereby giving this command syntax the name ''Assembly LanaguageLanguage''.

Next came a series of ''high level'' languages designed to make it easier for humans to write programs. These programs are written using a human readable syntax and then either compiled to machine code by a ''compiler'' or interpreted on behalf of the processor by an ''interpreter''. Such languages include BASIC, COBOL, Pascal and Fortran. One other such language is called ''C'' which was created at AT&T Bell Labs in the late 1960's 1960s and early 1970's1970s. In the late 1970's 1970s and early 1980's 1980s work started on an object oriented approach to C programming culminating in a new, object oriented variant of C known as ''C++''.

In order to address the short-comings of C and C++, Sun Microsystems started work on a new programming language and execution environment in the 1990's1990s. The end result was called Java. Java consists of a programming language with many of the pitfalls of C++ removed, a portable intermediate byte code format, a runtime environment (called the ''virtual machine'') that executes the byte code and handle issues such as memory management, and a vast suite of libraries providing all the functionality required to develop enterprise class applications (such as networking, file handling, database access, graphics etc).

"What does all this history have to do with C#?" I hear you ask. Well, the origins of the C# programming syntax can be traced right back to C and C++. If you are already familiar with C or C++ then you have a big head start in terms of learning C#. In fact the same can be said of syntax similarities between Java, C, C++ and C# syntax. In addition, C# also inherits many of the benefits of Java in terms of memory handling (better known as ''garbage collection'') and an intermediate byte code that negates the need to recompile an application for each target hardware platform. C# is also accompanied by a vast framework of libraries designed to provide the programmer with ready made readymade solutions to just about every imaginable scenario.

Despite these similarities there are differences between the Java and C# infrastructures work. The remainder of this chapter will be dedicated to providing and an overview of the C# infrastructure.

C# is an object oriented programming language. It is essentially a standard defining what constitutes valid syntax. On its own C# is actually of little use because it is dependent upon something called the ''Common Language Infrastructure'' (CLI) both for compilation and execution of applications. The CLI in turn, is actually a standard which defines specifications for the following components:

Unlike the C and C++ compilers which compile source code down to the machine code understood by the target microprocessor, the C# compiler compiles to an intermediate byte code format known as the Common Intermediate Language (CIL). This code can, in theroytheory, be take taken to any system where there is a CLI compliant Virtual Execution System (VES) and executed. There is, therefore, no need to compile an application for each and every target platform.

The VES (usually referred to as the ''runtime'') is the environment in which the CIL byte code is executed. The VES reads the byte code generated by the C# compiler and uses something called a ''Just in Time (JIT)'' compiler to compile the byte code down to the native machine code of the processor on which it is running. While this code is executing it does so in conjunction with a runtime agent which essentially manages the execution process. As a result, this executing code is known as ''managed code'' and the process handles issues such as garbage collection (to handle memory allocation and deallocationde-allocation), memory access and type safety to ensure that the code does not do anything it is not supposed to do.

It is worth noting that the JIT process can introduce a startup delay on execution of an application. One option available with .Net to avoid this problem is to pre-compile CLI byte code down to native machine code using the ''NGEN'' compiler. Because the NGEN compilation must take place on the target processor architecture this is step is often performed at the point that the application in question is installed by the user.

The CLS is a essentially a subset of the CTS aimed at creating interoperable libraries.

Other significant open source implementations are the Mono and DotGNU projects targeted at Windows, Linux and Unix UNIX platforms.