The Common Language Runtime

Central to the .NET Framework is its runtime execution environment, known as the Common Language Runtime (CLR) or the .NET runtime. Code running under the control of the CLR is often termed managed code.

However, before it can be executed by the CLR, any source code that you develop (in C# or some other language) needs to be compiled. Compilation occurs in two steps in .NET:

1. Compilation of source code to IL

2. Compilation of IL to platform-specific code by the CLR

This two-stage compilation process is very important, because the existence of the IL (managed code) is the key to providing many of the benefits of .NET.

Advantages of Managed Code

Microsoft Intermediate Language shares with Java byte code the idea that it is a low-level language with a simple syntax (based on numeric codes rather than text), which can be very quickly translated into native machine code. Having this well-defined universal syntax for code has significant advantages.

Platform Independence

First, it means that the same file containing byte code instructions can be placed on any platform; at runtime the final stage of compilation can then be easily accomplished so that the code will run on that particular platform. In other words, by compiling to IL you obtain platform independence for .NET, in much the same way as compiling to Java byte code gives Java platform independence.

You should note that the platform independence of .NET is only theoretical at present because, at the time of writing, a complete implementation of .NET is only available for Windows. However, a partial implementation is available (see, for example, the Mono project, an effort to create an open source implementation of .NET, at www.go-mono.com).

Performance Improvement

Although we previously made comparisons with Java, IL is actually a bit more ambitious than Java byte code. IL is always Just-in-Time compiled (known as JIT compilation), whereas Java byte code was often interpreted. One of the disadvantages of Java was that, on execution, the process of translating from Java byte code to native executable resulted in a loss of performance (with the exception of more recent cases, where Java is JIT compiled on certain platforms).

Instead of compiling the entire application in one go (which could lead to a slow startup time), the JIT compiler simply compiles each portion of code as it is called (just in time). When code has been compiled once, the resultant native executable is stored until the application exits, so that it does not need to be recompiled the next time that portion of code is run. Microsoft argues that this process is more efficient than compiling the entire application code at the start, because of the likelihood that large portions of any application code will not actually be executed in any given run. Using the JIT compiler, such code will never be compiled.

This explains why we can expect that execution of managed IL code will be almost as fast as executing native machine code. What it doesn’t explain is why Microsoft expects that we will get a performance improvement. The reason given for this is that, because the final stage of compilation takes place at runtime, the JIT compiler will know exactly what processor type the program will run on. This means that it can optimize the final executable code to take advantage of any features or particular machine code instructions offered by that particular processor.

Traditional compilers will optimize the code, but they can only perform optimizations that are independent of the particular processor that the code will run on. This is because traditional compilers compile to native executable before the software is shipped. This means that the compiler doesn’t know what type of processor the code will run on beyond basic generalities, such as that it will be an x86-compatible processor or an Alpha processor. Visual Studio 6, for example, optimizes for a generic Pentium machine, so the code that it generates cannot take advantage of hardware features of Pentium III processors. On the other hand, the JIT compiler can do all the optimizations that Visual Studio 6 can, and in addition it will optimize for the particular processor that the code is running on.

Language Interoperability

The use of IL not only enables platform independence; it also facilitates language interoperability. Simply put, you can compile to IL from one language, and this compiled code should then be interoperable with code that has been compiled to IL from another language.

You’re probably now wondering which languages aside from C# are interoperable with .NET, so the following sections briefly discuss how some of the other common languages fit into .NET.

Visual Basic 2005

Visual Basic .NET 2002 underwent a complete revamp from Visual Basic 6 to bring it up to date with the first version of the .NET Framework. The Visual Basic language itself had dramatically evolved from VB6, and this meant that VB6 was not a suitable language for running .NET programs. For example, VB6 is heavily integrated into Component Object Model (COM) and works by exposing only event handlers as source code to the developer - most of the background code is not available as source code. Not only that; it does not support implementation inheritance, and the standard data types Visual Basic 6 uses are incompatible with .NET.

Visual Basic 6 was upgraded to Visual Basic .NET in 2002, and the changes that were made to the language are so extensive you might as well regard Visual Basic as a new language. Existing Visual Basic 6 code does not compile to the present Visual Basic 2005 code (or to Visual Basic .NET 2002 and 2003 for that matter). Converting a Visual Basic 6 program to Visual Basic 2005 requires extensive changes to the code. However, Visual Studio 2005 (the upgrade of Visual Studio for use with .NET) can do most of the changes for you. If you attempt to read a Visual Basic 6 project into Visual Studio 2005, it will upgrade the project for you, which means that it will rewrite the Visual Basic 6 source code into Visual Basic 2005 source code. Although this means that the work involved for you is heavily cut down, you will need to check through the new Visual Basic 2005 code to make sure that the project still works as intended because the conversion might not be perfect.

One side effect of this language upgrade is that it is no longer possible to compile Visual Basic 2005 to native executable code. Visual Basic 2005 compiles only to IL, just as C# does. If you need to continue coding in Visual Basic 6, you can do so, but the executable code produced will completely ignore the .NET Framework, and you’ll need to keep Visual Studio 6 installed if you want to continue to work in this developer environment.

Visual C++ 2005

Visual C++ 6 already had a large number of Microsoft-specific extensions on Windows. With Visual C++ .NET, extensions have been added to support the .NET Framework. This means that existing C++ source code will continue to compile to native executable code without modification. It also means, however, that it will run independently of the .NET runtime. If you want your C++ code to run within the .NET Framework, you can simply add the following line to the beginning of your code:

  #using <mscorlib.dll> 

You can also pass the flag /clr to the compiler, which then assumes that you want to compile to managed code, and will hence emit IL instead of native machine code. The interesting thing about C++ is that when you compile to managed code, the compiler can emit IL that contains an embedded native executable. This means that you can mix managed types and unmanaged types in your C++ code. Thus the managed C++ code

  class MyClass { 

defines a plain C++ class, whereas the code

  __gc class MyClass { 

will give you a managed class, just as if you’d written the class in C# or Visual Basic 2005. The advantage of using managed C++ over C# code is that you can call unmanaged C++ classes from managed C++ code without having to resort to COM interop.

The compiler raises an error if you attempt to use features that are not supported by .NET on managed types (for example, templates or multiple inheritance of classes). You will also find that you will need to use nonstandard C++ features (such as the _gc keyword shown in the previous code) when using managed classes.

Because of the freedom that C++ allows in terms of low-level pointer manipulation and so on, the C++ compiler is not able to generate code that will pass the CLR’s memory type safety tests. If it’s important that your code be recognized by the CLR as memory type safe, you’ll need to write your source code in some other language (such as C# or Visual Basic 2005).

Visual J# 2005

The latest language to be added to the mix is Visual J# 2005. Prior to .NET Framework 1.1, users were able to use J# only after making a separate download. Now the J# language is built into the .NET Framework. Because of this, J# users are able to take advantage of all the usual features of Visual Studio 2005. Microsoft expects that most J++ users will find it easiest to use J# if they want to work with .NET. Instead of being targeted at the Java runtime libraries, J# uses the same base class libraries that the rest of the .NET-compliant languages use. This means that you can use J# for building ASP.NET Web applications, Windows Forms, XML Web services, and everything else that is possible - just as C# and Visual Basic 2005 can.

Scripting Languages

Scripting languages are still around, although in general, their importance is likely to decline with the advent of .NET. JScript, on the other hand, has been upgraded to JScript .NET. You can now write ASP.NET pages in JScript .NET, run JScript .NET as a compiled rather than an interpreted language, and write strongly typed JScript .NET code. With ASP.NET there is no reason to use scripting languages in server-side Web pages. VBA is, however, still used as a language for Microsoft Office and Visual Studio macros.

COM and COM+

Technically speaking, COM and COM+ aren’t technologies targeted at .NET, because components based on them cannot be compiled into IL (although it’s possible to do so to some degree using managed C++, if the original COM component was written in C++). However, COM+ remains an important tool, because its features are not duplicated in .NET. Also, COM components will still work - and .NET incorporates COM interoperability features that make it possible for managed code to call up COM components and vice versa (this is discussed in Chapter 23, “COM Interoperability”). In general, however, you will probably find it more convenient for most purposes to code new components as .NET components, so that you can take advantage of the .NET base classes as well as the other benefits of running as managed code.