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 , 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 we obtain platform independence for .NET, in much the same way as compiling to Java byte code gives Java platform independence. Performance improvement
IL is actually a bit more ambitious than Java byte code. IL is always Just-In-Time compiled (known as JIT), 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.
Instead of compiling the entire application in one go (which could lead to a slow start-up time), the JIT compiler simply compiles each portion of code as it is called (just-in-time). When code has , 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, since 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.
实际上，IL比Java字节代码的作用还要大。IL总是即时编译的(简称JIT)，而Java字节代码常常是解释型的，Java的一个缺点是，在运行应用程序时，把Java字节代码转换为内部可执行代码的过程可可能导致性能的损失。
JIT编译器并不是把整个应用程序一次编译完(这样会有很长的启动时间)，而是只编译它调用的那部分代码。代码编译过一次后，得到的内部可执行代码就存储起来，直到退出该应用程序为止，这样在下次运行这部分代码时，就不需要重新编译了。Microsoft认为这个过程要比一开始就编译整个应用程序代码的效率高得
多，因为任何应用程序的大部分代码实际上并不是在每次运行过程中都执行。使用JIT编译器，从来都不会编译这种代码从来都不会被编译。
visual studio和vs code的区别这解释了为什么托管IL代码的执行几乎和内部机器代码的执行速度一样快，但是并没有说明为什么Microsoft认为这会提高性能。其原因是编译过程的最后一部分是在运行时进行的，JIT编译器确切地知道程序运行在什么类型的处理器上，利用该处理器提供的任何特性或特定的机器代码指令来优化最后的可执行代码。
传统的编译器会优化代码，但它们的优化过程是独立于代码所运行的特定处理器的。这是因为传统的编
译器是在发布软件之前编译为内部机器可执行的代码。即编译器不知道代码所运行的处理器类型，例如该处
理器是x86兼容处理器或Alpha处理器，这超出了基本操作的范围。例如Visual Studio 6优化了一台一般的Pentium机器，所以它生成的代码就不能利用Pentium III处理器的硬件特性。相反，JIT编译器不仅可以进行Visual Studio 6所能完成的优化工作，还可以优化代码所运行的特定处理器。
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 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 interop
erable 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 let’s briefly discuss how some of the other common languages fit into .NET.
Visual Basic .NET
Visual Basic .NET has undergone a complete revamp from Visual Basic 6 to bring it up-to-date with
.NET. The way that Visual Basic has evolved over the last few years means that in its previous version, Visual Basic 6, it was not a suitable language for running .NET programs. For example, it is heavily integrated into 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, and the changes that were made to the language are so extensive you might as well regard Visual Basic .NET as a new language. Existing Visual Basic 6 code does not compile as Visual Basic .NET code. Converting a Visual Basic 6 program to Visual Basic .NET requires extensive changes to the code. However, Visual Studio .NET (the upgrade of VS 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 .NET, it will upgrade the project for you, which means that it will rewrite the Visual Basic 6 source code into Visual Basic .NET 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 .NET 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 .NET to native executable code. Visual Basic .NET compiles only to IL, just as C# does. If you need to continue coding in Visual Basic 6, you may 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++ .NET
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, then 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 .NET. The advantage
of using managed C++ over C# code is that we 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 is recognized by the CLR as memory type safe, then you’ll need to write your source code
in some other language (such as C# or Visual Basic .NET).
Visual J# .NET
The latest language to be added to the mix is Visual J# .NET. 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 .NET. 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, XMLWeb services, and everything else that is possible—just as C# and Visual
Basic .NET 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. We 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 serverside
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 29). 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.
托管代码的优点
Microsoft中间语言与Java字节代码共享一种理念：它们都是一种低级语言，语法很简单，可以非常快速地转换为机器码。对于代码来说，这种精心设计的通用语法，有很大的优点。
1. 平台无关性

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