2: Managed Code and the Common Language Runtime in C#

Drawer ECC200 in C# 2: Managed Code and the Common Language Runtime

2: Managed Code and the Common Language Runtime
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As you ll recall from 1, with Active Server Pages (ASP), the two scripting language choices were Visual Basic Scripting Edition (VBScript) and JScript. ASP pages were placed in directories with the proper Internet Information Services (IIS) permissions, and when a client requested a page, any script code on the page was interpreted and the finished HTML for the page was returned to the client. This provided a convenient development model even for nonprogrammers. Unlike with Internet Server Application Programming Interface (ISAPI) or COM+ components, the code on an ASP page could be changed as needed. Barring client-side caching of a page, any change was immediately visible to clients. One of the most significant improvements that ASP.NET offers is the way code is used at runtime. As mentioned in 1, although the ASP.NET and ASP development models are similar, the two technologies differ quite a bit behind the scenes. Instead of interpreting the page source each time a client requests a page, ASP.NET seamlessly compiles the page to Microsoft intermediate language (MSIL) code the first time the page is requested. Once the page is compiled in MSIL, the just-in-time (JIT) compiler converts the MSIL to native code. Note Rather than wait for an ASP.NET page to be compiled in MSIL on first use, it s also possible to force all pages in a site to be compiled in MSIL at one time, thus catching syntax errors and other compile-time errors in a single step. The easiest way to compile all files at once is to build the application using Microsoft Visual Studio .NET. In this chapter, I ll first introduce you to the .NET Framework, and then we ll look more closely at MSIL and the JIT compiler. Once you re familiar with MSIL code, I ll explain how the JIT compiler enables ASP.NET to use the same development model as ASP. Next we ll look at managed code and data, including some of the benefits and costs of using managed code. Finally, I ll talk about unsafe code, code that exists outside the managed runtime.
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The .NET Framework is designed from the ground up to allow developers of both Web and traditional applications to build their applications more efficiently and enable them to work more flexibly. One of the most significant features of the .NET Framework is that it enables code written in multiple languages to work together seamlessly. Figure 2-1 shows the structure of the .NET Framework at a very high level.
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Figure 2-1 : The .NET Framework architecture Underlying the entire framework are system services. In the current implementation, this base is the Win32 API and COM+ services, although the abstraction would allow any operating system to provide the services, in theory if not in practice. Traditionally, applications have called the operating system s API directly. In the Win32 programming world, this model is difficult for Visual Basic programmers because some APIs require using data structures that are convenient for C/C++ programmers but much less convenient for Visual Basic programmers. Layered on top of the system services is the common language runtime. The runtime loads and runs code written in any language that targets the runtime. Code targeted to the runtime is called managed code. (I ll describe managed code in detail later in this chapter.) The runtime also provides integrated, pervasive security. Previous Win32 environments provided security only for file systems and network resources, if at all. For example, file security on Microsoft Windows NT and Microsoft Windows 2000 is available only for volumes formatted using NTFS. The runtime provides code access security that allows developers to specify the permissions required to run the code. At load time and as methods are called, the runtime can determine whether the code can be granted the access required. Developers can also explicitly specify limited permissions, meaning that code designed to do something simple and not very dangerous can seek the minimal permissions. Compare this situation to today s VBScript-enabled mail readers, such as Microsoft Outlook, that have been targeted by virus developers. Even on a secure system, if a user with Administrator rights opens a VBScript virus, the script can do whatever the administrator can do. The role-based security that the runtime provides allows permissions to be set based on the user on whose behalf the code is running. Relying on the runtime are the .NET Framework classes. The .NET Framework classes provide classes that can be called from any .NET-enabled programming language. The classes follow a coherent set of naming and design guidelines in mind, making it easier for developers to learn the classes quickly. We ll introduce the class libraries in 3; they cover virtually all the areas a developer would expect, from data access services to threading and networking. On the top of the .NET Framework class library is ADO.NET and XML data. ADO.NET is a set of classes that provide data access support for the .NET Framework. ADO.NET is based on ADO but is designed to work with XML and to work in a disconnected environment. On top of ADO.NET and XML lies specific support for two different types of applications. One is the traditional client application that uses Windows Forms, a combination of what Visual Basic and the Microsoft Foundation Class Library (MFC) had to offer. The other type of application available is ASP.NET, including Web Forms, and XML Web services.
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On top of ASP.NET and the Windows Forms is the common language specification (CLS) and the languages that follow the CLS. The CLS is a set of rules that a CLS compliant language needs to follow, ensuring that each language has a common set of features.
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