Understanding .NET (2nd Edition)

The heart of .NET is the .NET Framework. First released in 2002, it brought enormous change to the lives of those who write Windows software and the people who manage them. Figure 1-1 shows the Framework's two main parts: the CLR and the .NET Framework class library. A .NET application always uses the CLR, and it can also use whatever parts of the class library it requires.

Every application written using the Framework depends on the CLR. Among other things, the CLR provides a common set of data types, acting as a foundation for C#, VB, and all other languages that target the .NET Framework. Because this foundation is the same no matter which language they choose, developers see a more consistent environment.

Applications written in any .NET language can use the code in the .NET Framework class library. Among the most important technologies provided in this library are the following:

• ASP.NET: Classes focused on building browser-accessible applications.

• Windows Forms: Classes for building Windows graphical user interfaces (GUIs) in any CLR-based programming language.

• ASP.NET Web Services (also called ASMX): Classes for creating applications that communicate with other applications using Web services.

• Enterprise Services: Classes that provide distributed transactions, object instance control, and other services useful for building reliable, scalable applications.

• ADO.NET: Classes focused on accessing data stored in relational database management systems (DBMS).

The .NET Framework class library contains much more than this short list indicates. Among the other services it provides are support for creating and working with XML documents, a variety of security services, and mechanisms for interoperating with applications built using older Windows technologies such as the Component Object Model (COM).

As this short description suggests, the .NET Framework class library can be used to create many different types of applications. And because all of the services in this library are built on the CLR, applications can combine them as needed. A browser application built using ASP.NET, for example, might use ADO.NET to access stored data and Enterprise Services to perform distributed transactions.

Software that uses the .NET Framework (and thus relies on the CLR) is referred to as managed code. As Figure 1-2 shows, an application can be built solely from managed code, relying entirely on the CLR and the relevant parts of the .NET Framework class library. An application can also be built from a combination of managed code and ordinary unmanaged code, with the two interacting as necessary. This second option, shown on the right side of the figure, is especially important for existing applications. Most new Windows applications created today are built wholly in managed code, but it can also be useful to extend pre-.NET applications with managed code. And although it's not shown in the figure, it's still possible to create new Windows applications entirely in unmanaged codeusing the .NET Framework isn't obligatory.

Managed code is typically object-oriented, so the objects it creates and uses are known as managed objects. A managed object can use and inherit from another managed object even if the two are written in different languages. This fact is a key part of what makes the .NET Framework class library an effective foundation: Objects written in any CLR-based language can inherit and use the class library's code. Given the fundamental role played by the CLR, understanding the .NET Framework begins with understanding this runtime environment.

The Common Language Runtime

Built from scratch to support modern applications, the CLR embodies a current view of what a programming environment should be. While it's hard to claim complete originality for any idea in computer science today, it is fair to say that this essential .NET technology takes an interesting new approach to programming languages.

What the CLR Defines

Think about how a programming language is typically defined. Each language commonly has its own unique syntax, its own set of control structures, a unique set of data types, its own notions of how classes inherit from one another, and much more. The choices a language designer makes are driven by the target applications for the language, who its users are meant to be, and the designer's own sensibilities.

Yet most people agree on much of what a modern general-purpose programming language should provide. While opinions on syntax differsome developers love curly braces, others abhor themthere's widespread agreement on what semantics a language should offer. Given this, why not define a standard implementation of those semantics, then allow different syntaxes to be used to express those semantics?

The CLR provides this standard implementation. By providing a common set of data types such as integers, strings, classes, and interfaces, specifications for how inheritance works, and much more, it defines a common set of semantics for languages built on it. The CLR says nothing about syntax, however. How a language looks, whether it contains curly braces or semicolons or anything else, is entirely up to the language designer. While it is possible to implement languages with varying behaviors on top of the CLR, the CLR itself provides a consistent, modern set of semantics for a language designer to build on.

Along with its standard types, the CLR provides other fundamental services. Those services include the following:

• Garbage collection, which automatically frees managed objects that are no longer referenced.

• A standard format for metadata, information about each type that's stored with the compiled code for that type.

• A common format, called assemblies, for organizing compiled code. An assembly can consist of one or more Dynamic Link Libraries (DLLs) and/or executables (EXEs), and it includes the metadata for the classes it contains. A single application might use code from one or more assemblies, and so each assembly can specify other assemblies on which it depends.

Using the CLR

The CLR was not defined with any particular programming language in mind. Instead, its features are derived largely from popular existing languages, such as C++, the pre-.NET version of VB, and Java. Today, Microsoft provides several CLR-based languages, including C#, the .NET version of VB, an extended version of C++, and others. Third parties also provide languages built on the CLR.

No matter what language it's written in, all managed code is compiled into Microsoft Intermediate Language (MSIL) rather than a machine-specific binary. MSIL (also referred to as just IL) is a set of CPU-independent instructions for performing typical operations such as loading and storing information and calling methods. Each DLL and EXE in an assembly contains MSIL rather than processor-specific code. Installing a .NET Framework application on your system really means copying to your disk files that contain MSIL rather than a machine-specific binary. When the application is executed, MSIL is transformed into native code before it's executed.

Figure 1-3 illustrates the process of compiling and executing managed code. Source code written in VB, C#, or another language that targets the CLR is first transformed into MSIL by the appropriate language compiler. As the figure shows, the compiler also produces metadata that's stored in the same file as the MSIL. Before execution, this MSIL is compiled into native code for the processor on which the code will run. By default, each method in a running application is compiled the first time that method is called. Because the method is compiled just in time to execute it, this approach is called just in-time (JIT) compilation.

One point worth noting is that any language built on the CLR should exhibit roughly the same performance as any other CLR-based language. Unlike the pre-.NET world, where the performance difference between VB and C++ was sometimes significant, a .NET Framework application written in C# isn't noticeably faster than the same application written in VB. While some compilers may produce better MSIL code than others, large variations in execution speed are unlikely.

The CLR is the foundation of everything else in the .NET Framework. All code in the .NET Framework class library depends on it, as do all Framework-based applications. Chapter 2 provides a more detailed look at the technology of the CLR.

The .NET Framework Class Library

The .NET Framework class library is exactly what its name suggests: a library of classes and other types that developers can use to make their lives easier. While these classes are themselves written in C#, they can be used from any CLR-based language. Code written in C#, VB, C++, or any other language supported by the .NET Framework can create instances of these classes and call their methods. That code can also rely on the CLR's support for inheritance to inherit from the library's classes.

Surveying the Library

The contents of the .NET Framework class library are organized into a tree of namespaces. Each namespace can contain types, such as classes and interfaces, and other namespaces. Figure 1-4 shows a very small part of the .NET Framework class library's namespace tree. The namespaces shown include the following:

• System: The root of the tree, this namespace contains all of the other namespaces in the .NET Framework class library. System also contains the core data types used by the CLR (and thus by languages built on the CLR). These types include several varieties of integers, a string type, and many more.

• System.Web: This namespace contains types useful for creating Web applications, and like many namespaces, it has subordinate namespaces. Developers can use the types in System.Web.UI to build ASP.NET browser applications, for example, while those in System. Web.Services are used to build ASP.NET Web Services applications.

• System.Data: The types in this namespace comprise ADO.NET. For example, the Connection class is used to establish connections to a database management system (DBMS), while an instance of the DataSet class can be used to cache and examine the results of a query issued against that DBMS.

• System.Windows.Forms: The types in this namespace make up Windows Forms, and they're used to build Windows GUIs. Rather than relying on language-specific mechanisms, such as the older Microsoft Foundation Classes (MFC) in C++, .NET Framework applications written in any programming language use this common set of types to build graphical interfaces for Windows.

• System.EnterpriseServices: The types in this namespace provide services required for some kinds of enterprise applications. Implemented by COM+ in the pre-NET world, these services include distributed transactions, object instance lifetime management, and more. The most important type in this namespace, one from which classes must inherit to use Enterprise Services, is the ServicedComponent class.

• System.XML: Types in this namespace provide support for creating and working with XML-defined data. The XmlDocument class, for instance, allows accessing an XML document using the Document Object Model (DOM). This namespace also includes support for technologies such as the XML Schema definition language (XSD) and XPath.

Many more namespaces are defined, providing support for file access, serializing an object's state, remote access to objects, and much more. In fact, the biggest task facing developers who wish to build on the .NET Framework is learning to use the many services that the library provides. There's no requirement to learn everything, however, so a developer is free to focus on only those things relevant to his or her world. Still, some parts will be relevant to almost everybody, and so the next sections provide a quick overview of some of this large library's most important aspects.

Building Web Applications: ASP.NET

Implemented in the System.Web namespace, ASP.NET is an important piece of the .NET Framework. The successor to the very popular Active Server Pages (ASP) technology, ASP.NET applications are built from one or more pages. Each page contains HTML and/or executable code, and typically has the extension .aspx. As Figure 1-5 shows, a request from a browser made via HTTP causes a page to be loaded and executed. Any output the page creates is then returned to the browser that made the request.

Building effective Web applications requires more than just the ability to combine code with HTML. Accordingly, ASP.NET provides a range of support, including the following:

• Web controls, allowing a developer to create a browser GUI in a familiar way. By dragging and dropping standard ASP.NET controls for buttons and other interface elements onto a form, it's possible to build GUIs for Web applications in much the same way as for local Windows applications.

• Mechanisms for managing an application's state information.

• Built-in support for maintaining information about an application's users, sometimes called membership information.

• Support for data binding, which allows easier access to information stored in a DBMS or some other data source.

Given the popularity of Web applications, ASP.NET probably impacts more developers than any other part of the .NET Framework class library. Chapter 5 provides more detail on this key component of the .NET Framework.

Accessing Data: ADO.NET

ADO.NET lets applications work with stored data. As Figure 1-6 shows, access to a DBMS relies on a .NET Framework data provider, written as managed code. Providers that allow access to SQL Server, Oracle, and other DBMS are included with the .NET Framework. They allow a client application to issue commands against the DBMS and examine any results those commands return. The result of a Structured Query Language (SQL) query, for example, can be examined in two ways. Applications that need only read the result a row at a time can do this by using a DataReader object to march through the result one record at a time. Applications that need to do more complex things with a query result, such as send it to a browser, update information, or store that information on disk, can instead have the query's result packaged inside a DataSet object.

As Figure 1-6 illustrates, a DataSet can contain one or more tables. Each table can hold the result of a different query, so a single DataSet might potentially contain the results of two or more queries, perhaps from different DBMS. In effect, a DataSet acts as an in-memory cache for data. As the figure shows, however, DataSets can hold more than just the result of a SQL query. It's also possible to read an XML document directly into a table in a DataSet without relying on a .NET Framework data provider. Data defined using XML has also become much more important in the last few years, so ADO.NET allows accessing it directly. While not all .NET Framework applications will rely on ADO.NET for data access, a large percentage surely will. ADO.NET is described in more detail in Chapter 6.

Building Distributed Applications

Creating software that communicates with other software is a standard part of modern application development. Yet different applications have different communication requirements. To meet these diverse needs, the .NET Framework class library includes three distinct technologies for creating distributed applications. Figure 1-7 illustrates these choices.

ASP.NET Web Services, mostly defined in System.Web.Services, allows applications to communicate using Web services. Since it's part of ASP.NET, this technology lets developers use a similar model for creating distributed software. As Figure 1-7 shows, applications that expose methods as Web services can be built from files with the extension .asmx, each of which contains only code. Clients make requests using the standard Web services protocol SOAP^[2], and the correct page is loaded and executed. Because this technology is part of ASP.NET, requests and replies also go through Internet Information Services (IIS), the standard Web server for Windows.

^[2] "SOAP" was originally an acronym for "Simple Object Access Protocol." Today, the standards group that owns this technology has decided that SOAP no longer stands for anythingit's just a name.

Communication via Web services is especially useful for interoperating with software built on platforms other than the .NET Framework, such as the Java environment. But it's not always the right solution. In some situations, the technology known as .NET Remoting, defined in the System.Runtime.Remoting namespace, is a better choice. Unlike ASP.NET Web Services, .NET Remoting focuses on direct communication between applications built on the .NET Framework. While it does support a version of SOAP, this technology also provides a binary protocol along with the ability to add extensions using any other protocol a developer needs. .NET Remoting isn't the most common choice for communication, but it can be important for some kinds of applications.

The third option for creating distributed applications using the .NET Framework is Enterprise Services. Defined in the System.EnterpriseServices namespace, it provides applications with services such as distributed transactions and more. Figure 1-7 illustrates this, showing a server application accessing two databases. If a single transaction included updates to both of these databases, Enterprise Services might well be the right choice for the application that used this transaction. Remote clients can communicate directly with an Enterprise Services application using DCOM, and it's also possible for an ASP.NET application to use Enterprise Services when necessary.

All three options make sense in different situations, and having a basic understanding of all three is useful. For a more detailed look at each of them, see Chapter 7.