.NET Tutorial for Beginners

.NET Tutorial for Beginners 3
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Published Date:12-07-2017
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India Community Initiative .NET Tutorial for Beginners Special thanks to the following who have put in sincere efforts to write and bring this tutorial together. Akila Manian (MVP) Ajay Varghese (MVP) Amit Kukreja Anand M (MVP) Aravind Corera (MVP) Arvind Rangan Balachandran Bipin Joshi (MVP) C S Rajagopalan G Gokulraj G Arun Prakash Gurneet Singh (MVP) Kunal Cheda (MVP) Manish Mehta (MVP) Narayana Rao Surapaneni (MVP) Pradeep Saurabh Nandu (MVP) Shankar N.S. Swati Panhale Reshmi Nair Content 1. Getting Ready .......................................................................................... 4 1.1 Tracing the .NET History..............................................................................4 1.2 Flavors of .NET...........................................................................................5 1.3 Features of .NET.......................................................................................10 1.4 Installing the .NET Framework SDK.............................................................12 2. Introduction to the .NET Initiative and the .NET Platform...................... 15 2.1 Understanding the Existing Development Scenario........................................15 2.2 Challenges faced by developers..................................................................18 2.3 NET Philosophy / Where does .NET fit in? ....................................................21 2.4 Understanding the .NET Platform and its layers ............................................25 2.5 Understanding the various components of the .NET Platform and the functions performed by them ........................................................................................30 2.6 Structure of a .NET Application...................................................................37 3. Code Management.................................................................................. 43 3.1 Introduction.............................................................................................43 3.2 First VB.NET / C program ........................................................................45 3.3 JIT (Just–in-Time Compiler) & Debugging....................................................51 3.4 Managed Vs. Unmanaged Methods/Transitions .............................................56 3.5 Summary ................................................................................................61 4. Language Features of C ....................................................................... 62 4.1 History of C ...........................................................................................62 4.2 Language Fundamentals in C ...................................................................63 4.3 Control Statements...................................................................................74 4.4 Arrays.....................................................................................................83 5. Language Features of VB.NET ................................................................ 88 5.1 History of VB.NET .....................................................................................88 5.2 Language Fundamentals in VB.NET.............................................................89 5.3 Features of VB.NET...................................................................................99 5.4 Control Statements................................................................................. 107 5.5 Arrays................................................................................................... 115 6. Object Oriented Programming Concepts .............................................. 122 6.1 Concept of Procedural Programming.......................................................... 123 6.2 Object Oriented Programming .................................................................. 126 6.3 Classes.................................................................................................. 127 6.4 Encapsulation......................................................................................... 127 6.5 Inheritance............................................................................................ 128 6.6 Polymorphism ........................................................................................ 129 6.7 Understanding CSharp and VB.NET as Object Oriented Programming languages ................................................................................................................. 132 6.8 Polymorphism ........................................................................................ 149 6.9 Abstract Classes (Virtual Class) ................................................................ 157 6.10 Interfaces ............................................................................................ 161 6.11 Delegates and Events............................................................................ 163 6.12 Structures............................................................................................ 168 6.13 Sample Application: OOPS ..................................................................... 170 7. Error and Exception Handling............................................................... 172 7.1 Need for Error Handling........................................................................... 172 7.2 Old-school unstructured exception handling in VB 6.0 and its disadvantages.. 173 7.3 Structured Exception Handling in C/VB.NET ............................................. 174 7.4 System.Exception: The mother of all exceptions......................................... 177 7.5 Handling exceptions that are not System.Exception compliant...................... 190 Catch.......................................................................................................... 191 7.6 Understanding Application exceptions (user-defined or custom exceptions).... 191 7.7 Nesting try/catch/finally blocks and re-throwing exceptions ......................... 198 7.8 Parting thoughts…................................................................................... 211 8. Assemblies and Application Domains ................................................... 212 8.1 Introduction........................................................................................... 212 8.2 Assembly Types...................................................................................... 212 8.3 Private Assemblies.................................................................................. 217 8.4 Shared Assemblies.................................................................................. 217 8.5 Application Domains................................................................................ 218 8.6 Conclusion............................................................................................. 223 1. Getting Ready Section Owner: Ajay Varghese (MVP) Content Contributors: Bipin Joshi (MVP) Welcome friends to the exciting journey of Microsoft .NET. If you are looking for information about what .NET is all about, what it can do for you or how it can help you and your customers, you have come to the right place. This section is intended to tell you about these and many more things. After covering this section you will be ready to delve into details of .NET. The section is divided into following sub-sections: 1) Tracing the .NET History 2) Flavors of .NET 3) Features of .NET 4) Installing .NET Framework SDK The first sub-section will introduce you with how .NET evolved and the path of .NET since its Beta releases. The second sub-section will introduce you with various flavors of...NET and their respective SDKs. It also gives overview of Visual Studio.NET – an excellent IDE for developing .NET applications. It is necessary to understand the features of .NET that make it robust, programmer friendly, powerful and flexible. The third sub-section is intended just for that. It gives overview of technical features that make .NET shine over traditional programming environments. The final sub-section tells you how to install .NET framework SDK, what are the system requirements and related topics. 1.1 Tracing the .NET History Sometime in the July 2000, Microsoft announced a whole new software development framework for Windows called .NET in the Professional Developer Conference (PDC). Microsoft also released PDC version of the software for the developers to test. After initial testing and feedback Beta 1 of .NET was announced. Beta 1 of the .NET itself got lot of attention from the developer community. When Microsoft announced Beta 2, it incorporated many changes suggested by the community and internals into the software. The overall ‘Beta’ phase lasted for more than 1 ½ years. Finally, in March 2002 Microsoft released final version of the .NET framework. One thing to be noted here is the change in approach of Microsoft while releasing this new platform. Unlike other software where generally only a handful people are involved in beta testing, .NET was thrown open to community for testing in it’s every pre-release version. This is one of the reasons why it created so many waves of excitement within the community and industry as well. Microsoft has put in great efforts in this new platform. In fact Microsoft says that its future depends on success of .NET. The development of .NET is such an important event that Microsoft considers it equivalent to transition from DOS to Windows. All the future development – including new and version upgrades of existing products – will revolve around .NET. So, if you want to be at the forefront of Microsoft Technologies, you should be knowing .NET Now, that we know about brief history of .NET let us see what .NET has to offer. 1.2 Flavors of .NET Contrary to general belief .NET is not a single technology. Rather it is a set of technologies that work together seamlessly to solve your business problems. The following sections will give you insight into various flavors and tools of .NET and what kind of applications you can develop. • What type of applications can I develop? When you hear the name .NET, it gives a feeling that it is something to do only with internet or networked applications. Even though it is true that .NET provides solid foundation for developing such applications it is possible to create many other types of applications. Following list will give you an idea about various types of application that we can develop on .NET. 1. ASP.NET Web applications: These include dynamic and data driven browser based applications. 2. Windows Form based applications: These refer to traditional rich client applications. 3. Console applications: These refer to traditional DOS kind of applications like batch scripts. 4. Component Libraries: This refers to components that typically encapsulate some business logic. 5. Windows Custom Controls: As with traditional ActiveX controls, you can develop your own windows controls. 6. Web Custom Controls: The concept of custom controls can be extended to web applications allowing code reuse and modularization. 7. Web services: They are “web callable” functionality available via industry standards like HTTP, XML and SOAP. 8. Windows Services: They refer to applications that run as services in the background. They can be configured to start automatically when the system boots up. As you can clearly see, .NET is not just for creating web application but for almost all kinds of applications that you find under Windows. • .NET Framework SDK You can develop such varied types of applications. That’s fine. But how? As with most of the programming languages, .NET has a complete Software Development Kit (SDK) - more commonly referred to as .NET Framework SDK - that provides classes, interfaces and language compilers necessary to program for .NET. Additionally it contains excellent documentation and Quick Start tutorials that help you learn .NET technologies with ease. Good news is that - .NET Framework SDK is available FREE of cost. You can download it from the MSDN web site. This means that if you have machine with .NET Framework installed and a text editor such as Notepad then you can start developing for .NET right now You can download entire .NET Framework SDK (approx 131 Mb) from MSDN web site at http://msdn.microsoft.com/downloads/default.asp?url=/downloads/sample.asp?url =/msdn-files/027/000/976/msdncompositedoc.xml • Development Tools If you are developing applications that require speedy delivery to your customers and features like integration with some version control software then simple Notepad may not serve your purpose. In such cases you require some Integrated Development Environment (IDE) that allows for Rapid Action Development (RAD). The new Visual Studio.NET is such an IDE. VS.NET is a powerful and flexible IDE that makes developing .NET applications a breeze. Some of the features of VS.NET that make you more productive are: - Drag and Drop design - IntelliSense features - Syntax highlighting and auto-syntax checking - Excellent debugging tools - Integration with version control software such as Visual Source Safe (VSS) - Easy project management Note that when you install Visual Studio.NET, .NET Framework is automatically installed on the machine. • Visual Studio.NET Editions Visual Studio.NET comes in different editions. You can select edition appropriate for the kind of development you are doing. Following editions of VS.NET are available: - Professional - Enterprise Developer - Enterprise Architect Visual Studio .NET Professional edition offers a development tool for creating various types of applications mentioned previously. Developers can use Professional edition to build Internet and Develop applications quickly and create solutions that span any device and integrate with any platform. Visual Studio .NET Enterprise Developer (VSED) edition contains all the features of Professional edition plus has additional capabilities for enterprise development. The features include things such as a collaborative team development, Third party tool integration for building XML Web services and built-in project templates with architectural guidelines and spanning comprehensive project life-cycle. Visual Studio .NET Enterprise Architect (VSEA) edition contains all the features of Visual Studio .NET Enterprise Developer edition and additionally includes capabilities for designing, specifying, and communicating application architecture and functionality. The additional features include Visual designer for XML Web services, Unified Modeling Language (UML) support and enterprise templates for development guidelines and policies. A complete comparison of these editions can be found at http://msdn.microsoft.com/vstudio/howtobuy/choosing.asp In addition to these editions, special language specific editions are available. They are: - Visual Basic.NET Standard Edition - Visual C Standard Edition - Visual C++ .NET Standard (soon to be released) These editions are primarily for hobbyist, student, or beginner who wants to try their hands on basic language features. A complete comparison of these standard editions with professional edition of VS.NET can be found at: http://msdn.microsoft.com/vcsharp/howtobuy/choosing.asp http://msdn.microsoft.com/vbasic/howtobuy/choosing.asp • .NET Redistributable In order to run application developed using .NET Framework the machine must have certain ‘runtime’ files installed. They are collectively called as .NET redistributable. This is analogous to traditional Visual Basic applications that required Visual Basic runtime installed on target computers. .NET redistributable provides one redistributable installer that contains the common language runtime (more on that later) and Microsoft .NET Framework components that are necessary to run .NET Framework applications. The redistributable is available as a stand-alone executable and can be installed manually or as a part of your application setup. You can download .NET redistributable at http://msdn.microsoft.com/downloads/default.asp?url=/downloads/sample.asp?url =/msdn-files/027/001/829/msdncompositedoc.xml More technical information about .NET redistributable can be found at http://msdn.microsoft.com/library/default.asp?url=/library/en- us/dnnetdep/html/dotnetfxref.asp Note that if you have installed .NET Framework SDK, there is no need of installing redistributable separately. Also, note that there is difference between .NET Framework SDK and .NET redistributable in terms of purpose and tools and documentation supplied. .NET Framework SDK is intended to ‘develop’ applications where as .NET redistributable is intended to ‘run’ .NET applications. • .NET and mobile development Now days the use of mobile and wireless devices is ever increasing. PDAs, mobile phones, Smartphones, handheld PCs and HTML pagers are becoming common. As compared to full blown desktop computers, Mobile devices are generally resource-constrained. There are limitations on what they can display and in which form. For example you can easily display graphical menus in desktop applications but the same may not be possible for cell phones. Today there are many vendors making CPUs and development tools for mobile devices. However, their standards are much varying. For example devices running Windows CE will have different tools and standards of development than Palm OS. Also, programming model for such devices is an issue of debate. For example, Wireless Application Protocol (WAP) was considered a ‘standard’ for mobile devices but it introduced disadvantages of its own such as requirement of continuous connectivity, lack in rich user interface and failure to utilize client – side resources effectively. Mobile devices can be broadly divided into two categories: 1) Mobile Devices that have certain client-side resources like PDAs, Smartphones and Handheld PCs. They can run stand-alone application with rich user interface. 2) Mobile Devices that lack even these client-side resources such as mobile phones. They can not run stand alone applications having rich and more interactive user interface. In order to encompass all possible devices from above categories Microsoft has developed two distinct technologies namely: - Microsoft .NET Compact Framework (.NET CF) - Microsoft Mobile Internet Toolkit (MMIT) o Microsoft .NET Compact Framework .NET compact framework is a sub set of entire .NET framework and is targeted at mobile devices having some client side resources. It provides support for managed code and XML Web services. Currently, .NET Compact Framework is in Beta 1 and is available on devices running the Windows CE or Windows CE .NET operating systems. However, Microsoft has promised support for other platforms in the future. As of now the framework supports Visual Basic.NET and C as development languages out of the box. Support for other languages is planned in near future. Microsoft is creating a set of extensions for Visual Studio .NET called Smart Device Extensions that will allow Visual Studio .NET developers to program for .NET Compact Framework. This means that developers familiar with Visual Studio.NET can start developing for mobile devices almost instantly. More information about .NET Compact Framework can be obtained at http://msdn.microsoft.com/vstudio/device/compact.asp o Microsoft Mobile Internet Toolkit Microsoft Mobile Internet Toolkit (MMIT) is designed to develop server side applications for mobile devices such as cell phones, PDAs, and pagers. It is different than .NET compact Framework in that it is a server side technology. It is ideal for devices that can not run stand alone applications. MMIT mainly uses ASP.NET as a technology for delivering markup to a wide variety of mobile devices. As we know that each mobile device has its own set of underlying standards and markup. MMIT shields these details from the developer and allows ‘uniform code’ for any target device. Based on the capabilities of target device the output is rendered. More information about MMIT can be obtained from http://msdn.microsoft.com/vstudio/device/mitdefault.asp 1.3 Features of .NET Now that we know some basics of .NET, let us see what makes .NET a wonderful platform for developing modern applications. • Rich Functionality out of the box .NET framework provides a rich set of functionality out of the box. It contains hundreds of classes that provide variety of functionality ready to use in your applications. This means that as a developer you need not go into low level details of many operations such as file IO, network communication and so on. • Easy development of web applications ASP.NET is a technology available on .NET platform for developing dynamic and data driven web applications. ASP.NET provides an event driven programming model (similar to Visual Basic 6 that simplify development of web pages (now called as web forms) with complex user interface. ASP.NET server controls provide advanced user interface elements (like calendar and grids) that save lot of coding from programmer’s side. • OOPs Support The advantages of Object Oriented programming are well known. .NET provides a fully object oriented environment. The philosophy of .NET is – “Object is mother of all.” Languages like Visual Basic.NET now support many of the OO features that were lacking traditionally. Even primitive types like integer and characters can be treated as objects – something not available even in OO languages like C++. • Multi-Language Support Generally enterprises have varying skill sets. For example, a company might have people with skills in Visual Basic, C++, and Java etc. It is an experience that whenever a new language or environment is invented existing skills are outdated. This naturally increases cost of training and learning curve. .NET provides something attractive in this area. It supports multiple languages. This means that if you have skills in C++, you need not throw them but just mould them to suit .NET environment. Currently four languages are available right out of the box namely – Visual Basic.NET, C (pronounced as C-sharp), Jscript.NET and Managed C++ (a dialect of Visual C++). There are many vendors that are working on developing language compilers for other languages (20+ language compilers are already available). The beauty of multi language support lies in the fact that even though the syntax of each language is different, the basic capabilities of each language remain at par with one another. • Multi-Device Support Modern lift style is increasingly embracing mobile and wireless devices such as PDAs, mobiles and handheld PCs. . . .NET provides promising platform for programming such devices. .NET Compact Framework and Mobile Internet Toolkit are step ahead in this direction. • Automatic memory management While developing applications developers had to develop an eye on system resources like memory. Memory leaks were major reason in failure of applications. .NET takes this worry away from developer by handling memory on its own. The garbage collector takes care of freeing unused objects at appropriate intervals. • Compatibility with COM and COM+ Before the introduction of .NET, COM was the de-facto standard for componentized software development. Companies have invested lot of money and efforts in developing COM components and controls. The good news is – you can still use COM components and ActiveX controls under .NET. This allows you to use your existing investment in .NET applications. .NET still relies on COM+ for features like transaction management and object pooling. In fact it provides enhanced declarative support for configuring COM+ application right from your source code. Your COM+ knowledge still remains as a valuable asset. • No more DLL Hell If you have worked with COM components, you probably are aware of “DLL hell”. DLL conflicts are a common fact in COM world. The main reason behind this was the philosophy of COM – “one version of component across machine”. Also, COM components require registration in the system registry. .NET ends this DLL hell by allowing applications to use their own copy of dependent DLLs. Also, .NET components do not require any kind of registration in system registry. • Strong XML support Now days it is hard to find a programmer who is unaware of XML. XML has gained such a strong industry support that almost all the vendors have released some kind of upgrades or patches to their existing software to make it “XML compatible”. Currently, .NET is the only platform that has built with XML right into the core framework. .NET tries to harness power of XML in every possible way. In addition to providing support for manipulating and transforming XML documents, .NET provides XML web services that are based on standards like HTTP, XML and SOAP. • Ease of deployment and configuration Deploying windows applications especially that used COM components were always been a tedious task. Since .NET does not require any registration as such, much of the deployment is simplified. This makes XCOPY deployment viable. Configuration is another area where .NET – especially ASP.NET – shines over traditional languages. The configuration is done via special files having special XML vocabulary. Since, most of the configuration is done via configuration files, there is no need to sit in front of actual machine and configure the application manually. This is more important for web applications; simply FTPing new configuration file makes necessary changes. • Security Windows platform was always criticized for poor security mechanisms. Microsoft has taken great efforts to make .NET platform safe and secure for enterprise applications. Features such as type safety, code access security and role based authentication make overall application more robust and secure. 1.4 Installing the .NET Framework SDK Now that you have fare idea of what .NET I and what it can do for you, it is time to install .NET framework SDK on your machine. Following sections will tell you everything you need to know for installing .NET framework. • Hardware Requirements In order to install .NET framework SDK following hardware is required: - Computer/Processor : Intel Pentium class, 133 megahertz (MHz) or higher - Minimum RAM Requirements : 128 megabytes (MB) (256 MB or higher recommended) - Hard Disk : o Hard disk space required to install: 600 MB o Hard disk space required: 370 MB - Display : Video: 800x600, 256 colors - Input Device : Microsoft mouse or compatible pointing device • Software Requirements - Microsoft Internet Explorer 5.01 or later is required - Microsoft Data Access Components 2.6 is also required (Microsoft Data Access Components 2.7 is recommended) - Operating System : o Microsoft Windows® 2000, with the latest Windows service pack and critical updates available from the Microsoft Security Web page o Microsoft Windows XP – (Microsoft Windows XP Professional if you want to run ASP.NET) o Microsoft Windows NT® 4.0 Note: If you want to simply run .NET applications then you can also run them on Microsoft Windows XP Home edition, Windows Millennium Edition (Windows ME) and Windows 98. Here are some URLs that you will find handy in making your system up-to-date for above software requirements. Internet Explorer 6 can be downloaded from http://www.microsoft.com/windows/ie/downloads/ie6/default.asp Microsoft Data Access Components 2.7 can be downloaded from http://www.microsoft.com/data/download_270RTM.htm Various Windows service packs and patches can be obtained from http://www.microsoft.com/downloads/search.asp • Where to get .NET Framework SDK As mentioned earlier .NET framework SDK is freely downloadable from MSDN site. Visit http://msdn.microsoft.com/downloads/default.asp?url=/downloads/sample.asp?url =/msdn-files/027/000/976/msdncompositedoc.xml and download it now. The total download size is 137,363,456 bytes (approximately 131 Mb). For your convenience Microsoft has provided multi-part version of the entire download. If you are unable to download the SDK from MSDN web site, check out popular PC magazines around. Many of them contain .NET Framework SDK on their companion CD. • Starting the installation Note: If you already have a previous version of .NET installed on the machine then it must first be uninstalled. Refer ReadMe files that ship with .NET framework SDK. These files contain valuable information related to installation, system requirements and trouble shooting. In order to start the installation, you need to run the setup program that is available with the download mentioned above. A wizard will guide you with necessary process. It will also allow you to select various components of the framework. After the installation is complete it is a good idea to apply .NET framework Service pack 1. The service pack fixes some of the bugs. It can be downloaded from: http://msdn.microsoft.com/netframework/downloads/sp1/default.asp • Installing Samples and Quick Start Tutorials .NET framework comes with an excellent set of tutorials that help you learn various technologies such as ASP.NET and windows forms. In order to configure the tutorials follow Start menu - Program - Microsoft .NET Framework SDK - Samples and Quick Start Tutorials. This will open up a HTML document that will guide you through the process of configuring the samples and tutorials. • Installing MSDE .NET framework samples and quick start tutorials require a Microsoft SQL Server Desktop Engine (MSDE). MSDE is scaled down version of SQL Server. The samples use databases from the MSDE. In order to work with the samples make sure you have started an instance of MSDE. You can use this MSDE for creating your own databases for testing applications. Coming Next… By the time you must have got idea about what .NET is and what it can do for you. You probably will have installed .NET on your machine waiting eagerly to try hands on it. However, before you go into the code level details, it is essential that you firmly understand certain fundamentals. In the next section we will demystify some intrinsic concepts and features of .NET framework. 2. Introduction to the .NET Initiative and the .NET Platform Section Owner: Saurabh Nandu (MVP) Content Contributors: Balachandran, Pradeep The Microsoft .NET initiative is a very wide initiative and it spans multiple Microsoft Products ranging from the Windows OS to the Developer Tools to the Enterprise Servers. The definition of .NET differs from context to context, and it becomes very difficult for you to interpret the .NET strategy. This section aims at demystifying the various terminologies behind .NET from a developer’s perspective. It will also highlight the need for using this new .NET Platform in your applications and how .NET improves over its previous technologies. 2.1 Understanding the Existing Development Scenario Windows DNA is a concept for building distributed applications using the Microsoft Windows operating system and related software products. First we will understand about the 2- tier, 3- tier and then move on to N- tier Windows DNA. Why to divide an application into logical layers? Factoring an application into logical parts is useful. Breaking a large piece of software into smaller pieces can make it easier to build, easier to reuse and easier to modify. It can also be helpful in accommodating different technologies or different business organizations. 2-Tier: Client Server Presentation Layer Win 32 Clients (VB Forms) Data Source Layer Sql File Mail Server System Server Fig Showing 2 – Tier Client Server Model Through the appearance of Local-Area-Networks, PCs came out of their isolation, and were soon not only being connected mutually but also to servers. Client/Server- computing was born. A two-tiered application is an application whose functionality can only be segmented into two logical tiers, presentation services and data services. The presentation services of a two-tiered application are responsible for gathering information from the user, interacting with the data services to perform the application's business operations, and presenting the results of those operations to the user. The Presentation services are also called the presentation layer because it presents information to the user. Things you might find in a presentation layer include a Web browser, a terminal, a custom-designed GUI, or even a character-based user interface. Client-Server architecture was a major buzzword in the early 90's, taking initially dumb terminal applications and giving them a fancy windows-like front end, using PCs with terminal emulators which presented pretty GUIs (Graphical user interface) or later Visual Basic etc front-ends. A web browser talking to a web server is an example of a client talking to a server. Here there is presentation logic (presentation tier) happening at the client, and data/file access (data access tier) and logic happening at the server. One reason why the 2-tier model is so widespread is because of the quality of the tools and middleware that have been most commonly used since the 90’s: Remote-SQL, ODBC, relatively inexpensive and well-integrated PC-tools (like Visual Basic, Power-Builder, MS Access, 4-GL-Tools by the DBMS manufactures). In comparison the server side uses relatively expensive tools. In addition the PC-based tools show good Rapid-Application- Development (RAD) qualities i.e. simpler applications can be produced in a comparatively short time. The 2-tier model is the logical consequence of the RAD-tools’ popularity. 3 – Tier: Client Server Presentation Layer Win32 Client Browser based Interface Applications html /xml (Vi l B i f ) Jl HTTP IIS / Apache Business Layer COM / COM / COM + ASP Business Rules and Process Data Service Layer Sql Oracle Mail File Server RDBMS Server System Fig Showing 3 – Tier or N- Tier Client Server Model In a three-tiered application, the presentation services are responsible for gathering information from the user, sending the user information to the business services for processing, receiving the results of the business services processing, and presenting those results to the user. The most popular architecture on the web currently, mostly taking the form of web browser processing client side presentation in the form of HTML/DHTML, etc, the web server using some scripting language (ASP) and the database server (SQL Server for example) serving up the data. The basic functionalities of 3 – Tier or N-Tier follows are The presentation services tier is responsible for: • Gathering information from the user • Sending the user information to the business services for processing • Receiving the results of the business services processing • Presenting those results to the user The business services tier is responsible for: • Receiving input from the presentation tier. • Interacting with the data services to perform the business operations. • Sending the processed results to the presentation tier. The data services tier is responsible for the: • Storage of data. • Retrieval of data. • Maintenance of data. • Integrity of data. In Windows DNA applications commonly implement their business logic using one or more of three implementation options. • Asp Pages • COM components • Stored procedures running in the DBMS Writing much business logic in ASP pages is a bad idea. Since simple languages are used, such as Microsoft Visual Basic Script, and the code is interpreted each time it is executed, which hurts the performance. Code in ASP pages is also hard to maintain, largely because business logic is commonly intermixed with presentation code that creates the user interface. One recommended approach for writing middle-tier business logic is to implement that logic as COM objects. This approach is a bit more complex than writing a pure ASP application. Wrapping business logic in COM objects also cleanly separates this code from the presentation code contained in ASP pages, making the application easier to maintain. The Third option for writing business logic is to create some of that code as stored procedures running in the database management system (DBMS). Although a primary reason for using stored procedures is to isolate the details of database schema from business logic to simplify code management and security, having code in such a close proximity to data can also help optimize performance. 2.2 Challenges faced by developers In Windows DNA, there are two major choices of user interfaces - Win32 clients and browser based clients. During the Internet revolution of the late 90s we saw the emergence of the browser and the Web Server. With the introduction of Internet, information started being available but with limited functionality. With the development of the Windows Distributed Internet Architecture, we started to see Web sites that allowed simple transactions to occur. Clients on browsers could access Web sites that had COM components available to them that allowed them to retrieve information from the database. So now we gained the capability to simulate the environment of the Win32 platform. The client software – the browser – can access information on a server. But as with the Win32 environment, we are limited in the way in which the information is presented to us. Customization is neither widespread nor broadly developed. Let us look into limitations of these technologies. Limitations in Win32 Clients In a client-server environment visual tool such as Visual Basic, are often used to create a rich user interface. The drawbacks is that such client software is difficult to deploy and maintain, requiring and install on every client and a change to every client when an upgrade is needed. DLL conflicts on the client are frequent because of variations in the version of the operating system and other software installed on the client. Visual Basic is the most common language used to write middle-tier components. This requires high level of expertise in COM. Since these middle-tire components are implemented using Microsoft Transaction Server on Windows NT or COM+ services on Windows 2000. These components use stateless designs, which can look very different from the stateful designs often used in client-based components. COM components, in the middle tier must work together, Versioning all the components properly so that they understand each other's interfaces can be a challenge. This requires a highly sophisticated skill level and a well - controlled deployment process. COM works well on Microsoft platforms. But it suffers from lack of interoperability with other platforms. One of the most important ways functionality can be reused is for a software component to inherit another component, But COM does not support inheritance. Visual Basic is the most popular language for developing applications with the DNA model, this is used in two major roles - forms based VB Clients and COM components. This VB6 language has its own limitations it doesn’t have the capability of multithreading, lack of OOPS concepts, Poor error handling ability and poor integration with other languages. Hence it makes it unsuitable for development of object-based frameworks. Today’s applications need to use the Win32 API for a variety of purposes like monitor widows messages, manipulate controls, reading and writing to INI files and socket programming etc. But these widows API are hard to program for variety of reasons, like it is not object oriented and complex calls to the functions with long lists of arguments, since Win32 API is written in C++ language, getting calling conventions right on data types is messy. Limitations in DNA-Based Internet Development or Browser based clients With DNA - based software development, creating software that is accessed by a user locally is done very differently from development for the Internet. The Visual Basic forms for client-server user interfaces versus the use of Active Server Pages for Internet user interfaces. Even though both situations involve designing and implementing GUI based user interfaces the tools and programming techniques used are quite different. ASP lacks in state management between post backs. Every time a page is rendered, the programmer must make sure that all the visual controls like text boxes, dropdowns have their information loaded. It is the programmer's responsibility to manage the state in the user interface and to transfer state information between pages. This causes developers to have to write a lot of code for the internet user interfaces that is not relevant to business problem being solved. If the Internet application is going to run on a group of Web Servers, then considerable additional work is necessary to design a state management system that is independent of particular server. Browser based clients are somewhat more difficult to create, and offer a more limited user interface with fewer controls and less control over layout of the screen and handling of screen events. It is possible to create rich user interfaces using DHTML, but it requires lot of coding and also browser compatibility issues rises, for which a separate coding or two version of the same page have to be maintained, keeping in mind, the browser we are targeting. The Internet has caused server-based applications to become much more popular than ever before and has made the connectionless request/response programming model common. But communicating between servers—especially among those running on different platforms—is difficult, and because most substantial Internet applications are Database-Centric, the ability to access a wide variety of data sources easily is more important than ever.

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