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Appendix I Introduction to HyperText Markup Language 4: Part 1 1377 1 3 4 5 6 7 8 9 10 11 12 13 14

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15 16 17 Fig. I.1 Fig. I.Fig.Fi I.1g. I.11Basic HTML file. Fig. I. Good Programming Practice I.3 Always include the tags in the beginning and end of your HTML document. Good Programming Practice I.4 Place comments throughout your code. Comments in HTML are placed inside the tags. Comments help other programmers understand the code, assist in debugging and list other useful information that you do not want the browser to render. Comments also help you understand your own code, especially if you have not looked at it for a while. We see our first comments (i.e., text that documents or describes the HTML markup) on lines 5 and 6 Comments in HTML always begin with . The browser ignores any text and/or tags inside a comment. We place comments at the top of each HTML document giving the figure number, the file name and a brief description of the purpose of the exam
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1376 Introduction to HyperText Markup Language 4: Part 1 Appendix I Common Programming Error I.1 Forgetting to include end tags for elements that require them is a syntax error and can grossly affect the formatting and look of your page. Unlike conventional programming languages, a syntax error in HTML does not usually cause page display in browsers to fail completely. I.3 Editing HTML In this appendix we show how to write HTML in its source-code form. We create HTML documents using a text editor and store them in files with either the.html or .htm file name extension. A wide variety of text editors exist. We recommend that you initially use a text editor called Notepad, which is built into Windows. Notepad can be found inside the Accessories panel of your Program list, inside the Start menu. You can also download a free HTML source-code editor called HTML-Kit at www.chami.com/html-kit. Unix users can use popular text editors like vi or emacs. Good Programming Practice I.2 Assign names to your files that describe their functionality. This practice can help you identify documents faster. It also helps people who want to link to your page, by giving them an easier-to-remember name for the file. For example, if you are writing an HTML document that will display your products, you might want to call it products.html. As mentioned previously, errors in conventional programming languages like C, C++ and Visual Basic often prevent the program from running. Errors in HTML markup are usually not fatal. The browser will make its best effort at rendering the page, but will probably not display the page as you intended. The file name of your home page (the first of your HTML pages that a user sees when browsing your Web site) should be index.html, because when a browser does not request a specific file in a directory, the normal default Web server response is to return index.html (this may be different for your server) if it exists in that directory. For example, if you direct your browser to www.deitel.com, the server actually sends the file www.deitel.com/index.html to your browser. I.4 Common Elements Throughout these HTML appendices, we will present both HTML source code and a sample screen capture of the rendering of that HTML in Internet Explorer. Figure I.1 shows an HTML file that displays one line of text. Lines 1 and 2 are required in every HTML document and are used to specify the document type. The document type specifies which version of HTML is used in the document and can be used with a validation tool, such as the W3C s validator.w3.org, to ensure an HTML document conforms to the HTML recommendation. In these examples we create HTML version 4.01 documents. All of the examples in these appendices have been validated through the Web site validator.w3.org. The HTML document begins with the opening tag (line 3) and ends with the closing tag (line 17).
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Appendix I Introduction to HyperText Markup Language 4: Part 1 1375 Outline I.1 Introduction I.2 Markup Languages I.3 Editing HTML I.4 Common Elements I.5 Headers I.6 Linking I.7 Images I.8 Special Characters and More Line Breaks I.9 Unordered Lists I.10 Nested and Ordered Lists I.11 Internet and World Wide Web Resources Summary Terminology Self-Review Exercises Answers to Self-Review Exercises Exercises I.1 Introduction In this appendix we introduce the basics of creating Web pages in HTML. We write many simple Web pages. In Appendix J, Introduction to HyperText Markup Language 4: Part 2, we introduce more sophisticated HTML techniques, such as tables, which are particularly useful for structuring information from databases. In this appendix, we do not present any C# programming. In this appendix, we introduce basic HTML elements and attributes. A key issue when using HTML is the separation of the presentation of a document (i.e., how the document is rendered on the screen by a browser) from the structure of that document. In this appendix and in Appendix J, we discuss this issue in depth. I.2 Markup Languages HTML is a markup language. It is used to format text and information. This marking up of information is different from the intent of traditional programming languages, which is to perform actions in a designated order. In HTML, text is marked up with elements, delineated by tags that are keywords contained in pairs of angle brackets. For example, the HTML element itself, which indicates that we are writing a Web page to be rendered by a browser, begins with the start tag and terminates with the end tag . These elements format your page in a specified way. Over the course of the next two appendices, we introduce many of the commonly used tags and how to use them. Good Programming Practice I.1 HTML tags are not case sensitive. However, keeping all the letters in one case improves program readability. Although the choice of case is up to you, we recommend that you write all of your code in lowercase. Writing in lowercase ensures greater compatibility with future markup languages that are designed to be written with only lowercase tags and elements.
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I Introduction to HyperText Markup Language 4: Part 1 Objectives To understand the key components of an HTML document. To be able to use basic HTML elements to create World Wide Web pages. To be able to add images to your Web pages. To understand how to create and use hyperlinks to traverse Web pages. To be able to create lists of information. To read between the lines was easier than to follow the text. Henry James Mere colour, unspoiled by meaning, and annulled with definite form, can speak to the soul in a thousand different ways. Oscar Wide High thoughts must have high language. Aristophanes I ve gradually risen from lower-class background to lowerclass foreground. Marvin Cohen
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Appendix H COM Integration 1373 The translation for each COM component is performed by a Runtime Callable Wrapper (RCW). The RCW is a proxy object created by the .NET runtime from the information in the object s primary interop assembly. The RCW manages the COM object and performs communication between .NET code and the COM object. When we instantiate a COM object in C#, we are actually creating a new instance of the object s RCW. The communication between the RCW and the COM component is entirely transparent, enabling the .NET developer to interact with the COM object as if it were a .NET object. TERMINOLOGY ActiveX monolithic executable COM (Component Object Model) .NET component COM component .OCX file COM limitation primary interop assembly Common Runtime Library proxy proxy Component Object Model (COM) RCW (Runtime Callable Wrapper) DLL (Dynamic Link Library) RegSvr32 utility .dll file Runtime Callable Wrapper (RCW) DLL hell shared library Dynamic Link Library (DLL) .tlb file Globally Unique Identifier (GUID) Windows Forms proxy GUID (Globally Unique Identifier) Windows Registry SELF-REVIEW EXERCISES H.1 Fill in the blanks in each of the following statements: a) Initially, DOS and Windows programs were designed as executables. b) Microsoft incorporated shared libraries, or , into Windows to allow program modularity and code reusability. c) The COM specification was designed as a uniform programming model that promotes , and . d) A contains language-independent descriptions of all interfaces, methods and data types defined in a COM component. e) When an ActiveX control is imported, two files are created: A(n) proxy and proxy. H.2 State whether each of the following statements is true or false. If false, explain why. a) The Runtime Callable Wrapper (RCW) allows .NET components to be accessed from COM components. b) A primary interop assembly contains information about COM components. c) .NET component GUIDs must be entered in the Windows Registry. d) The Component Object Model eliminates DLL hell. e) ActiveX controls are COM components. ANSWERS TO SELF-REVIEW EXERCISES H.1 a) monolithic. b) Dynamic Link Libraries (DLLs). c) language independence, backwards compatibility and version control. d) type library e) Common Language Runtime, Windows Forms proxy. H.2 a) False. The RCW allows .NET components to access COM components. b) True. c) False. Unlike COM components, .NET components contain their GUIDs internally. d) False. COM DLLs may not be correctly representing in the windows registry causing misidentification and resulting in DLL hell. e) True.
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1372 COM Integration Appendix H The partition of programs into smaller pieces makes it easier to distribute application upgrades, because only modified DLLs must be redistributed. Often, developers packaged DLLs with their applications to ensure that users were running the library version designed for their software. However, the packaged DLLs could overwrite preexisting libraries on users systems, possibly breaking previously installed software. The problems introduced by shared libraries were so difficult to locate and fix that their effects became known as DLL hell. In an attempt to combat DLL hell, Microsoft developed the Component Object Model (COM). COM is a specification that controls library versions, backwards compatibility and language interoperability. The COM specification, defined by Microsoft, is detailed and strict to ensure that COM developers create compatible libraries. Microsoft implemented the COM architecture on a large scale. Today, virtually all Windows libraries adhere to the COM specification. When implemented correctly, COM ensures highly organized and reusable libraries, but it does have limitations. COM is difficult to program and deploy, because developers must guarantee that new COM components are both compatible with previous versions and registered with the system. Microsoft .NET components retain the benefits of COM while avoiding many of its associated problems. To facilitate developers migration from COM to .NET, Microsoft created a set of utilities designed to integrate existing COM components into the .NET platform. ActiveX controls are a commonly used COM component. The RegSvr32 utility program registers COM components with the operating system. ActiveX can be imported into the IDE s toolbox. The Customize Toolbox option lists all the COM components that are registered on a specific machine. Windows components contain Globally Unique Identifiers (GUID) for the component and its internal classes as well as language-independent descriptions of all classes and types that are defined in the component. Language-independent descriptions help to enable component reuse across multiple programming languages. A GUID consists of a long string of numbers that a computer generates on the basis of the computer s current time, hardware and a random number. The GUID algorithm never generates the same identifier twice; thus, GUIDs enable unique component identification across all computers. When a COM component is registered, its GUID is stored in the Windows Registry; programs then can use the registry to locate and identify the component. A type library describes all of a COM component s interfaces, types and methods; the type library is included in either the component .dll file or in a separate .tlb file. The separation of component identifiers (located in the Windows Registry) from the data represented on the disk (the library file) is the source of many problems associated with the COM architecture. .NET components problems by maintaining all identification information internally. A primary interop assembly is used by .NET to locate COM methods and to translate component data types between the .NET platform types and COM component types.
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Appendix H COM Integration 1371 74 total = additionObject.AddFunction( ref firstInteger, 75 ref secondInteger ); 76 77 resultBox.Text = total.ToString(); 78 } // end method calculateButton_Click 79 } // end class Addition 80 } // end namespace Addition Fig. H.6 COM DLL component in C#. (Part 3 of 3.) H.4 Internet and World Wide Web Resources www.microsoft.com/com The Microsoft COM Web page provides technical white papers, documentation and developer support. This Web page is an essential resource for COM developers. www.cs.umd.edu/~pugh/com This Web site presents a high-level technical overview of the COM architecture. msdn.microsoft.com/msdnmag/issues/01/08/Interop/Interop.asp This Web site provides an introduction to integration services provided in .NET. The Web site includes introductory examples and describes .NETs COM Interopability capabilities. SUMMARY Initially, applications created for Windows or DOS were designed as single monolithic executables entire applications packaged in single executable files. As applications grew larger and more complex, it became impractical for developers to construct and distribute all the necessary components of an application, which resulted in longer development times and more costly distribution mechanism. Microsoft incorporated dynamic link libraries (DLLs) in Windows to allow developers to modularize and reuse code. A shared library, or dynamic link library, is a file containing compiled code that an application loads at execution time. Runtime loading allows developers to modify a single library and immediately test the results without rebuilding the entire application. Shared libraries increase the modularity of programs by allowing multiple applications to access a single code library.
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1370 COM Integration Appendix H 21 // integer text boxes 22 private System.Windows.Forms.TextBox resultBox; 23 private System.Windows.Forms.TextBox firstIntegerBox; 24 private System.Windows.Forms.TextBox secondIntegerBox; 25 26 // calculates addition 27 private System.Windows.Forms.Button calculateButton; 28 29 private CAddition additionObject = new CAddition(); 30 31 // Required designer variable. 32 private System.ComponentModel.Container 33 components = null; 34 35 public Addition() 36 { 37 InitializeComponent(); 38 } 39 40 // Visual Studio .NET generated code 41 42 // The main entry point for the application. 43 [STAThread] 44 static void Main() 45 { 46 Application.Run( new Addition() ); 47 } 48 49 // event handler to enable calculateButton 50 private void integerBox_TextChanged( object sender, 51 System.EventArgs e ) 52 { 53 // enable calculate button if both boxes 54 // contain text 55 if ( firstIntegerBox.Text != “” && 56 secondIntegerBox.Text != “” ) 57 calculateButton.Enabled = true; 58 else 59 calculateButton.Enabled = false; 60 } // end method integerBox_TextChanged 61 62 // event handler that displays sum when calculate 63 // is clicked 64 private void calculateButton_Click( object sender, 65 System.EventArgs e ) 66 { 67 int firstInteger, secondInteger, total; 68 69 firstInteger = Int32.Parse( firstIntegerBox.Text ); 70 secondInteger = Int32.Parse( secondIntegerBox.Text ); 71 72 // addition object invokes AddFunction that 73 // returns integer value Fig. H.6 COM DLL component in C#. (Part 2 of 3.)
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Appendix H COM Integration 1369 Performance Tip H.1 The .NET and COM architectures are fundamentally different in terms of memory management and object representation. Method calls to COM objects can degrade program performance because the RCW must convert all data types between the managed (.NET) context and the unmanaged (COM) context. When we instantiate a COM object in C#, we are actually creating a new instance of the object s RCW. The communication between the RCW and the COM component is entirely transparent, enabling the .NET developer to interact with the COM object as if it were a .NET object. We created an application (Fig. H.6) demonstrating how to use the Simple Addition DLL that we want to integrate into .NET. This program includes three text boxes and a button. After entering an intinto each of the first two text boxes, the user clicks the Calculate button, and the program displays the sum of the two int in the third text box. Line 29 creates the RCW additionObject for COM component Deitel_DLL.CAddition. Lines 55 59 enable the calculateButton button if both text boxes contain values, and disable the button if the text boxes are empty. When the user clicks Calculate, the button fires the event handler calculateButton_Click, which obtains the content of the text boxes and adds the values. The event handler calls COM method addFunction, which returns the sum as an int (lines 74 75). The result then is displayed in txtResultBox (line 77). In this appendix, we demonstrated the use of COM libraries and ActiveX controls from a .NET application. In addition, we briefly explored the history of COM and the differences between its architecture and that of .NET. After reading this appendix, students should have a basic understanding of COM and should be able to use COM components in .NET applications. To learn more about .NET and COM, consult the Web resources described in Section H.4. 1 // Fig. H.6: Addition.cs 2 // Uses a COM component to add two integers. 3 4 using System; 5 using System.Drawing; 6 using System.Collections; 7 using System.ComponentModel; 8 using System.Windows.Forms; 9 using System.Data; 10 using Deitel_DLL; 11 12 namespace Addition 13 { 14 // Adds two integers using a COM component 15 public class Addition : System.Windows.Forms.Form 16 { 17 // display labels 18 private System.Windows.Forms.Label SecondLabel; 19 private System.Windows.Forms.Label FirstLabel; 20 Fig. H.6 COM DLL component in C#. (Part 1 of 3.)
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1368 COM Integration Appendix H Fig. H.5 Fig. H.5Fig. H.FiFi5g. H.5g. H.5Add Reference dialog DLL Selection. When a COM component is registered, its GUID is stored in the Windows Registry; programs then can use the registry to locate and identify the component. Once a program has located a desired component, it uses the component s type library to find and use the library s objects and methods. A type library describes all of a COM component s interfaces, types and methods; the type library is included in either the component .dllfile or in a separate .tlbfile. The separation of component identifiers (located in the Windows Registry) from the data represented on the disk (the library file) is the source of many problems associated with the COM architecture. By contrast, .NET components avoid these problems by maintaining all identification information internally. When Visual Studio imports a COM component, it creates a file that contains all identification and data-description information internally. Visual Studio obtains the component GUID from the Windows Registry and converts the data description from the type library-format into the .NET assembly format. This processing creates a new DLL file, called a primary interop assembly, which then is placed into the applications bindirectory. The primary interop assembly is used by .NET to locate COM methods and to translate component data types between the .NET platform types and COM component types. The translation for each COM component is performed by a Runtime Callable Wrapper (RCW). The RCW is a proxy object created by the .NET runtime from the information in the object s primary interop assembly. The RCW manages the COM object and performs communication between .NET code and the COM object.
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