ptg 724 CHAPTER 17 Building Components </div> </form> </body> </html> In the Page_Load() event handler, an instance of the HelloWorld component is created. Next, the result returned by a call to the SayMessage() method is assigned to a Label control. When you open the page in your browser, you see the message Hello World!. This process of creating the component is simple. You don’t need to perform any special registration, and you don’t need to compile anything explicitly. Everything just works magically. Components and Dynamic Compilation You are not required to explicitly compile (build) the component because ASP.NET Framework automatically compiles the component for you. Any component that you add to the App_Code folder is compiled dynamically in the same way as an ASP.NET page. If you add a new component to the App_Code folder and request any page from your website, the contents of the App_Code folder are compiled into a new assembly and saved to the Temporary ASP.NET Files folder, located at the following path: C:\WINDOWS\Microsoft.NET\Framework\[version]\ Temporary ASP.NET Files\[application name] Whenever you modify the component, the existing assembly in the Temporary ASP.NET Files folder is deleted. The App_Code folder is compiled again when you make a new page request. NOTE An assembly is the dll file (or dll files) in which components are stored. You can add as many subfolders to the App_Code folder as you need to organize your components. The ASP.NET Framework finds your component no matter how deeply you nest the component in a subfolder. One significant drawback of this process of dynamic compilation is that any errors in any component contained in the App_Code folder prevent any pages from executing. Even if a page does not use a particular component, any syntax errors in the component raise an exception when you request the page. From the Library of Wow! eBook ptg 725 Building Basic Components TIP If a component contains an error, and you want to temporarily hide the component from ASP.NET Framework, change the file extension to an extension that ASP.NET Framework does not recognize, such as HelloWorld.cs.exclude. Visual Web Developer uses this method to hide a component when you right-click a component and select the menu option Exclude From Project. Mixing Different Language Components in the App_Code Folder You don’t have to do anything special if all the components in the App_Code folder are written in the same language. For example, if you use Visual Basic .NET to create all your components, the ASP.NET Framework automatically infers the language of your compo- nents and everything just works. However, if you mix components written in more than one language in the App_Code folder—for example, Visual Basic .NET and C#— you must perform some extra steps. First, you need to place components written in different languages in different subfolders. You can name the subfolders anything you want. The point is to not mix different language components in the same folder. Furthermore, you need to modify your web configuration file to recognize the different subfolders. For example, if you create two subfolders in the App_Code folder named VBCode and CSCode, you can use the web configuration file in Listing 17.3 to use compo- nents written in both VB.NET and C#. LISTING 17.3 Web.Config <?xml version=”1.0”?> <configuration> <system.web> <compilation> <codeSubDirectories> <add directoryName=”VBCode” /> <add directoryName=”CSCode” /> </codeSubDirectories> </compilation> </system.web> </configuration> When the contents of the App_Code folder are compiled, two assemblies are created: one that corresponds to the VBCode folder and one that corresponds to the CSCode folder. You don’t need to indicate the language used for each folder—ASP.NET Framework infers the language for you. 17 From the Library of Wow! eBook ptg 726 CHAPTER 17 Building Components There is nothing wrong with mixing components written in different languages in the same ASP.NET page. After a component is compiled, .NET Framework treats VB.NET and C# components in the same way. Declaring Methods The simple HelloWorld component in Listing 17.1 contains a single method named SayMessage(), which returns a string value. When writing components with Visual Basic .NET, you create methods by creating either a subroutine or a function. Use a subroutine when a method does not return a value, and use a function when a method does return a value. The SayMessage() method in Listing 17.1 is an instance method. In other words, you must create a new instance of the HelloWorld class before you can call the SayMessage() method like this: HelloWorld objHelloWorld = new HelloWorld(); lblMessage.Text = objHelloWorld.SayMessage(); In the first line, a new instance of the HelloWorld component is created. The SayMessage() method is called from this instance. For this reason, the SayMessage() method is an instance method. As an alternative to creating an instance method, you can create a static method. The advantage of a static method is that you do not need to create an instance of a compo- nent before calling it. For example, the SayMessage() method in the modified HelloWorld component in Listing 17.4 is a static method. NOTE Static methods are called shared methods in Visual Basic .NET. LISTING 17.4 StaticHelloWorld.cs public class StaticHelloWorld { public static string SayMessage() { return “Hello World!”; } } The StaticHelloWorld component defined in Listing 17.3 is exactly the same as the HelloWorld component created in Listing 17.1 with one change: The SayMessage() method includes a static modifier. From the Library of Wow! eBook ptg 727 Building Basic Components The page in Listing 17.5 uses the StaticHelloWorld component to display the Hello World! message. LISTING 17.5 ShowStaticHelloWorld.aspx <%@ Page Language=”C#” %> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.1//EN” “http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd”> <script runat=”server”> void Page_Load() { lblMessage.Text = StaticHelloWorld.SayMessage(); } </script> <html xmlns=”http://www.w3.org/1999/xhtml” > <head id=”Head1” runat=”server”> <title>Show Shared Hello World</title> </head> <body> <form id=”form1” runat=”server”> <div> <asp:Label id=”lblMessage” Runat=”server” /> </div> </form> </body> </html> The page in Listing 17.5 does not create an instance of the StaticHelloWorld component. The SayMessage() method is called directly from the StaticHelloWorld class. The advantage of using static methods is that they save you typing. You don’t have to go through the pain of instantiating a component before calling the method. Many classes in .NET Framework include static methods. For example, String.Format(),Int32.Parse(), and DateTime.DaysInMonth() methods are all static methods. There is nothing wrong with mixing both static and instance methods in the same component. For example, you might want to create a Product component that has a static GetProducts() method and an instance SaveProduct() method. 17 From the Library of Wow! eBook ptg 728 CHAPTER 17 Building Components The one significant limitation of using a static method is that a static method cannot refer to an instance field or property. In other words, static methods must be stateless. Declaring Fields and Properties You can define a property for a component in two ways: the lazy way and the virtuous way. The lazy way to create a property is to create a public field. If you declare any field with the Public access modifier, the field can be accessed from outside the component. For example, the component in Listing 17.6 contains a public field named Message. LISTING 17.6 FieldHelloWorld.cs public class FieldHelloWorld { public string Message; public string SayMessage() { return Message; } } The Message field is declared near the top of the FieldHelloWorld class definition. The Message field is returned by the SayMessage() method. The page in Listing 17.7 uses the FieldHelloWorld component to display a message. LISTING 17.7 ShowFieldHelloWorld.aspx <%@ Page Language=”C#” %> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.1//EN” “http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd”> <script runat=”server”> void Page_Load() { FieldHelloWorld objFieldHelloWorld = new FieldHelloWorld(); objFieldHelloWorld.Message = “Good Day!”; lblMessage.Text = objFieldHelloWorld.SayMessage(); } </script> <html xmlns=”http://www.w3.org/1999/xhtml” > <head id=”Head1” runat=”server”> <title>Show Field Hello World</title> </head> From the Library of Wow! eBook ptg 729 Building Basic Components <body> <form id=”form1” runat=”server”> <div> <asp:Label id=”lblMessage” Runat=”server” /> </div> </form> </body> </html> In the Page_Load() event handler in Listing 17.7, an instance of the FieldHelloWorld class is created, a value is assigned to the Message field, and the SayMessage() method is called. There are a couple of serious disadvantages to creating properties by creating public fields. First, .NET Framework recognizes properties as separate entities. Several methods in .NET Framework recognize properties but not fields. For example, you can refer to component properties and not fields when using the Eval() method in a databinding expression. If you want to bind a collection of Product objects to a GridView control, you should expose the properties of the Product component as true properties and not as fields. The other disadvantage of fields is that they do not provide you with a chance to validate the value assigned to the field. For example, imagine that a property represents a database column and the column accepts no more than five characters. In that case, you should check whether the value assigned to the property is less than five characters. The component in Listing 17.8 uses a property instead of a field. (It does things the virtuous way.) LISTING 17.8 PropertyHelloWorld.cs using System; public class PropertyHelloWorld { private string _message; public string Message { get { return _message; 17 From the Library of Wow! eBook ptg 730 CHAPTER 17 Building Components } set { if (value.Length > 5) throw new Exception(“Message too long!”); _message = value; } } public string SayMessage() { return _message; } } The component in Listing 17.8 contains a property named Message and a private backing field named _message. The Message property contains both a getter (get) and a setter (set). The getter is called when you read the value of the Message property, and the setter is called when you assign a value to the Message property. The getter simply returns the value of the private _message field. The setter assigns a value to the private _message field. The setter throws an exception if the length of the value assigned to the _message field exceeds five characters. NOTE In Listing 17.8, the private field is named _message. The underscore character (_) has no programmatic significance. By convention, private members of a class are named with a leading underscore, but there is nothing wrong with following some other convention. NOTE The version of C# included with .NET Framework 3.5 and 4 supports the automatic properties feature. Automatic properties provide you with a shorthand syntax for creat- ing a property with a backing field. To learn more about automatic properties, see Chapter 20, “Data Access with LINQ to SQL.” The page in Listing 17.9 uses the PropertyHelloWorld component. From the Library of Wow! eBook ptg 731 Building Basic Components LISTING 17.9 ShowPropertyHelloWorld.aspx <%@ Page Language=”C#” %> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.1//EN” “http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd”> <script runat=”server”> void Page_Load() { PropertyHelloWorld objPropertyHelloWorld = new PropertyHelloWorld(); objPropertyHelloWorld.Message = “Hello World!”; lblMessage.Text = objPropertyHelloWorld.SayMessage(); } </script> <html xmlns=”http://www.w3.org/1999/xhtml” > <head id=”Head1” runat=”server”> <title>Show Property Hello World</title> </head> <body> <form id=”form1” runat=”server”> <div> <asp:Label id=”lblMessage” Runat=”server” /> </div> </form> </body> </html> If you open the page in Listing 17.9 in your web browser, you get a big, fat error message (see Figure 17.2). Because a string longer than five characters is assigned to the Message property in the Page_Load() method, the Message property raises an exception. You can also create read-only properties when the situation warrants it. For example, the component in Listing 17.10 returns the current server time. It would not make sense to assign a value to this property, so the property is declared as read-only. 17 From the Library of Wow! eBook ptg 732 CHAPTER 17 Building Components LISTING 17.10 ServerTime.cs using System; public class ServerTime { public string CurrentTime { get { return DateTime.Now.ToString(); } } } FIGURE 17.2 Assigning more than five characters. From the Library of Wow! eBook ptg 733 Building Basic Components 17 NOTE You c an create static fields a nd proper tie s in the same way a s you create shared methods, by using the static keyword. Any value you assign to a static field or prop- erty is shared among all instances of a component. I recommend that you avoid using static fields and properties when building ASP.NET applications. Using static fields and properties raises nasty concurrency issues in a multithreaded environment such as ASP.NET. If you insist on creating a static field or property, make it read-only. Declaring Constructors A constructor is a special class method called automatically when you create a new instance of a class. Typically, you use the constructor to initialize private fields contained in the class. When creating a constructor in C#, you create a method with the same name as the class name. For example, the class in Listing 17.11 displays a random quotation (see Figure 17.3). The collection of random quotations is created in the component’s constructor. FIGURE 17.3 Displaying a random quotation. From the Library of Wow! eBook . path: C:WINDOWSMicrosoft .NET Framework[version] Temporary ASP. NET Files[application name] Whenever you modify the component, the existing assembly in the Temporary ASP. NET Files folder is deleted. The App_Code. automatically compiles the component for you. Any component that you add to the App_Code folder is compiled dynamically in the same way as an ASP. NET page. If you add a new component to the App_Code. and 4 supports the automatic properties feature. Automatic properties provide you with a shorthand syntax for creat- ing a property with a backing field. To learn more about automatic properties,