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Custom Web Controls Demystified, Part 1 : Page 3

Many ASP.NET developers use Web controls, but few have taken the time to explore how to develop their own custom Web controls, thereby potentially missing some very useful, reusable functionality.


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Let's Code Already!
I'm going to show you how to develop three Web controls in the rest of this article. I will go into as much detail as I can considering that this is an article and not a book. Some of the things I will cover will be more general than others and I'm hoping that you get intrigued enough to read further on the subject from the books I've listed in the sidebar called "Good Books for Learning How to Develop Custom ASP.NET Controls."

The first custom Web control you'll learn to develop is a button that will have the ability to pop up a confirmation dialog when it is clicked. This will give the user the ability to cancel whatever processing would have taken place when the button was clicked. I'll show you how to develop this Web control as an inherited control that I'll call the ConfirmationButton.

Next you'll learn to develop a second custom Web control I'll call the FormField control. This control was in my article on Declarative Programming. This control is a combination of a label, a textbox, and a button that you can arrange in a variety of ways. The idea behind this control is that it serves as the basis for data entry on a Web page. Because of the heavy use this control may receive on a single Web page, I'll develop this as a rendered control.

The third and last custom Web control that you'll lean to develop is an EmailContact control. You no doubt have seen many Web sites that provide the user with a "Contact Us" page. This page usually includes a small form that allows the visitor of the site to send an e-mail to the site's Webmaster. You can see similar forms in today's most popular type of Web site among the develop community—the blog. You may see this kind of form when you leave feedback for a blog posting. I'll show you how to encapsulate everything necessary to build this kind of form into a custom Web control. You'll even see how to build the actual e-mailing functionality directly into the control. I'll show you how to develop this control as a composite control and will contain within it both the FormField rendered control and the ConfirmationButton inherited control.

The WebControl class inherits form the Control class and adds some styling properties to it.
You will enable all these controls with certain functionality during the development steps in this article. At the end of this article, I will list additional functionality that the final versions of the controls include in the downloadable code.

So let's get to work...

The ConfirmationButton Control
The first custom Web control will also be the simplest. The ConfirmationButton control will be a simple class that starts by inheriting from the built-in Button Web control that ships with Visual Studio.

In VB.NET:

Public Class ConfirmationButton Inherits Button End Class

In C#:

public class ConfirmationButton : Button { }

The parent Button class of this control resides in the System.Web.UI.WebControls namespace, which you obtain by referencing the System.Web.dll in your project. If this class is compiled as-is, it will duplicate the functionality of the Button class that .NET provides. In fact, you can compile this class, add it to the toolbox for a Web Form, then drop it on a Web Form and it will be no different than dropping a Button control from the Web Form toolbox.

Properties
You will make only two simple changes to your new class. The first is the addition of a property. I want to describe this process in detail, however, because this is the way you'll add most properties in all three controls you'll build in this article. I say most properties because when you build the composite control, you'll code "mapped" properties a bit differently, but I'll get to that later. You want to add a ConfirmationMessage property to your ConfirmationButton class, and a value other than an empty string will indicate that you want a confirmation dialog box to pop up when a user clicks a button. Here is the code for your new property.

In VB.NET:



Public Property ConfirmationMessage() As String Get If CType( _ VieState("ConfirmationMessage"), _ Object) Is Nothing Then Return String.Empty End If Return CType( _ ViewState("ConfirmationMessage"), String) End Get Set(ByVal Value As String) ViewState("ConfirmationMessage") = Value End Set End Property

In C#:

public string ConfirmationMessage { get { if((object) ViewState["ConfirmationMessage"] == null) return String.Empty; return (string) ViewState["ConfirmationMessage"]; } set { ViewState["ConfirmationMessage"] = value; } }

Look closely and you'll notice that this property is not much different than properties you write every day in your classes. The typical procedure for a property is to expose a member variable of some kind. You've no doubt done this time and time again. The ConfirmationMessage property works similarly with the difference being that you aren't using a privately declared member variable. Instead, you're using a ViewState variable as the internal storage for the ConfirmationMessage property; so let me talk a little more about this.

The get accessor in the property returns the value of the internal variable. In the property, this is stored in ViewState["ConfirmationMessage"], and the set accessor sets the ViewState variable to whatever you set the property to. ViewState is a variable of type StateBag. Like the famous Session variables and Cookie variables that you have used in Web applications, it is a dictionary that contains one or more values accessed by a key. In this case, I've standardized the naming conventions so the key is equivalent to the name of the property. Keep this in mind because you're going to see it a lot more through the rest of this article. The contents of the ViewState variable are stored in the __ViewState hidden field on your Web page in an encrypted format and are used to save and retrieve values that you need to persist in between page postbacks. The rest of the code around the 'return' statement in the get accessor checks to see if the specific ViewState value exists in the ViewState array. Because ViewState is a dictionary type (actually, StateBag is), checking for an entry in it that does not exist does not raise an exception; instead, it adds an empty object. The if statement checks the ViewState value for Nothing (or null). If this is the case, then it will return whatever you want the default value to be. The check for Nothing (or null) is done with an object-cast of the ViewState variable. The reason for this is due to the nature of the StateBag object, where an attempted access of a non-existing value automatically adds a null value.

Standard Attributes
You'll also have a few additions for this property that will be "standard operating procedure" for all properties you write in custom Web controls. These consist of certain attributes with which you'll decorate your property. Some properties will receive more attribute decorations than others, but all will receive at least four attributes: Category, Description, Bindable, and DefaultValue. All are found in the System.ComponentModel namespace.

In VB.NET:

< _ Category("Behavior"), _ Description("Gets or sets the message to be " & _ "displayed in a confirmation dialog. The " & _ "dialog will present itself if this property " & _ "is not empty."), _ DefaultValue(""), _ Bindable(True) _ > _

In C#:

[ Category("Behavior"), Description(""), DefaultValue(""), Bindable(true) ]

Category Attribute
This attribute specifies the category that your property will be listed under when you view it in the property browser. Indulge me in a comical tangent: Make sure you do not accidentally misspell the category name. I can't tell you how many times I have misspelled the word "Appearance" when using it as a Category. Instead, I've typed "Apperance" and guess what the property browser shows? That's right—two categories, "Appearance" and "Apperance."

Description Attribute
This attribute determines the description of the property that is displayed at the bottom of the property browser. It provides simple instructions for programmers using your Web control.

DefaultValue Attribute
Contrary to what the name of this attribute makes you assume, it does not determine what gets stored in the property by default. As you saw when I described the Property statement, you're setting the default value for the ViewState variable in your get accessor. This attribute lets Visual Studio know what the default value of the property will be for two main reasons. The first is that it lets the property browser know whether to display the value of the property in bold letters or not. If you haven't noticed before, when you drop a control on a Web Form and refer to the property browser, all the property values are displayed in normal font until you change their values, after which they are displayed in bold letters. This lets you know that the entered value is not the default value for the property. The second reason is that only properties with non-default values appear in the ASPX tags that define your Web control on a Web Form.

Bindable Attribute
This attribute determines if the property will participate in data binding functionality. The data source for the property can be defined in an expression. This topic is beyond the scope of this article, but if you want further information, look at the Data category of a typical Web control in the property browser and click on the DataBindings value, then click the Help button for a full explanation.

This is the technique for creating properties that you'll see in many of the properties going forward in all your Web controls, so this will be the only place where I will get into a lot of detail for Web control properties.



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