Working with Custom Provider Controls
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by Brian Mains
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Custom Server Controls Overview

Within the ASP.NET architecture, there are a variety of kinds of server controls.  There are controls that don't render any type of user interface but that have an important function such as data source controls, AJAX control extenders, etc.  There are web or HTML controls that map to existing HTML controls like the TextBox, CheckBox, and others.

There are more advanced controls that have additional capabilities, such as template controls, or controls that their interface is created from the template that is provided.  There are data-bound controls that render an interface based on the underlying data source, or validation controls to validate user input.

These are the various types of controls that can be created.  The base class for all controls is the System.Web.UI.Control class, which is often used for controls that don't render a user interface.  For more specific functionality, server controls in ASP.NET inherit from the System.Web.UI.WebControls.WebControl class, which provides added capabilities to server-control rendering.

For controls that consist of one or more child controls, the CompositeControl class can be used.  All of the child controls that make up the interface are added to the Controls collection, which this control can directly expose properties/methods of the children, grouping the entire multiple server control logic into one logical unit.  For data-bound controls, the DataBoundControl and CompositeDataBoundControl classes define methods for receiving a data source, performing the binding and rendering the final user interface based on the data provided.

This article assumes you have an understanding of these controls, and a basic understand of custom control development.  However, if you need more information, the following resources are helpful.

·         Developing ASP.NET Server Controls Overview on MSDN

·         A Crash Course on Creating Controls

Throughout the article, you will notice in the code segments statements like the following: "this.DesignMode". Because some of the code (like the CreateChildControls) method runs in the designer, there may be some problems that come up in the designer.  Wrapping certain code in an "if" statement, with this as its conditional, would prevent those errors from occurring.  Plus, using this statement within code means that I don't have to provide a custom designer, which saves some coding effort for the time being.

Custom controls also have to check the EnableViewState property within it, because the control has to run with or without ViewState enabled, and will perform certain actions more when it is disabled, such as data binding to a data source control.  There is another option of using ControlState instead, since it guarantees state storage for control values, but you should only use this for critical data.  You will also see the use of the ViewState collection in many of the property definitions.  Before returning an object, the control must ensure the value exists within ViewState; returning a value and converting it to the proper type without checking it first will raise an exception.

Rendering occurs through the HtmlTextWriter class; not all controls need to customize the rendering process, but when custom rendering is needed, this class does the job.  It has a Write() method to write whatever text it needs to, but it also has a better process for writing HTML tags; RenderBeginTag() and RenderEndTag() methods that allow you to dynamically create the tag by passing in the tag name only.  Any attributes that you want to add to an element being rendered with RenderBeginTag() need to be added before you call this method.  That means all of the AddAttribute() and AddStyleAttribute() definitions occur before RenderBeginTag() is called.

Controls also make use of attributes, but this will be discussed later.

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