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Creating ASP.NET Custom Controls with Style : Page 3

Getting your custom control to behave the way you want it to is only half the work. You have to create the logic that generates the actual HTML shown in the browser. It's tedious work, especially if you're supporting multiple browsers.


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Building Composite Controls
When you build composite controls, the Render method takes on a different form. Since the control tree is already built once you enter the Render method, all you can do is provide the surrounding formatting, and apply formatting to the controls already in the tree. You still need to use the child control's rendering capabilities to render the control. Not doing so makes the child control invisible to the user and probably useless within the control. For the formatting surrounding the controls you can use the same methods as before—you can format the child controls either explicitly or by setting properties on the controls, or by applying a style. To apply a style, you again need a Styleobject. You can use the ControlStyle if you just want to propagate the main style to the child controls, or one defined in a public Style property. Listing 6 shows the body code for a composite control that shows a TextBox with a caption. The Caption takes the formatting of the control itself, but you can format the TextBox through a separate style. Notice that I defined the TextBox globally because multiple methods need to access it—first when the code adds the control to the control tree, and later to apply the style and render the control. You could also apply the style in the CreateChildControls method, but that would cause the style of the child control to be stored in the ViewState, in addition to the Style object that has been applied. This would result in a larger ViewState for no reason whatsoever. The ApplyStyle method copies all properties from the originating style to the ControlStyle of the child control, overriding any existing style properties already set on the child control. If you only want to override blank properties, you should use the MergeStyle method instead. You can apply formatting to controls in several ways. The best way differs per control, but in most cases you will want to keep the outer tags of the control as defined by the parent control. This means you should override the RenderContents method instead of the Render method. When you work with composite controls, keep in mind that you can tweak the formatting of the child controls during the Render phase. In addition, you can add additional HTML during this phase so you can actually create a sparse control tree surrounded by HTML generated in the Render or RenderContents method. This is more efficient than adding more controls to the control tree.


Michiel van Otegem is the co-founder and president of ASPNL.com, a consulting and training firm based in Amsterdam, the Netherlands. He teaches advanced ASP.NET and XML classes, and is a frequent seminar speaker. He is also the author of Sams Teach Yourself XSLT in 21 Days. Michiel is a respected member in the international ASP/ASP.NET community, helping on forums and mailing lists. In his spare time he is also the president of the Dutch .NET user group, Stichting dotNED.
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