The ParseChildren PersistChildren and PersistenceMode.InnerProperty

After a while of non-web control development, ParseChildren and PersistChildren attributes are important attributes to remember when trying to get the desired results your looking for in the Visual Studio designer.

So, this post should clear up (and serve as a reminder for me) how and why these attributes are important.  Let’s explore what these two attributes are used for.  Let’s start with ParseChildren.

The [ParseChildrenAttribute]

The ParseChildren Attribute is probably, the most important attribute you should pay attention to when developing web controls.  It’s actually used by the ASP.NET Parser and ControlBuilder object to figure out how to parse the ASP.NET code you write.  Visual Studio also uses this attribute to figure out what valid sub-controls and components are allowed within the contents of a server control.

Let’s say, I want to create an AggregateFeeds control that displays an aggregate list of RSS feeds.

A Basic and Boring Control Syntax

You’ll notice that the RssResource is the only available option that is allowed as a child from the AggregateFeeds control.  Here’s the code behind the AggregateFeeds control:

113     [
114     ParseChildren(
115         typeof(RssResource),
116         DefaultProperty = "Feeds",
117         ChildrenAsProperties = true
118         )
119     ]
120     public class AggregateFeeds : Control
121     {
122         public AggregateFeeds()
123         {
124             this.Feeds = new RssFeedCollection();
125         }
126         public RssFeedCollection Feeds
127         {
128             get;
129             private set;
130         }
131         protected override void Render(HtmlTextWriter writer)
132         {
133             this.Feeds
134                 .ForEach( rssRes => writer.Write( rssRes.Url ) );
135         }
136     }
137 
138     public class RssFeedCollection : List<RssResource>
139     {
140 
141     }
142 
143     public class RssResource
144     {
145         public string Url { get; set; }
146     }

The ParseChildren attribute on AggregateFeeds tells the ASP.NET, that any children within the AggregateFeeds control should be typeof(RssResource).  ChildrenAsProperties=true let’s ASP.NET know that it should STOP parsing server controls with “runat=server”, and switch to instantiating objects into the properties of the ArggregateFeeds control.  DefaultProperty says, that the results of the parsed objects should go into the default property Feeds.

Syntax Goodness With InnerProperty

The previous example was great, it’s simple and get’s the job done.  But let’s say, the requirements have changed, our control is growing, and we need to allow more customization, and extensibility for the consumers of our AggregateFeeds control.

Let’s clean up the markup and allow our developers to create markup like this:

To get this type of syntactical behavior, check out the code below:

113     [
114     ParseChildren(
115         ChildrenAsProperties = true
116         )
117     ]
118     public class AggregateFeeds : Control
119     {
120         public AggregateFeeds()
121         {
122             this.Feeds = new RssFeedCollection();
123         }
124 
125         [PersistenceMode(PersistenceMode.InnerProperty)]
126         public RssFeedCollection Feeds
127         {
128             get;
129             private set;
130         }
131         [PersistenceMode(PersistenceMode.InnerProperty)]
132         public AggregateSettings Settings
133         {
134             get;
135             private set;
136         }
137         protected override void Render(HtmlTextWriter writer)
138         {
139             this.Feeds
140                 .ForEach( rssRes => writer.Write( rssRes.Url ) );
141         }
142 
143     }
144 
145     public class AggregateSettings
146     {
147         public int TimeOut { get; set; }
148         public bool CacheResults { get; set; }
149     }
150 
151     public class RssFeedCollection : List<RssResource>
152     {
153 
154     }
155 
156     public class RssResource
157     {
158         public string Url { get; set; }
159     }

Notice, we’ve removed DefaultProperty and typeof(RssResource) from ParseChildren attribute.  We’re no longer working with a simple control that has simple children objects that need to be parsed, we’re now working with a complex control with more than one property that we’re setting in the markup, so we’ve removed the “default” stuff.  The syntactical magic happens with PersitanceMode attribute on the properties.  PersistanceMode.InnerProperty allows us to specify our cool <Feeds> and <Settings> tags.  How does Visual Studio know what members are available?  It does so by Reflection.

Get fancy, more than one child type

Also, I want to point out, suppose, we want to support multiple types of Feed objects.  We could use an enum in RssResource, or we could use inheritance to achieve the following:

All we would have to do is simply mark RssResource as an abstract class. Then, subclass for each type.

156     public abstract class RssResource
157     {
158         public string Url { get; set; }
159     }
160     public class MediaRss : RssResource
161     {
162 
163     }
164     public class ITunesRss : RssResource
165     {
166 
167     }

Again, I’m just showing that it’s possible, but following my mantra of “less code, less maintenance,” I’d use an enum to describe the type of rss feed on RssResource.

Where is [PersistChildren]?

Nowhere!  Is PersistChildren attribute needed?  No, it’s not a required attribute to create your custom control.  The PersistChildrenAttribute only provides designer support for your control with Visual Studio and has no “processing” affect in ASP.NET, but remember ParseChildren does.

I’m a   source-view only guy.  I really don’t remember the last time I’ve used the Visual Studio “Design View”, it’s a waste, crashes all the time, so I’ve pretty much given up on it.  Besides, “Design View” is for n00bs anyway.  Just kidding!  If you plan on using the Design View, then you’ll probably need your PersistChildren attribute…

In general, PersistChildren and ParseChildren are exclusive complementary attributes to describe the same semantic operation.  The rule of thumb goes:

If ParseChildren(true), then PersistChildren(false). 

If ** ParseChildren(false), then PersistChildren(true).

Following the PersistChildren guideline above should keep your code out of trouble.  But again, I wouldn’t use PersistChildren only until you actually need it.  Less code, less maintenance.

Here’s a nice list of attributes you should consider when writing your custom controls:

http://wdevs.blogspot.com/2007/10/attributes-to-consider-applying-when.html

Hope that helps! Happy coding!

Brian Chavez