Wednesday, October 25, 2006

[ASP.NET] Basic of Custom Server Control Development Part II - ParseChildren(), PersistChildren() and PersistenceMode()

Let us boils down to the three attributes that important in implementing simple properties and subproperties in the custom control development - ParseChildrenAttribute, PersistChildrenAttribute and PersistenceModeAttribute.

Default and Inner Property Persistence
Default persistence is in constrast to Inner persistence, but they are identically same for its functionality. In our control development, we may have complex properties (class). To persist them, we can use hyphenated syntax (Default Property Persistence) or nesting them within your control tags (Inner Property Persistence). Example of Inner Property Persistence,

<asp:GridView ID="GridView1" runat="server">
<HeaderStyle BorderWidth="12" />

In the example given, the BorderWidth is the subproperties of the HeaderStyle property of GridView. To enable this inner property persistence, you need to set the ParseChildren(true) and PersistChildren(false) in your control class.

public class MyWebControl : WebControl

1. ParseChildrenAttribute
This attribute tells the parser to parse the parse the contents within the control tag as properties but not child controls.

To enable the inner default property persistence, you have to specify the inner default property to the second parameter of ParseChildrenAttribute.

[ParseChildren(true, "<DefaultPropertyName>")]
public class MyControl : WebControl

2. PersistChildrenAttribute
On the other hand, this attribute tells the designer whether the child controls of the server control should be persisted as inner child controls. Setting to false indicates inner content corresponds to properties but not to child controls.

* Therefore, the ParseChildrenAttribute and the PersistChildrenAttribute attributes use opposing conventions for the semantics of their argument. Check more difference of these two attributes at Justin Lovell's Blog - The ParseChildren and PersistChildren Attributes

3. PersistenceModeAttribute
This attribute specifies how the property of server control is persisted declaractively in ASP.NET pages. It has 3 enumerations :

EncodedInnerDefaultProperty - Specifies that the property persists as the only inner text of the ASP.NET server control. The property value is HTML encoded. Only a string can be given this designation.

InnerDefaultProperty - Specifies that the property persists in the ASP.NET server control as inner text. Also indicates that this property is defined as the element's default property. Only one property can be designated the default property.

InnerProperty - Specifies that the property persists in the ASP.NET server control as a nested tag. This is commonly used for complex objects, those that have persistable properties of their own.

Picked from

public virtual TableItemStyle HeaderStyle { ... }

This indicates that the designer to persist the property as inner property. By default, it is persisted by using hyphenated syntax in control tag. To persist inner default property, you need to have

[ParseChildren(true, "<DefaultPropertyName>")]
public class MyControl : WebControl


public virtual ListItemCollection Items { ... }

These three attributes are confusing, but are not undifferentiable.


Rem said...

Nice post...

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