Define a property of a base class (ascx) in a custom UserControl, and instanciate a child class in the aspx

I defined 3 custom usercontrols like that :

public partial abstract class MyAbstractControl : Usercontrol{
  // Base class definition here, with common property and methods
  public string CommonAttribute {get; set;}
}

public partial class MyConcreteControl1 : MyAbstractControl{
  // Some specific stuff here
}

public partial class MyConcreteControl2 : MyAbstractControl{
  // Other specific but different stuff here
}

Then I defined another UserControl that have a property of the base class :

public partial class MyBeautifulControl : UserControl{
  [PersistenceMode(PersistenceMode.InnerProperty)]
  public MyAbstractControl ChildElement{get;set;}
}

In the aspx file, I'm using this control, but I'd like to define an instance of MyConcreteControl1 instead of MyAbstractControl

But If I write :

<MyBeautifulControl ru开发者_开发问答nat="server" id="beautiful">
  <ChildElement commonAttribute="value" />
</MyBeautifulControl>

The ChildElement can only be defined as a MyAbstractControl instance. I'd like to create a MyConcreteControl1 or an MyConcreteControl2 instance, depending on the context, and I don't know how.

---

Use the System.Web.UI.TemplateContainerAttribute:

[PersistenceMode(PersistenceMode.InnerProperty), 
 TemplateContainer(typeof(MyConcreteControl))]
public MyAbstractControl ChildElement{get;set;}

see the documentation for details. There is also a HOWTO about.