Question

I was reading a post at VS 2008, ASP.NET: Generate Local Resources.
Mehdi Golchin showed us a beautiful job of StateManagedCollection.
However I was wondered about using multiple classes of IStateManager in one StateManagedCollection.
As you can see below:
public class MenuItemCollection : StateManagedCollection
{

public MenuItem this[int index] 
{ 
    get { return (MenuItem)((IList)this)[index]; } 
} 

public int Add(MenuItem item) 
{ 
    return ((IList)this).Add(item); 
} 

public void Remove(MenuItem item) 
{ 
    ((IList)this).Remove(item); 
} 

// Write Insert and RemoveAt methods 

protected override void SetDirtyObject(object o) 
{ 
    ((MenuItem)o).SetDirty(); 
} 

}
This MenuItemCollection class can have only one child class(“MenuItem”).
If I want to use another class as well as MenuItem class, for example MenuItem2 class, how do I have to write the codes?
Anyone can help me?
Thanks in advance.

Answer 1

Write a generic version – for example,

public class GenericStateManagedCollection<T> : StateManagedCollection 
   where T: IStateManager, new()
{

    public T this[int index]
    {
        get { return (T)((IList)this)[index]; }
    }

    public int Add(T item)
    {
        return ((IList)this).Add(item);
    }

    public void Remove(T item)
    {
        ((IList)this).Remove(item);
    }

    // Write Insert and RemoveAt methods

    protected override void SetDirtyObject(object o)
    {
        ((T)o).SetDirty();
    }

    protected override object CreateKnownType(int index)
    {
        return Activator.CreateInstance<T>();
    }

    protected override Type[] GetKnownTypes()
    {
        return new Type[] { typeof(T) };
    }

}

And use it as

public class MenuItemCollection : GenericStateManagedCollection<MenuItem> { }
public class XyzItemCollection : GenericStateManagedCollection<XyzItem> { }

EDIT:

I have most probably mis-understood your question! Assuming now that you want to put two different type of objects into the StateManagedCollection. From usage perspective, it doesn’t make sense to have objects of completely unrelated types into the collection – you need to have some base class. For example, consider DataControlFieldCollection which holds instances of (abstract) type ‘DataControField. BoundField, ButtonField etc inherits fromDataControField`.

So you need to go via similar route – for example,

public class MenuItemBase : IStateManager
{
   // Use implementation from link you quoted (Mehdi Golchin's answer)
   ...
}

public class MenuItem : MenuItemBase
{
  ...
}

public class MenuItem2 : MenuItemBase
{
  ...
}

public class MenuItemCollection : StateManagedCollection
{

    public MenuItemBase this[int index]
    {
        get { return (MenuItemBase)((IList)this)[index]; }
    }

    public int Add(MenuItemBaseitem)
    {
        return ((IList)this).Add(item);
    }

    public void Remove(MenuItemBaseitem)
    {
        ((IList)this).Remove(item);
    }

    // Write Insert and RemoveAt methods

    protected override void SetDirtyObject(object o)
    {
        ((MenuItemBase)o).SetDirty();
    }

    // important to override CreateKnownType and GetKnownTypes
    private static readonly Type[] _knownTypes = new Type[] {typeof(MenuItem), typeof(MenuItem2) }

    protected override Type[] GetKnownTypes()
    {
       return _knownTypes;
    }

    protected override object CreateKnownType(int index)
    {
       switch (index)
       {
          case 0:
             return new MenuItem();

          case 1:
              return new MenuItem2();

          default:
              throw new Exception("Invalid Index");
       }
     }
}

Note: Untested code