Hi Matt and thanks for your answer.

 Should I do like that?

class CustomUltraExpandableGroupBox : UltraExpandableGroupBox
{
    protected override void OnPanelInitialized()
    {
        base.OnPanelInitialized();

        //Ajout du panel
        this.Panel = new UltraExpandableGroupBoxPanel();
    }
}

I got the same problem anymore...

Sorry for the confusion.  This method is called when the UltraExpandableGroupBox creates its panel; you cannot assign your own panel to this property.  So, now that you have the panel in this method, you can simply do "this.Panel.Controls.Add(myControl);".

-Matt

 No problem ;)

But there's still a problem.

Your method is the first I tried, but the property .Panel is null when i do a [UltraExpandableGroupBox uegb = new UltraExpandableGroupBox();]

So I can't access the .Controls attribute and no more the .Add() method...

[EDIT] If I modify the Panel attribute even in the class Custom, I've got:  Property or indexer (...) cannot be assigned to -- it is read only 

I think that you're misunderstanding the point of the OnPanelInitialized method.  You will never be able to assign anything to the Panel property, this is created by the UltraExpandableGroupBox.  You need to use the OnPanelInitialize method to populate the contents of your control, such as:

 public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        MyExGroupBox grp = new MyExGroupBox();
        grp.Dock = DockStyle.Fill;
        this.Controls.Add(grp);
    }

    private class MyExGroupBox : UltraExpandableGroupBox
    {
        protected override void OnPanelInitialized()
        {
            base.OnPanelInitialized();

            Label label = new Label();
            label.BackColor = Color.Blue;
            label.Dock = DockStyle.Fill;
            this.Panel.Controls.Add(label);
        }
    }
}

-Matt

Hi have a similar problem. When I create the group box at run time and then try and add another control into it the Panel property is always null.  I am using NetAdvantage for .NET 2007 Volume 1 CLR 1.x. When I create the class above that inherits the expandable group box I do not have visibility to OnPanelInitialized(). Is there another event that I can override or listen to in order to then populate the controls into the panel?

thanks in advance