As MSDN advises, you will need to iterate the controls as children of the GroupBox. Also, note that you usually need to add a Grid into your GroupBox to be able to add new controls into that GroupBox. So you will need to get the children of the Grid in that GroupBox and iterate through them, someething like this:
//iterate through the child controls of "grid"
int count = VisualTreeHelper.GetChildrenCount(grid);
for (int i = 0; i < count; i++)
{
Visual childVisual = (Visual)VisualTreeHelper.GetChild(grid, i);
if (childVisual is TextBox)
{
//write some logic code
}
else
{
}
}
You might find this useful: http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/93ebca8f-2977-4c36-b437-9c82a22266f6