Change Source to several Labels at the same Time, C #

You could create a read-only property in your UserControl that returns the list of controls added to it. Not including those of the user control.

According to the code of the control on which you have based, it would be something like that

 public IEnumerable<Control> PanelControls
{
    get
    {
        var panelControls = new List<Control>();
        foreach (Control control in Controls)
        {
            if (control != titlePanel) panelControls.Add(control);
        }
        return panelControls;
    }
}

In this way the PanelControls property only returns the controls added to the user control, not your own, and you could go through them and change the source or any other property you want.

The cambiar_fuentes method could be changed by a generic method in which you indicate the type of control to which you want to apply the change, and pass the source:

private void CambiarFuentes<T>(IEnumerable<Control> controls, Font newFont) where T: Control
{
    var controlList = controls as Control[] ?? controls.ToArray();
    if (!controlList.Any()) return;
    foreach (var toChangeControl in controlList.OfType<T>())
    {
        toChangeControl.Font = newFont;
    }
    CambiarFuentes<T>(controlList.SelectMany(x => x.Controls.OfType<Control>()), newFont);
}

In this way to change all the sources of the Labels included in the controls CollapsiblePanel you could do:

foreach (var collapsiblePanel in Controls.OfType<CollapsiblePanel>())
{
    CambiarFuentes<Label>(collapsiblePanel.PanelControls, new Font("Arial", 10, FontStyle.Regular));
}

If you prefer to keep the current format you can check in the cambiar_fuentes method if the container control is a type control CollapsiblePanel . If so, you go through the collection PanelControls and if not the collection Controls :

private void cambiar_fuentes(Control contenedor)
{
    var panel = contenedor as CollapsiblePanel;
    var controlList = panel != null
        ? panel.PanelControls
        : contenedor.Controls.OfType<Control>();
    foreach (Control control in controlList)
    {
        if (control.Controls.Count > 0)
            cambiar_fuentes(control);
        else
        {
            if (control is Label) ((Label)control).Font = new Font("Arial", 10, FontStyle.Regular);
        }
    }
}

This way you could continue making the same call:

cambiar_fuentes(panel1);

It occurs to me that you could discriminate the usercontrols when you evaluate whether the control has internal controls in the following way:

private void cambiar_fuentes(Control contenedor)
{
    foreach (Control control in contenedor.Controls)
    {
        if (control.Controls.Count > 0 && !(control is usercontrol))
           cambiar_fuentes(control);
        else
        {
           if (control is Label) ((Label)control ).Font = new Font("Arial", 10, FontStyle.Regular);
        }
   }
}

Keep in mind that you have to replace usercontrol with the name of the class in your user control.

This way you avoid having user controls inspected.

Hello, you can do the following Recover from a control, in this case your form those of type Label in the following way

private void cambiar_fuentes(Control contenedor)
{
  foreach (var c in contenedor.Controls.OfType<Label>())
  {
    c.Font = new Font("Arial", 10, FontStyle.Regular);

  } 
}

Then you would call it that in your form

cambiar_fuentes(this);