Go through all textboxes, comboboxes,... and clear their content

You are correct: the other responses here do not deal with Controls inside Container Controls, like Panel, and GroupBox, in the Form.

To find all the TextBoxes, and ComboBoxes, on the Form, no matter how "deeply" they are "nested" inside Container Controls, requires a recursive search.

In my response to a previous QA question I provided code for recursively building a collection of all Controls on a Form using the "stack-based" technique recommended by Eric Lippert: [^].

You can take that code and use it, like this:

// requires Linq

// clear Text in all TextBoxes
foreach (TextBox theTextBox in (SpecialMethods.GetAllControls(this)).OfType<TextBox>().ToList())
{
    theTextBox.Clear();
}

// clear ComboBoxItems in all ComboBoxes
foreach (ComboBox theComboBox in (SpecialMethods.GetAllControls(this)).OfType<ComboBox>().ToList())
{
    theComboBox.Items.Clear();
}

foreach (Control objControl in this.Controls)
            {

                if (objControl.GetType() == typeof(TextBox))
                {
                    TextBox objTxt = (TextBox)objControl;
                    objTxt.Text = "";
                }
            }

Try others control with same way.
Thanks

Those two solutions do the same thing - only working for controls that are put directly on a form. If controls are inside groupbox or panel, it does nothing.
If i use this:

foreach (Control c in f1.groupBox1.Controls)
       {

           if (c is TextBox || c is ComboBox)
           {
               c.Text = "";
           }

       }

deletes controls inside groupBox1. But i don't want to write the code for every groupbox, panel,...