Try another function:
public static void ClearChildControls( this Control parentControl )
{
foreach( Control control in parentControl.Controls )
{
switch( control )
{
case TextBox textbox: textbox.Text = string.Empty; break;
case TextBoxBase txt: txt.Text = string.Empty; break;
case ComboBox comboBox: comboBox.Text = String.Empty; break;
case CheckBox checkBox: checkBox.Checked = false; break;
case RadioButton radioButton: radioButton.Checked = false; break;
case DateTimePicker dateTimePicker: dateTimePicker.Value = DateTime.Now; break;
default: control.ClearChildControls( ); break;
}
}
}