Reset a form to its original state with a button

I created a new button to reset the form which consists of

combobox,

textbox,

richtextbox,

disabled button

So, i created a utilities.cs class but its not working and gives an error.

ERROR: The type or namespace name 'ComboBox' could not be found.

namespace example
{
class Utilities
{
    public static void ResetAllControls(Mainform form)
    {
        foreach (Mainform control in form.Controls)
        {
            if (form is ComboBox)
            {
                ComboBox comboBox = (ComboBox)control;
                if (comboBox.Items.Count > 0)
                    comboBox.SelectedIndex = 0;
            }
        }
    }

}

}