How to write test case using nunit

public class ClassName 
{
private void SetBillingContactEmail()
        {
            //Return if billing contact and its emails are null
            if (_companyview.BillingContact == null || _companyview.BillingContact.Emails == null)
                return;

            //Sets the primary email address of billing contact to the CustomerContactAREmail
            _customermodel.CustomerContactARemail = _companyview.BillingContact.Emails.Where(x => x.IsPrimary == true).Select(y => y.Address).FirstOrDefault();
        }
}