var request = new CreateInvoiceRequest();
request.AuthenticationData.ApiKey = "<YOUR AGENT KEY>";

request.Seller.BankName = "Our Bank";
request.Seller.BankAccount = "11111111-22222222-33333333";

request.Customer.Name = "Good Friend Inc.";
request.Customer.CustomerAddress.PostalCode = "1132";
request.Customer.CustomerAddress.City = "Budapest";
request.Customer.CustomerAddress.StreetAddress = "Tárnoki út 23.";
request.Customer.TaxNumber = "12345678-1-42";

request.Items = new List<InvoiceItem> {
    new InvoiceItem {
        Name = "Something we sell",
        Quantity = 1,
        UnitOfQuantity = "piece",
        UnitPrice = 1000,
        VatRate = "EU",
        NetPrice = 1000,
        VatAmount = 0,
        GrossAmount = 1000
    }
};

var api = new SzamlazzHuApi();
var response = await api.CreateInvoice(request);

//Optionally write invoice PDF to the disk
File.WriteAllBytes("invoice.pdf", response.InvoicePdf);

var request = new QueryTaxpayerRequest();
request.AuthenticationData.ApiKey = "<YOUR AGENT KEY>";
request.TaxpayerId = "<CORE TAX NUMBER (FIRST 8 DIGIT OF THE VAT NUMBER)>"

var api = new SzamlazzHuApi();
var response = await api.QueryTaxpayer(request);

public class QueryTaxpayerResponse
{
    // True, if the query was successful and there were no technical error in communication
    public bool Success { get; set; }
    // Contains error code about request. It is -1, if the query was successful
    public int ErrorCode { get; set; }
    // Contains error message. It is null if query was successful
    public string ErrorMessage { get; set; }
    // Holds information about the validity of the VAT number
    public bool TaxpayerValid { get; set; }
    // Contains everything about the holder of the queried VAT number. It can be null in case the TaxpayerValid is false
    public Taxpayer Taxpayer { get; set; }       
}

public class Taxpayer
{
    // Contains the legal name of the taxpayer
    public string TaxpayerName { get; set; }
    // Contains the short name of the taxpayer, which can be null               
    public string TaxpayerShortName { get; set; }
    // Contains tax number of the taxpayer
    public TaxNumber TaxNumberDetail { get; set; }
    // Contains tax number of the VAT group, if taxpayer has group membership
    public TaxNumber VatGroupMembership { get; set; }
    // Contains the incorporation type of the taxpayer
    public string Incorporation { get; set; }
    // Contains a list of the taxpayer addresses. This can be empty list   
    public List<TaxpayerAddressItem> AddressList { get; set; }
}

public class TaxNumber
{
    // This contains the core tax number (XXXXXXXX)-X-XX
    public string TaxpayerId { get; set; }
    // This contains the VAT code, it can be null if it is not presented (eg. group VAT number) XXXXXXXX-(X)-XX
    public string VatCode { get; set; }
    // This contains the country code, it can be null if it is not presented (eg. group VAT number) XXXXXXXX-X-(XX)
    public string CountryCode { get; set; }
}

public class TaxpayerAddressItem
{
    // This holds information about the type of the address (eg. HQ, SITE, etc.). See NAV documentation about the possible codes.
    public string AddressType { get; set; }
    // This contains the address    
    public Address Address { get; set; }
}