Billecta.Connector.DebtorConnector dc = new Billecta.Connector.DebtorConnector("", "", true);
Guid creditorPublicId = /* Enter the owner creditor public id here */;

DebtorView debtor = new DebtorView
{
    CreditorPublicId = creditorPublicId,
    Name = "Acme AB",
    Address = "Address 1",
    ZipCode = "11173",
    City = "Stockholm",
    ContactEmail = "john.doe@acme.com",
    ContactName = "John Doe",
    CountryCode = "SE",
    Email = "invoice@acme.com",
    OrgNo = "556677-8899",
    VatNumber = "SE556677889901",
};
var created = dc.CreateDebtor(debtor);
Guid debtorPublicId = Guid.Parse(created.PublicId); // save debtorPublicId on you side

Billecta.Connector.InvoiceConnector ic = new Billecta.Connector.InvoiceConnector("", "", true);
Guid creditorPublicId = /* Enter the owner creditor public id here */;
Guid shoesProductPublicId = /* Retreive the shoes product public id here */;
Guid socksProductPublicId = /* Retreive the socks product public id here */;

InvoiceActionEntryView debtor = new InvoiceActionEntryView
{
    CreditorPublicId = creditorPublicId,
    CommunicationLanguage = LanguageTypeView.SV,
    DebtorPublicId = debtorPublicId,
    DeliveryMethod = DeliveryMethodTypeView.Email,
    InvoiceDate = DateTime.Now,
    DueDate = DateTime.Now.AddDays(20),
    InvoiceNumber = null, // when set to null Billecta api will generate invoice number for you
    InvoiceFee = new AmountView(2500, "SEK"), // 25 SEK invoice fee
    InvoicePDF = null, // when set to null Billecta wil generate the invoice pdf
    OurReference = "John Doe", // you contact person
    YourReference = "Customer contact name",
    InterestPercentage = 8,
    InterestTermsInDays = 365, // interest ticks 8% every 360 days
    InterestType = InterestTypeView.AboveEffectiveReference,
    InterestStartInDaysAfterDueDate = 0, // start ticking invoice from first due date
    Records = new List
    {
        new InvoiceActionRecordView
        {
            SequenceNo = 0,
            ArticleNumber = 1,
            ArticleDescription = "Socks",
            ProductPublicId = socksProductPublicId,
            Quantity = 10,
            Units = "pairs",
            RecordType = RecordTypeView.Standard,
            UnitPrice = new AmountView(10000, "SEK"), // costs 100 SEK each pair
            VAT = 25,
        },
        new InvoiceActionRecordView
        {
            SequenceNo = 1,
            ArticleNumber = 4,
            ArticleDescription = "Shoes",
            ProductPublicId = shoesProductPublicId,
            Quantity = 2,
            Units = "pairs",
            RecordType = RecordTypeView.Standard,
            UnitPrice = new AmountView(50000, "SEK"), // costs 500 SEK each pair
            VAT = 25,
        },
        new InvoiceActionRecordView
        {
            SequenceNo = 3,
            ArticleDescription = "Thank you for purchasing at Acme",
            RecordType = RecordTypeView.Message
        }
    }
};
var created = ic.CreateInvoice(debtor);
string actionPublicId = created.PublicId;