example

nested.cs

var contact = new Contact();
contact.Name = Name;
      
var line = new LineItem() {
  Description = LineDescription,
  Quantity = float.Parse(LineQuantity),
  UnitAmount = float.Parse(LineUnitAmount),
  AccountCode = LineAccountCode
};

var lines = new List<LineItem>() {
  line
};

var invoice = new Invoice() {
  Type = Invoice.TypeEnum.ACCREC,
  Contact = contact,
  Date = DateTime.Today,
  DueDate = DateTime.Today.AddDays(30),
  LineItems = lines
};

var invoiceList = new List<Invoice>();
invoiceList.Add(invoice);

var invoices = new Invoices();
invoices._Invoices = invoiceList;

var AccountingApi = new AccountingApi();
var response = await AccountingApi.CreateInvoicesAsync(accessToken, xeroTenantId, invoices);