tx.cs

List<Invoice> Invoices = new List<Invoice>();

Invoice invoice = new Invoice()
{
    Customer = new Customer() {
        Name = "Peter Thiel",
        City = "Charlotte",
        Street = "123 Microsoft Way",
        Zip = 22282 },

    InvoiceNumber = "I778977622",

    Items = new List<Product>()
    {
        new Product() {
            Name = "Product 1",
            Description = "Product 1 Description"
        },
        new Product() {
            Name = "Product 2",
            Description = "Product 2 Description"
        }
    }
};

Invoices.Add(invoice);

var json = JsonConvert.SerializeObject(Invoices, Formatting.Indented);