test.cs

public static string GetDummyInvoiceData()
{
	Invoice invoice = new Invoice
	{
		InvoiceID = "123456789",
		InvoiceNumber = "123456789",
		InvoiceDate = "2020-01-01",
		Customer = new Customer
		{
			CustomerID = "123456789",
			Name = "John Doe",
			Email = ""
		},
		LineItems = new List<LineItem>
		{
			new LineItem
			{
				LineItemID = "123456789",
				Description = "Item 1",
				Quantity = "1",
				Price = "100.00",
				Tax = "10.00",
				Total = "110.00"
			},
			new LineItem
			{
				LineItemID = "123456789",
				Description = "Item 2",
				Quantity = "1",
				Price = "100.00",
				Tax = "10.00",
				Total = "110.00"
			},
			new LineItem
			{
				LineItemID = "123456789",
				Description = "Item 3",
				Quantity = "1",
				Price = "100.00",
				Tax = "10.00",
				Total = "110.00"
			}
		}
	};

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

	return JsonConvert.SerializeObject(invoices);
}