InvoiceExportKit.Abstractions 1.0.0

InvoiceExportKit

A lightweight, open-source .NET 8 library for exporting professional invoice documents to Excel (.xlsx) using the Open XML SDK. Designed as a reusable, NuGet-friendly toolkit with clean separation of concerns.

Features

• Export invoices to .xlsx with full Open XML SDK support
• Seller and buyer information blocks (name, address, tax ID, email, phone)
• Invoice metadata: number, issue date, due date, currency, notes
• Line items with code, description, quantity, unit, unit price, discount %, and totals
• Totals section: subtotal, discount, tax, and final total
• Visual formatting: merged cells, custom column widths, borders, bold headers, number/date formatting, horizontal alignment
• Two built-in templates: Simple (clean blue) and Corporate (dark header, row banding)
• Pluggable template system — implement IInvoiceTemplate and pass it in
• Validation layer with descriptive error messages
• Auto-calculation of line totals and invoice totals
• ASP.NET Core Web API sample with POST /api/invoices/export/excel
• Console sample generating both templates to an output folder

Project Structure

InvoiceExportKit/
├── src/
│   ├── InvoiceExportKit.Abstractions      # Models, contracts, options (no dependencies)
│   ├── InvoiceExportKit.Core              # Validation + totals calculation (→ Abstractions)
│   ├── InvoiceExportKit.Excel.OpenXml     # Open XML rendering (→ Abstractions, Templates)
│   └── InvoiceExportKit.Templates         # Built-in templates: Simple & Corporate (→ Abstractions)
├── samples/
│   ├── InvoiceExportKit.ConsoleSample     # CLI sample: writes .xlsx files to /output
│   └── InvoiceExportKit.WebApiSample      # ASP.NET Core API sample
└── tests/
    ├── InvoiceExportKit.Core.Tests        # Calculation & validation unit tests
    └── InvoiceExportKit.Excel.OpenXml.Tests # Export integration tests

Dependency Graph

Abstractions  ←  Core
Abstractions  ←  Templates
Abstractions  ←  Excel.OpenXml  ←  Templates

Business logic (Core) has zero dependency on the rendering layer (Excel.OpenXml).

Quick Start

1. Build the invoice model

var invoice = new InvoiceModel
{
    InvoiceNumber = "INV-2024-001",
    IssueDate     = new DateTime(2024, 11, 1),
    DueDate       = new DateTime(2024, 11, 30),
    Currency      = "USD",
    TaxRate       = 21m,
    Notes         = "Payment within 30 days. Thank you for your business.",

    Seller = new ContactModel
    {
        Name    = "Acme Software LLC",
        TaxId   = "US-12-3456789",
        Email   = "billing@acme.com",
        Address = new AddressModel
        {
            Street = "350 Fifth Avenue", City = "New York",
            PostalCode = "NY 10118", Country = "United States"
        }
    },

    Buyer = new ContactModel
    {
        Name    = "Globex Corp.",
        Address = new AddressModel
        {
            Street = "742 Evergreen Terrace", City = "Springfield",
            PostalCode = "62701", Country = "United States"
        }
    },

    Items =
    [
        new InvoiceItemModel
        {
            Code = "LIC-ENT", Description = "Enterprise License (annual)",
            Quantity = 1, Unit = "license", UnitPrice = 9_999.00m
        },
        new InvoiceItemModel
        {
            Code = "SVC-IMPL", Description = "Implementation Services",
            Quantity = 20, Unit = "hrs", UnitPrice = 150.00m, DiscountPercent = 10m
        }
    ]
};

2. Export

var exporter = new ExcelInvoiceExporter();
var service  = new InvoiceExportService(exporter);

byte[] bytes = service.Export(invoice, new ExportOptions
{
    TemplateName      = "Corporate",  // or "Simple"
    RecalculateTotals = true,
    SheetName         = "Invoice"
});

File.WriteAllBytes("invoice.xlsx", bytes);

3. ASP.NET Core DI

builder.Services.AddSingleton<IInvoiceExporter, ExcelInvoiceExporter>();
builder.Services.AddSingleton<IInvoiceValidator, InvoiceValidator>();
builder.Services.AddScoped<InvoiceExportService>();

Then POST /api/invoices/export/excel with the ExportRequest JSON body (see InvoicesController).

Templates

Custom template

public class BrandedTemplate : IInvoiceTemplate
{
    public string Name => "Branded";
    public TemplateRenderOptions RenderOptions { get; } = new()
    {
        HeaderBackgroundColor = "C00000",
        HeaderFontColor       = "FFFFFF",
        FontName              = "Arial",
        TitleFontSize         = 22,
        ColumnWidths          = new() { [3] = 42 }
    };
}

var exporter = new ExcelInvoiceExporter(customTemplates: [new BrandedTemplate()]);

Running Tests

dotnet test

Running the Console Sample

cd samples/InvoiceExportKit.ConsoleSample
dotnet run
# Output files written to: bin/Debug/net8.0/output/

Running the Web API Sample

cd samples/InvoiceExportKit.WebApiSample
dotnet run
# Swagger UI available at: https://localhost:5001/swagger

Example cURL request:

curl -X POST https://localhost:5001/api/invoices/export/excel \
  -H "Content-Type: application/json" \
  -d '{
    "invoice": {
      "invoiceNumber": "INV-001",
      "issueDate": "2024-11-01",
      "dueDate": "2024-11-30",
      "currency": "USD",
      "taxRate": 20,
      "seller": { "name": "Seller Co", "address": { "street": "1 Main", "city": "NYC", "postalCode": "10001", "country": "US" } },
      "buyer":  { "name": "Buyer Inc", "address": { "street": "2 Main", "city": "LA",  "postalCode": "90001", "country": "US" } },
      "items": [{ "description": "Consulting", "quantity": 5, "unit": "hrs", "unitPrice": 200 }]
    },
    "options": { "templateName": "Corporate" }
  }' \
  --output invoice.xlsx

Roadmap

• PDF export via a pluggable IPdfInvoiceExporter (QuestPDF / iText)
• Multiple currency symbol rendering in cells (not just ISO code)
• Logo / image embedding support for Corporate template
• Localization support (date formats, decimal separators) via CultureInfo
• Async export API (ExportAsync) for streaming large workbooks
• IBAN / payment-details section in the template
• Additional built-in templates: Minimal, A4Formal
• NuGet packages published to nuget.org
• GitHub Actions CI pipeline (build + test)

License

MIT — see LICENSE for details.