C# SQL API & XML Export

First, I want the data layer to communicate with external accounting services through a REST-style API. The module should issue authenticated HTTP requests using only the methods I plan to expose—GET, POST and PUT—to push or pull records between the database and providers such as InFakt or Betterfly. I expect clean, asynchronous calls via HttpClient (or another modern .NET approach), clear error handling and concise helper classes that make future endpoint additions straightforward.

Second, the system must generate invoice exports. When I select ‘Faktury’ in the UI, the code should collect the relevant rows and create XML files that fully respect the FA(3) KSeF structure. The output has to validate against the official XSD so that I can upload it to KSeF without manual fixes.

Deliverables
• C# source code for the API integration layer (GET, POST, PUT implemented)
• C# exporter that writes FA(3) KSeF-compliant XML for invoices
• Sample configuration, schema validation proof

As data source we have prepared SQL views.

If you are comfortable with SQL Server, .NET and XML serialization, your tidy, well-commented pull request is exactly what I am waiting for.