C# GST Details and E-Invoice Library

I need a clean, well-documented C# ClassLibrary that I can drop into multiple .NET solutions to handle India-specific GST tasks.

Scope of work
• Generate e-invoices strictly in JSON format that conforms to the standard schema released by the GSTN.
• Fetch a firm’s legal name, trade name, address and registration status when I pass a valid GSTIN.
• Validate any GST number for length, checksum and structural rules before the service calls are made.

I have not locked myself to a particular API yet, so design the HTTP layer so I can point it either to the official Government GST API (FREE) or any future third-party endpoint (FREE) by swapping a single configuration setting .

Key expectations
• .NET Standard 2.0+ so the DLL works in both legacy and modern projects.
• Async/await pattern throughout; no blocking calls.
• Clear interfaces (e.g., IGstInvoiceService, IGstLookupService) and dependency-injection friendly design.
• Unit tests covering the core logic.
• XML and inline summary comments that make IntelliSense useful.

Deliverables
1. Compiled DLL and full source code in a Git repo.
2. README with setup steps, sample code snippets and API configuration instructions.
3. Test project demonstrating all three features working end-to-end.

Acceptance
I will run your demo console app, point it at my test GSTINs, generate a sample invoice and see the JSON schema validate against the official XSD. All tests must pass in my environment before we mark the milestone complete.

Tools & tech that will likely help: .NET, C#, System.Text.Json, HttpClient, FluentValidation, xUnit/NUnit.

If you already have experience with GST integrations and e-invoice schemas, let me know in your proposal along with a short outline of your approach and timeline.