InvoicesController.cs

[Route("api/[controller]")]
[ApiController]
public class InvoicesController : ControllerBase
{
    private readonly IEnumerable<IInvoicingService> invoicingServices;

    public InvoicesController(IEnumerable<IInvoicingService> invoicingServices)
    {
        this.invoicingServices = invoicingServices;
    }

    [HttpGet]
    public ActionResult<string> Get(float hoursPerformed)
    {
        var builder = new StringBuilder();

        builder.AppendLine($"Invoice {DateTime.Now.ToShortDateString()}");
        builder.AppendLine($"");
        builder.AppendLine($"Hours performed : {hoursPerformed}");
        builder.AppendLine($"");

        foreach (var service in invoicingServices)
        {
            builder.AppendLine($"{service.CreateInvoice(hoursPerformed)}");
        }

        builder.AppendLine($"---------------------");

        return builder.ToString();
    }
}