[Route("{id}")]
[HttpGet]
public async Task<HttpResponseMessage> GetInvoiceById(int id)
{
    var result = await _invoice_service.GetInvoiceById(id);
    if (result == null)
        return Request.CreateResponse(HttpStatusCode.NotFound);

    return Request.CreateResponse(HttpStatusCode.OK, result);
}

public async Task<Invoice> GetInvoiceById(int id)
        {
            return await _invoice_repo.GetInvoiceById(id);

        }

namespace Accounting.Infrastructure.Repositories
{
    public class InvoiceRepository : BaseRepository, IInvoiceRepository
    {
        private readonly IQueryable<Invoice> _dbSet;

        public InvoiceRepository(IUnitOfWork unit_of_work) : base(unit_of_work)
        {
            _dbSet = unit_of_work.Set<Invoice>()
                .Include(m => m.Articles)
        }

        public Task<Invoice> GetInvoiceById(int id)
        {
            return _dbSet.SingleOrDefaultAsync(m => m.id == id);
        }

        public void Insert(Invoice invoice)
        {
            _unitOfWork.Set<Invoice>().Add(invoice);
        }
    }
}