GetCustomer(string id)
SaveCustomer(...)

GetCustomer(string id)->db.GetCustomer(string strConnect, string id)
SaveCustomer(...)->db.SaveCustomer(string strConnect, ....)

ConfigureServices(IServiceCollection services)
{
//...
 services.AddSingleton<IDataContextService>(new DataContextService()
            {
                ConnectionString = Configuration.GetSection("Data")["ConnectionString"]
            });
}

public class HomeController : Controller
{
    private readonly IDataContextService _dataService;

    public HomeController(IDataContextService dataService)
    {
        _dataService = dataService;
    }

    [HttpGet]
    public IActionResult Index([FromServices] IDataContextService dataService2)
    {
        IDataContextService  dataService3 = HttpContext.RequestServices.GetService<IDataContextService>();
        return View();
    }
}