My ApplicationDbContext is null for some reason - asp.net-core

Question

I get the error System.NullReferenceException. Object reference not set to an instance of an object. At Club.BLL.MaintenanceBLL.Maintenance.DoMaintenance()
But the context code is the same as on scaffolded pages, so not sure what I need to do. Any help is appreciated.
The code is:
namespace Club.BLL.MaintenanceBLL
{
public class Maintenance
{
private readonly Club.Data.ApplicationDbContext Context;
public Maintenance(ApplicationDbContext context)
IN DEBUG MODE context IS INDICATED
AS OBJECT REFERENCE NOT SET TO AN INSTANCE OF AN OBJECT.
HOW DO I FIX THIS.
{
Context = context;
}
public Maintenance()
{
}
public void DoMaintenance()
{
//Parse Maintenance table and action those items
//where Active=True and ActionDate has passed
//==================================
//Retrieve list of rows in Maintenance table
var maintenances = Context.Maintenance; PROGRAM FAILS ON THIS LINE.
I imagine I am missing something fundamental as I am 'learning by doing'. The lines that call up the DoMaintenance routine are located in root/Pages/Index, which is a scaffolded page. The DoMaintenance routine is called from the following lines in the root/Pages/index.cshtml page:
public void OnGet()
{
Maintenance maintenance = new Maintenance();//Create new instance
maintenance.DoMaintenance();
}
AND startup.cs includes the lines
public void ConfigureServices(IServiceCollection services)
{
services.AddDbContext(options =>
options.UseSqlServer(
Configuration.GetConnectionString("DefaultConnection")));
services.AddDefaultIdentity(options =>
options.SignIn.RequireConfirmedAccount = true)
.AddRoles()
.AddEntityFrameworkStores();
ANY HELP WOULD BE APPRECIATED.
Thanks and cheers....Alan

Answer 1

According to your description and codes, I found you have register the dbcontext as a service in your application that means if you want to use it, you should inject the dbcontext into your Maintenance class.
Normally, we will create a service as BLL service in asp.net core.
Then we could register the service in the startup.cs and inject it in the razor page.
I suggest you could try to modify the Maintenance class as below codes shows:
You could create a interface called IMaintenance:
public interface IMaintenance
{
public void DoMaintenance();
}
Then you could let Maintenance inherit IMaintenance as below:
public class Maintenance : IMaintenance
{
private readonly TestDbcontext Context;
public Maintenance(TestDbcontext testDbcontext ) {
Context = testDbcontext;
}
public void DoMaintenance() {
var maintenances = Context.Employees.ToList() ;
}
}
At last, you could register the Maintenance as service in startup.cs ConfigureServices method:
public void ConfigureServices(IServiceCollection services)
{
services.AddControllersWithViews();
services.AddDbContext<TestDbcontext>(ServiceLifetime.Transient);
services.AddTransient<IMaintenance, Maintenance>();
}
You could directly use the Maintenance in razor pages like below, the asp.net core dependency injection will inject the dbcontext automatically into Maintenance.
public class IndexModel : PageModel
{
private readonly ILogger<IndexModel> _logger;
private readonly IStringLocalizer<IndexModel> _localizer;
private readonly IMaintenance _maintance;
public CustomerModel Customer { get; set; }
public IndexModel(ILogger<IndexModel> logger, IStringLocalizer<IndexModel> localizer, IMaintenance maintance)
{
_logger = logger;
_localizer = localizer;
_maintance = maintance;
}
public void OnGet()
{
_maintance.DoMaintenance()
}
}
namespace Club.Pages
{
[AllowAnonymous]
public class IndexModel : PageModel
{
private readonly IMaintenance _maintenance;
public IndexModel (IMaintenance<IndexModel> maintenance)
{
_maintenance = maintenance;
}
public void OnGet()
{
_maintenance.DoMaintenance();
}
}
}

Answer 2

You might have missed to inject your context and since there's default ctor, everything works fine without setting Context property.
public void ConfigureServices(IServiceCollection services)
{
services.AddDbContext<Context>(() => ...));
}