Adding console app project to a solution with .Net Core web api - asp.net-core

Question

I added reference to the web api project. The added the following to Program.cs
class Program
{
static void Main(string[] args)
{
var builder = new ConfigurationBuilder();
BuildConfig(builder);
var host = Host.CreateDefaultBuilder()
.ConfigureServices((context, services) =>
{
services.AddDbContext<ModelContext>(
options => options.UseOracle(connectionString)); // dbcontext from web api project
services.AddTransient<IEmailReminderService, EmailReminderService>();
}).Build();
var svc = ActivatorUtilities.CreateInstance<EmailReminderService>();
var er = new EmailReminderService();
er.OpenIncidentReminder();
}
static void BuildConfig(IConfigurationBuilder builder)
{
builder.SetBasePath(Directory.GetCurrentDirectory())
.AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
.AddJsonFile($"appsettings{Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT")}.json", optional: true)
.AddEnvironmentVariables();
_configuration = builder.Build();
}
}
I get System.NullReferenceException: 'Object reference not set to an instance of an object.'
_context was null.
Edit
public class EmailReminderService : IEmailReminderService
{
private ModelContext _context;
//private readonly IExceptionLogService _exceptionLogService;
public EmailReminderService(ModelContext con)
{
_context = con;
//_exceptionLogService = exceptionLogService;
}
public void OpenIncidentReminder()
{
var openIncidents = _context.Incidents.ToList();
}
}

Answer 1

#terodaktil is right about the configuration of the DbContext. You need to configure it properly to specify the provider. See Microsoft's docs about this topic.
var host = Host.CreateDefaultBuilder(args)
.ConfigureServices((context, services) => {
// here i'm using in-memory DB, you'd use SqlServer, Oracle, etc.
services.AddDbContext<AppDbContext>(o => o.UseInMemoryDatabase("db"));
services.AddTransient<IEmailReminderService, EmailReminderService>();
}).Build();
That said, you need to create a scope to resolve a scoped service like DbContext.
Then you can use the scope's service provider directly to resolve IEmailReminderService, or ActivatorUtilities.CreateInstance to create an instance of the concrete implementation EmailReminderService.
using var scope = host.Services.CreateScope();
// now you can resolve the dbcontext
var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
// or any service that depends on a scoped service
var reminder = scope.ServiceProvider.GetRequiredService<IEmailReminderService>();
// var reminder = ActivatorUtilities.CreateInstance<EmailReminderService>(scope.ServiceProvider);
reminder.OpenIncidentReminder();
References:
https://learn.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection?view=aspnetcore-5.0#call-services-from-main