DatabaseContext object is disposed ASP.NET Core - asp.net-core

Question

When I try to retrieve data from a table from database using Entity Framework Core in class, I get an exception:
System.ObjectDisposedException: 'Cannot access a disposed context instance. A common cause of this error is disposing a context instance that was resolved from dependency injection and then later trying to use the same context instance elsewhere in your application. This may occur if you are calling 'Dispose' on the context instance, or wrapping it in a using statement. If you are using dependency injection, you should let the dependency injection container take care of disposing context instances.
My code looks like this:
Startup.cs:
public class Startup
{
public Startup(IConfiguration configuration)
{
Configuration = configuration;
}
public IConfiguration Configuration { get; }
public void ConfigureServices(IServiceCollection services)
{
services.AddControllersWithViews();
services.AddDbContext<DatabaseContext>(
options => options.UseSqlServer(Configuration.GetConnectionString("DatabaseContextConection")));
services.AddIdentity<ApplicationUser, IdentityRole>(config => {
config.User.RequireUniqueEmail = true;
config.Password.RequiredLength = 8;
config.Password.RequireNonAlphanumeric = false;
config.Password.RequireUppercase = false;
config.Password.RequireLowercase = false;
// config.COO
}).AddEntityFrameworkStores<DatabaseContext>();
services.AddScoped<IDatabaseChangeNotificationService, SqlDependencyService>();
services.AddSignalR();
}
public void Configure(IApplicationBuilder app, IWebHostEnvironment env,IDatabaseChangeNotificationService notificationService)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
else
{
app.UseExceptionHandler("/Home/Error");
app.UseHsts();
}
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseAuthentication();
app.UseRouting();
app.UseAuthorization();
app.UseEndpoints(endpoints =>
{
endpoints.MapControllerRoute(
name:"",
pattern: "{controller=Home}/{action=Index}/{id?}");
endpoints.MapHub<ChatHub>("/ChatHub");
});
notificationService.Config();
}
}
DatabaseContext.cs:
public class DatabaseContext: IdentityDbContext<ApplicationUser> ,IApplicationDbContext
{
public DatabaseContext(DbContextOptions<DatabaseContext> options):base(options)
{
}
public DbSet<Message> Messages { get; set; }
public new async Task<int> SaveChanges()
{
return await base.SaveChangesAsync();
}
}
SqlDependencyService.cs:
public interface IDatabaseChangeNotificationService
{
void Config();
}
public class SqlDependencyService : IDatabaseChangeNotificationService
{
private UserManager<ApplicationUser> _userManager;
public string FullName;
public string UserId;
private readonly IConfiguration configuration;
private readonly IHubContext<ChatHub> chatHub;
private DatabaseContext _DBContext;
public SqlDependencyService(DatabaseContext DbContext, IConfiguration _configuration, IHubContext<ChatHub> _chatHub, UserManager<ApplicationUser> userManager)
{
_DBContext = DbContext;
configuration = _configuration;
chatHub = _chatHub;
_userManager = userManager;
}
public void Config()
{
TableUsersAvailabilitySensor();
}
private void TableUsersAvailabilitySensor()
{
string connectionString = configuration.GetConnectionString("DatabaseContextConection");
using (var conn = new SqlConnection(connectionString))
{
if (conn.State != System.Data.ConnectionState.Open)
{
conn.Open();
}
using (var cmd = new SqlCommand(#"Select IsActive from [dbo].AspNetUsers", conn))
{
cmd.Notification = null;
SqlDependency dependency = new SqlDependency(cmd);
dependency.OnChange += TableUsersChanged;
SqlDependency.Start(connectionString);
cmd.ExecuteReader();
}
}
}
List<string> AvailableUsers = new List<string>();
private void TableUsersChanged(object sender, SqlNotificationEventArgs e)
{
if (e.Type == SqlNotificationType.Change)
{
string text = checkAvailabilityChanged(e);
if (text == "Updated" || text == "Inserted")
{
var list = _DBContext.Users.Where(a => a.IsActive == true).ToList();
AvailableUsers.AddRange(list.Select(a => a.UserName));
var currentUserId = _userManager.GetUserId(AccountController.currentUser);
var _currentUser = _DBContext.Users.Find(currentUserId);
FullName = _currentUser.FirstName + " " + _currentUser.LastName;
UserId = currentUserId;
chatHub.Clients.Clients(AvailableUsers).SendAsync("AddMeToYourContacts", FullName, UserId);
}
}
TableUsersAvailabilitySensor();
}
private string checkAvailabilityChanged(SqlNotificationEventArgs e)
{
switch (e.Info)
{
case SqlNotificationInfo.Update:
return "Updated";
case SqlNotificationInfo.Delete:
return "Deleted";
case SqlNotificationInfo.Insert:
return "Inserted";
default:
return "Nothing occurred";
}
}
}
The exception is thrown on this line of code:
var list = _DBContext.Users.Where(a => a.IsActive == true).ToList();