I have three projects in my solution, with the .net core 5:

• Demo.DAL (Library)
• Demo.BLL (Library)
• Demo.PL (MVC)

Demo.DAL:
Contains the DbContext class in the Data folder.

namespace Demo.DAL.Data
{
    public class App_DbContext : DbContext
    {
        public App_DbContext(DbContextOptions<DbContext> options): base(options)
        {

        }
        public DbSet<Department> Departments { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.ApplyConfigurationsFromAssembly(Assembly.GetExecutingAssembly());
        }
    }
}

Demo.PL:
This is the ConfigureServices method in the Startup class.

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews();

    services.AddDbContext<App_DbContext>(optionsBuilder =>
    {
        optionsBuilder.UseSqlServer(Configuration.GetConnectionString("myConnection"));
    }
    );

}

I was trying to apply the dependency injection with the App_DbContext class.

When I remove this part from the ConfigureServices method

services.AddDbContext<App_DbContext>(optionsBuilder =>
    {
        optionsBuilder.UseSqlServer(Configuration.GetConnectionString("myConnection"));
    }
    );

the program runs with no exceptions and when I return it back the following exceptions appear.

AggregateException: Some services are not able to be constructed (Error while validating the service descriptor ‘ServiceType: Demo.DAL.Data.App_DbContext Lifetime: Scoped ImplementationType: Demo.DAL.Data.App_DbContext’: Unable to resolve service for type ‘Microsoft.EntityFrameworkCore.DbContextOptions`1[Microsoft.EntityFrameworkCore.DbContext]’ while attempting to activate ‘Demo.DAL.Data.App_DbContext’.)
Microsoft.Extensions.DependencyInjection.ServiceProvider..ctor(IEnumerable serviceDescriptors, IServiceProviderEngine engine, ServiceProviderOptions options)

InvalidOperationException: Error while validating the service descriptor ‘ServiceType: Demo.DAL.Data.App_DbContext Lifetime: Scoped ImplementationType: Demo.DAL.Data.App_DbContext’: Unable to resolve service for type ‘Microsoft.EntityFrameworkCore.DbContextOptions`1[Microsoft.EntityFrameworkCore.DbContext]’ while attempting to activate ‘Demo.DAL.Data.App_DbContext’.
Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.ValidateService(ServiceDescriptor descriptor)

I found relevant questions about similar issues, I tried most of the mentioned solutions in their answers but none of them worked with me.