Entity Framework Core DbContext inheritance problem with DbOptions in constructor

Question

In our project we have a database where we use the DB first approach. Since we use features which do not get scaffolded I have a second DBContext which inherits from the generated one. This allows me to avoid manually manipulating the generated db context each time a database change occurs.

So the class definitions and usage in StartUp looks like:

  // the db context generated by scaffolding the database
  public partial class ApplicationDatabaseContextGenerated : DbContext
  {
    public ApplicationDatabaseContextGenerated() {}
    public ApplicationDatabaseContextGenerated(DbContextOptions<ApplicationDatabaseContextGenerated> options) : base(options) {}

    // the db sets scaffolded
  }

  // the db context used by the app with the extended functionality
  public class ApplicationDatabaseContext : ApplicationDatabaseContextGenerated
  {

    public ILogger<ApplicationDatabaseContext> Logger { get; protected set; }

    public ApplicationDatabaseContext() : base() {}

    public ApplicationDatabaseContext(DbContextOptions<ApplicationDatabaseContext> options, ILogger<ApplicationDatabaseContext> logger) : base(options)
    {
      Logger = logger ?? throw new ArgumentNullException(nameof(logger));
    }

    // the extended functionality like views and functions

  }

  // how I use it in startup
  public void ConfigureServices(IServiceCollection services)
  {
    // other things ...

    services.AddDbContext<ApplicationDatabaseContext>(options => options.UseNpgsql(Configuration.GetConnectionString("db1-connection")));

    // other things ...
  }

This unfortunatly results in a compilation error higlighting the base(options) of the ApplicationDatabaseContext constructor stating:

Error CS1503 Argument 1: cannot convert from 'Microsoft.EntityFrameworkCore.DbContextOptions<... ApplicationDatabaseContext>' to 'Microsoft.EntityFrameworkCore.DbContextOptions<... ApplicationDatabaseContextGenerated>'

---

I thought, let's be smart, a ApplicationDatabaseContextGenerated is basically a database context and changed the constructor of the ApplicationDatabaseContextGenerated to:

  public ApplicationDatabaseContextGenerated(DbContextOptions<DbContext> options) : base(options) {}

Nope, that's not allowed neither and results in:

Error CS1503 Argument 1: cannot convert from 'Microsoft.EntityFrameworkCore.DbContextOptions<... ApplicationDatabaseContext>' to 'Microsoft.EntityFrameworkCore.DbContextOptions<Microsoft.EntityFrameworkCore.DbContext>'

---

Ok, let' take both as DbContext

  // the db context generated by scaffolding the database
  public partial class ApplicationDatabaseContextGenerated : DbContext
  {
    public ApplicationDatabaseContextGenerated() {}
    public ApplicationDatabaseContextGenerated(DbContextOptions<DbContext> options) : base(options) {}

    // the db sets scaffolded
  }

  // the db context used by the app with the extended functionality
  public class ApplicationDatabaseContext : ApplicationDatabaseContextGenerated
  {

    public ILogger<ApplicationDatabaseContext> Logger { get; protected set; }

    public ApplicationDatabaseContext() : base() {}

    public ApplicationDatabaseContext(DbContextOptions<DbContext> options, ILogger<ApplicationDatabaseContext> logger) : base(options)
    {
      Logger = logger ?? throw new ArgumentNullException(nameof(logger));
    }

    // the extended functionality like views and functions

  }

Pretty, that compiles. Let's start it and we get:

  System.InvalidOperationException: No database provider has been configured for this DbContext. A provider can be configured by overriding the DbContext.OnConfiguring method or by using AddDbContext on the application service provider. If AddDbContext is used, then also ensure that your DbContext type accepts a DbContextOptions<TContext> object in its constructor and passes it to the base constructor for DbContext.
    at Microsoft.EntityFrameworkCore.Internal.DbContextServices.Initialize(IServiceProvider scopedProvider, IDbContextOptions contextOptions, DbContext context)
    at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider()
    at Microsoft.EntityFrameworkCore.DbContext.get_DbContextDependencies()
    at Microsoft.EntityFrameworkCore.DbContext.get_Model()
    at Microsoft.EntityFrameworkCore.Internal.InternalDbQuery`1.get_EntityType()
    at Microsoft.EntityFrameworkCore.Internal.InternalDbQuery`1.get_EntityQueryable()
    at Microsoft.EntityFrameworkCore.Internal.InternalDbQuery`1.System.Linq.IQueryable.get_Provider()
    at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ExecuteAsync[TSource,TResult](MethodInfo operatorMethodInfo, IQueryable`1 source, CancellationToken cancellationToken)
    at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.SingleAsync[TSource](IQueryable`1 source, CancellationToken cancellationToken)
    at ... MyMethod(ApplicationDatabaseContext dbContext, CancellationToken cancellationToken) in myfile.cs:line 51
    at ... MyOtherMethod in myfile.cs:line 61

---

Well, how to convert DbContextOptions or how to inherit DbContexts?

Answer 1

DbContextOptions<TContext> is a generic class, and as any class it doesn't support covariance, hence DbContextOptions<TDerivedContext> cannot be treated as DbContextOptions<TBaseContext>.

The solution is to use the non generic DbContextOptions class in the base context constructor (similar to the DbContext class constructor with options):

public ApplicationDatabaseContextGenerated(DbContextOptions options) : base(options) { }

Since the AddDbContext<ApplicationDatabaseContext> will create an instance of the DbContextOptions<ApplicationDatabaseContext> class, the ApplicationDatabaseContext class constructor can use either DbContextOptions<ApplicationDatabaseContext> or DbContextOptions options parameter.