CODEWITHSUNDEEP
c#entity-frameworkgenericsentity-framework-core
grokky
grokky

Reputation: 9265

Design of abstract generic DbContext class in EF Core

I'm having trouble implementing an abstract generic base class, using EF Core. It's probably something obvious, but I'm not seeing it.

base class:

public abstract class AContext<TUser> : IdentityDbContext<TUser> 
  where TUser : IdentityUser {

    public AContext(DbContextOptions<AContext<TUser>> options, ILogger<AContext<TUser>> logger) 
      : base(options) {
    }

}

subclass:

public class Context : AContext<User> {

    public Context(DbContextOptions<Context> options, ILogger<Context> logger)
      : base(options, logger) {   // problem is here with "options"
    }

}

Error: Cannot convert from 'DbContextOptions<Context>' to 'DbContextOptions<AContext<User>>'.

I tried casting but that doesn't work. Is the problem due to it not being covariant? Can I redesign it somehow?

Upvotes: 2

Views: 1141

Answers (1)

Sefe
Sefe

Reputation: 14007

A DbContextOptions<Context> is not assignable to DbContextOptions<AContext<TUser>>. The reason is that there could be internal write operations to objects of type T, which for DbContextOptions<Context> would be Context. DbContextOptions<AContext<TUser>> can not provide that, because it's T is an AContext<TUser>.

Covariance is not an answer for you, since co- and contravariance is only available for interfaces, not for classes.

If you change the parameter type in your constructor from DbContextOptions<Context> to DbContextOptions<AContext<User>>, everything should work fine.

Upvotes: 3

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