Decorating EF Core's DbContext

[leoshusar]

Hi!
I am currently trying this library for DB audit logging.
I currently have it working for MongoDB - I am using IMongoRepository here.
But is there any way to decorate EF Core's DbContext too?

I tried this decorator

class AuditLogDbContextDecorator : DbContext
{
    private readonly DbContext _decorated;

    public AuditLogDbContextDecorator(DbContext decorated)
    {
        _decorated = decorated;
    }

    public override int SaveChanges()
    {
        LogChanges();
        return _decorated.SaveChanges();
    }

    private void LogChanges()
    {
        ...
    }
}

and decorated it like this: builder.Services.Decorate(typeof(DbContext), typeof(AuditLogDbContextDecorator));
but this fails with

Scrutor.MissingTypeRegistrationException: 'Could not find any registered services for type 'Microsoft.EntityFrameworkCore.DbContext'.'

When I decorate my context implementation: builder.Services.Decorate(typeof(MyContext), typeof(AuditLogDbContextDecorator));
it works, but then it (probably) deadlocks when I want to retrieve my context either in constructor or by GetService<MyContext>(). No error, no exception, it just hangs. Even Ctrl + C won't stop the app, I have to close it using the stop button in VS.

Do you know what can be causing this? I was thinking about circular dependency, could it be? Is it even possible to decorate partial abstract class like DbContext?

[khellang]

Hmm, yeah, that's a tricky one. You're essentially decorating a concrete registration as opposed to an interface registration 🤔

I think what's causing the hang might be the injected DbContext in the decorator.

The rule of thumb should basically be that you should inject the exact same type as you decorate. That means you probably should inject MyContext instead.

I haven't really tried this scenario myself, but I can't see why it shouldn't work 😅

[leoshusar]

If you meant like this:

public class AuditLogDbContextDecorator<TContext> : DbContext where TContext : DbContext
{
    private readonly TContext _decorated;

    public AuditLogDbContextDecorator(TContext decorated)
    {
        _decorated = decorated;
    }
}

and register like this

builder.Services.Decorate(typeof(MyContext), typeof(AuditLogDbContextDecorator<MyContext>));

this doesn't work either :( still hangs

[khellang]

Hmm, yeah, that's exactly what I meant 😅 I'm not really sure what's going on. Need to take a look at this and do some debugging...

[khellang]

Closed by #155