Can I configure an interceptor in EntityFramework Core?

Here is an example found on github from ajcvickers on how to use an Interceptor in EF CORE (2.2 at the time of answering this question):

public class NoLockInterceptor : IObserver<KeyValuePair<string, object>>
{
    public void OnCompleted()
    {
    }

    public void OnError(Exception error)
    {
    }

    public void OnNext(KeyValuePair<string, object> value)
    {
        if (value.Key == RelationalEventId.CommandExecuting.Name)
        {
            var command = ((CommandEventData)value.Value).Command;

            // Do command.CommandText manipulation here
        }
    }
}

Next, create a global listener for EF diagnostics. Something like:

public class EfGlobalListener : IObserver<DiagnosticListener>
{
    private readonly NoLockInterceptor _noLockInterceptor = new NoLockInterceptor();

    public void OnCompleted()
    {
    }

    public void OnError(Exception error)
    {
    }

    public void OnNext(DiagnosticListener listener)
    {
        if (listener.Name == DbLoggerCategory.Name)
        {
            listener.Subscribe(_noLockInterceptor);
        }
    }
}

And register this as part of application startup:

DiagnosticListener.AllListeners.Subscribe(new EfGlobalListener());

It is coming for EntityFramework Core 3.0: https://github.com/aspnet/EntityFrameworkCore/issues/15066

It will work the same way as the one in EF 6

Update: Interception of database operations is now available in EF Core 3.0.

Original answer:

---

EF Core does not have "interceptors" or similar lifecycle hooks yet. This feature is tracked here: https://github.com/aspnet/EntityFramework/issues/626.

Overriding a low-level component may be unnecessary if all you want is log output. Many low-level EF Core components already produce logging, logging including query execution. You can configure EF to use a custom logger factory by calling DbContextOptionsBuilder.UseLoggerFactory(ILoggerFactory factory). (See https://docs.asp.net/en/latest/fundamentals/logging.html and https://github.com/aspnet/Logging for more details on this logger interface.) EF Core produces some notable log events with well-define event IDs. (See Microsoft.EntityFrameworkCore.Infrastructure.CoreLoggingEventId in 1.0.0-rc2, which was renamed to justMicrosoft.EntityFrameworkCore.Infrastructure.CoreEventId for 1.0.0 RTM.) See https://docs.efproject.net/en/latest/miscellaneous/logging.html for examples of doing this.

If you need additional logging beyond what EF Core components already produce, you will need to override EF Core's lower-level components. This is best done by overriding the existing component and added this overridding version to EF via dependency injection. Doing this requires configuring a custom service provider for EF to use internally. This is configured by DbContextOptionsBuilder.UseInternalServiceProvider(IServiceProvider services) See https://docs.efproject.net/en/latest/miscellaneous/internals/services.html for more details on how EF uses services internally.