Detect lazy-load in Entity Framework Core

Question

Entity Framework Core 3.1.2 - I have enabled UseLazyLoadingProxies on my DbContext to ensure data integrity, but I want to throw an exception during development if it is used.

How can I execute some code every time EF Core loads a relationship lazily?

Answer 1

The only way I know is diagnostic messages. See an example here: https://www.domstamand.com/getting-feedback-from-entityframework-core-through-diagnostics.

The event class you want is https://learn.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.diagnostics.lazyloadingeventdata.

In the application's DBContext

#if DEBUG
    static ApplicationDbContext()
    {
        // In DEBUG mode we throw an InvalidOperationException
        // when the app tries to lazy load data.
        // In production we just let it happen, for data
        // consistency reasons.
        DiagnosticListener.AllListeners.Subscribe(new DbContextDiagnosticObserver());
    }
#endif

Then a class to hook into EF notifications

internal class DbContextDiagnosticObserver : IObserver<DiagnosticListener>
    {
        private readonly DbContextLazyLoadObserver LazyLoadObserver =
            new DbContextLazyLoadObserver();

        public void OnCompleted() { }

        public void OnError(Exception error) { }

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

And then finally the class that throws exceptions whenever a lazy load occurrs

internal class DbContextLazyLoadObserver : IObserver<KeyValuePair<string, object>>
    {
        public void OnCompleted() { }
        public void OnError(Exception error) { }

        public void OnNext(KeyValuePair<string, object> @event)
        {
            // If we see some Lazy Loading, it means the developer needs to
            // fix their code!
            if (@event.Key.Contains("LazyLoading"))
                throw new InvalidOperationException(@event.Value.ToString());
        }
    }