EF Core Include is Still Lazy Loading? - asp.net-core-webapi

Question

In an Asp.Net Core 2.2 project with EF core (latest everything, ran all NuGet updates today) I have this operation:
return Ok(_db.GlobalRoles
.Include(gr => gr.GlobalRoleFeatures)
.ThenInclude(grf => grf.Feature)
.Include(gr => gr.GlobalRoleCompanyGroupRoles)
.ThenInclude(grcgr => grcgr.CompanyGroupRole)
.ThenInclude(cgr => cgr.CompanyGroupRoleFeatures)
.ThenInclude(cgrf => cgrf.Feature)
.ToList());
For the most part the details aren't important, suffice to say it's a tree of entities I want to eager-load. When I profile the DB this ultimately results in 4 queries. At first I found that unexpected, but shrugged it off as perhaps just how EF has optimized fetching these results. No big deal. And the resulting data is correct.
But when I wrap it in an IMemoryCache:
return Ok(_cache.GetOrCreate(nameof(GlobalRole), entry =>
{
entry.SlidingExpiration = TimeSpan.FromMinutes(_appSettings.DataCacherExpiryMinutes);
return _db.GlobalRoles
.Include(gr => gr.GlobalRoleFeatures)
.ThenInclude(grf => grf.Feature)
.Include(gr => gr.GlobalRoleCompanyGroupRoles)
.ThenInclude(grcgr => grcgr.CompanyGroupRole)
.ThenInclude(cgr => cgr.CompanyGroupRoleFeatures)
.ThenInclude(cgrf => cgrf.Feature)
.ToList();
}));
While the first fetch of this data works as expected, subsequent fetches from the cache result in an exception:
Newtonsoft.Json.JsonSerializationException: Error getting value from 'GlobalRoleCompanyGroupRoles' on 'Castle.Proxies.GlobalRoleProxy'. ---> System.InvalidOperationException: Error generated for warning 'Microsoft.EntityFrameworkCore.Infrastructure.LazyLoadOnDisposedContextWarning: An attempt was made to lazy-load navigation property 'GlobalRoleCompanyGroupRoles' on entity type 'GlobalRoleProxy' after the associated DbContext was disposed.'. This exception can be suppressed or logged by passing event ID 'CoreEventId.LazyLoadOnDisposedContextWarning' to the 'ConfigureWarnings' method in 'DbContext.OnConfiguring' or 'AddDbContext'.
It appears that when serializing the object, the eager-loaded lists of contained entities aren't there. (Or perhaps they are but it's still trying to load them again? Or in some way query the context?) Naturally the context instance has long since been disposed, just the fully materialized list should be cached.
When I debug, the top-level list is indeed returned from the cache. But upon inspection the GlobalRoleFeatures and GlobalRoleCompanyGroupRoles properties of any object therein result in the same above exception.
Note: Same behavior using .ToListAsync() on the query and async all the way up through .GetOrCreateAsync() and through the controller action.
Am I overlooking something? Is there a way to get the fully materialized list, no longer dependent on the DB context, into a memory cache?

Answer 1

The problem is using IMemoryCache. You are not, in fact, serializing the items into your cache. Objects are cached directly in memory, which means that the ties they have to things like a DbContext persist, even though the DbContext does not.
Specifically, the way lazy loading works is that EF actually creates a dynamic proxy of your entity class and overrides (hence the need for the virtual keyword) the reference or collection property with a customer getter that checks the EF object cache for the items, and if they cannot be found, makes a query to get them. Because you're caching directly in memory, you're caching these proxy class instances, which still have this logic on them.
It's a bad idea to use IMemoryCache regardless. Instead, you should always use IDistributedCache. There is a MemoryDistributedCache provider (which is actually the default) if you still want to actually cache in memory, but using IDistributedCache does two things for you:
It's more generic than IMemoryCache, so you can later sub in any cache provider (Redis, SQL Server, etc.) without changing you app code.
Specifically to your issue here, it will force you to actually serialize the cache value, even if using the memory cache provider, meaning you won't have this same non-obvious problem.
That does mean it's a little more work. You'll need to use something like JsonConvert to serialize and deserialize to/from the cache, but you can add extensions to IDistributedCache to take care of that for you.