Question

I’m doing this on a table with ~43k rows:

MyDbContext.Stores.Load();
MyDbContext.Stores.Local.Count.Dump(); //horrible performance!

I can see through the profiler that the first instruction fires up the select statement to fetch all rows. Actually the second instruction returns the correct value but after ~12 seconds, and it is not what I was expecting considering that all data should be in memory.
What is wrong (or what is its real purpose) with .Local in Entity Framework?

Answer 1

I think you should do this:

var stores = MyDbContext.Stores.ToList();

// stores is in memory after executing .ToList()
var count = stores.Count();

DbSet.Local Property

This property returns an ObservableCollection that contains all
Unchanged, Modified, and Added objects that are currently tracked by
the context for the given DbSet. The returned observable collection
stays in sync with the underlying DbSet collection and the contents of
the context. This means that you can modify the observable collection
or add/remove entities to/from the underlying DbSet collection (that
includes adding entities by executing a query) and both collections
will be synchronized.

This property is often used in data binding applications.