使用 AsNoTracking() 延迟加载实体框架

Question

We are currently using lazy loading for Entity Framework and running into out of memory exception . The reason why we're running into this exception is because the Linq query loads a lot of data and at latter stages it's using lazy loading to load navigation properties. But because we don't use NoTrackingChanges Entity Framework cache builds up really quickly which results in out of memory error.

My understanding with EF is the we should always use NoTrackingChanges on query unless you want to update the returned object from the query.

I then tested using NoChangeTracking :

var account = _dbcontext.Account
                        .AsNoTracking()
                        .SingleOrDefault(m => m.id == 1); 
var contactName = account.Contact.Name

but I get the following error:

System.InvalidOperationException: When an object is returned with a NoTracking merge option, Load can only be called when the EntityCollection or EntityReference does not contain objects.

Answer 1

You've specified for EF to not track your instantiated Account value:

var account = _dbcontext.Account.AsNoTracking().SingleOrDefault(m=>m.id == 1);

Thus trying to access navigation properties off of them will never work:

var contactName = account.Contact.Name

You can explicitly include navigation properties you want by using the Include() . So the following should work:

var account = _dbcontext.Account
  .Include(a => a.Contact)
  .AsNoTracking()
  .SingleOrDefault(m=>m.id == 1);

var contactName = account.Contact.Name;  // no exception, it's already loaded

I'm really not convinced that using AsNoTracking prevents from using lazy loading

It can be tested really quickly:

DotNetFiddle Full Example

public static void Main()
{
    var actor1 = new Actor { Id = 1, Name = "Vin Diesel" }; 
    var movie1 = new Movie { Id = 1, Title = "Fast and Furious", PrimaryActor = actor1 };
    using (var context = new MovieDb())
    {

        Console.WriteLine("========= Start Add: movie1 ==============");
        context.Movies.Add(movie1);
        context.SaveChanges();
        Console.WriteLine("========= END Add: movie1 ==============");

        var m1 = context.Movies.First();
        Console.WriteLine(m1.PrimaryActor.Name);

        var m2 = context.Movies.Include(m => m.PrimaryActor).AsNoTracking().First();
        Console.WriteLine(m2.PrimaryActor.Name);

        var m3 = context.Movies.AsNoTracking().First();
        Console.WriteLine(m3.PrimaryActor.Name);
    }
}

Output:

========= Start Add: movie1 ==============
========= END Add: movie1 ==============
Vin Diesel
Vin Diesel
Run-time exception (line 31): Object reference not set to an instance of an object.

The variable m1 is tracked by the context, thus it can Lazy Load the navigation property and prints the value. m2 is not tracked, but I've explicitly included the navigation property so it prints the value. m3 is not tracked and I have not included it explicitly thus the value is null and we get a NRE.