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Lazy Loading in Entity Framework Core

Microsoft has emphasized that, while LINQ code is "copy and paste" compatible from Entity Framework 6 to Entity Framework Core, you should do a lot of testing to make sure that any code you copy behaves the same way in its new environment as it did in the old (you really get the impression that Microsoft doesn't think you can do enough testing).

Part of the reason for those warnings is Entity Framework 6's quiet support for lazy loading. If you've put the virtual keyword on your navigation properties and omitted an include method in your LINQ query, lazy loading will ensure that rows are downloaded to your code ... and you might not know that's what's happening. When you migrate that code to Entity Framework Core then, in the absence of lazy loading, a whole bunch of formerly working code is going to find a bunch of empty navigation properties.

The good news is that Entity Framework Core's Microsoft.EntityFrameworkCore.Proxies NuGet package provides support for lazy loading. After you add the package to your project, you just need to configure Entity Framework Core to use it. To do that, go to your DbContext object and, in the OnConfiguring method, call the UseLazyLoadingProxies method on the DbContextOptionsBuilder object that's passed to the method.

Typical code for a DbContext class already working with SQL Server would look like this:

protected override void OnConfiguring(
               DbContextOptionsBuilder optionsBuilder)
{
 optionsBuilder
    .UseLazyLoadingProxies()
    .UseSqlServer(
             Configuration.GetConnectionString(" ... connection string name ... "));
}

I'm not suggesting that, after making these changes, you still shouldn't do a lot of testing; I am saying that you're less likely to be surprised if you do make these changes.

Posted by Peter Vogel on 01/08/2019 at 8:08 AM


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