Migrating to dotnetcore 3.1 (mostly EFCore)

Dotnetcore 3 has been out for a little while now, and there were some changes I wanted in the project I was working on, so off I went and gave it a burl.
Needless to say, there were some issues, but in fairness, some of it was our fault.

This is predominantly going to be about the migration from EFCore 2 –> 3, since that was by far the biggest pain point.


Dotnetcore 2 –> 3 is easy.
EFCore 2 –> 3 is harder.
Need to think about how the Linq will translate into SQL.
Can’t really do:

Migrating dotnetcore 2 –> 3

The framework migration was fairly trivial for the ASPNetCore 2.2 project I had.
Update all projects to use dotnetcore 3.1 in the project file.
<TargetFramework>netcoreapp2.1</TargetFramework> –> <TargetFramework>netcoreapp3.1</TargetFramework>
Easy as that!

There were also some simple changes to Program.cs and Startup.cs (in my case, YMMV).


var webHostBuilder = WebHost.CreateDefaultBuilder(args)
  .ConfigureAppConfiguration(builder => additionalConfig.Invoke(builder))
  .ConfigureServices(services => services.AddAutofac())


var webHostBuilder = Host.CreateDefaultBuilder(args)
  .UseServiceProviderFactory(new AutofacServiceProviderFactory())
  .ConfigureWebHostDefaults(webBuilder =>
      webBuilder.ConfigureAppConfiguration(builder => additionalConfig.Invoke(builder))

Note the different method of registering Autofac.

In ConfigureServices, UseMvc changed to AddControllers. As I understand it, there can be a few ways to configure that method, but in my case that’s all I needed (basic endpoint configuration)
Configure was getting an IHostingEnvironment, that’s now IWebHostEnvironment.




app.UseAuthorization();  <-- Added
app.UseEndpoints(endpoints => endpoints.MapControllers());  <-- instead of UseMvc

Test config needed adjusting:


Server = new TestServer(Program
  .GetWebHostBuilder<TestServerStartup<TestUserIdentity>>(appRootPath, null, TestConfiguration.AddTestConfig));


var server = await Program
  .GetWebHostBuilder<TestServerStartup<TestUserIdentity>>(appRootPath, null, TestConfiguration.AddTestConfig)
  .ConfigureWebHost(webBuilder => webBuilder.UseTestServer()).StartAsync();

Server = server.GetTestServer();

In the tests, we would resolve services with Server.Host.Services.GetService, which now drops the Host to just Server.Services.GetService

Update all nugets as required to support the new framework, and that was it!
That hasn’t caused any issues so far. Nice and simple and successful.

The migration from EFCore 2 –> 3, not so much :(

Migrating EFCore 2 –> 3

The major sticking point is that EFCore 2 silently performed client-side evaluation when it wasn’t able to translate a query. This means that you could write pretty much any valid linq query and EFCore would “just do it”.
For us, EFCore tended to grab tables and pull them back or make n+1 subqueries, doing fun things to performance and general understanding of when things were going to happen.

In EFCore 3, this has been disallowed and queries that can’t be translated now give an exception at runtime. Much better!
EFCore now operates in a more SQL fashion. You can’t write any Linq and expect it to “just work”. For example, GroupJoin doesn’t really exist in SQL, so EFCore won’t translate it.
This is good news! No more weird query behaviours due to the EFCore black box (well less of them)

The migration was not easy though. The three main errors I got were:

The first is easy to understand, but the latter two are a bit of a mystery for me, but they both pertain to operations performed on navigation properties.

Essentially, you can’t do the following anymore:

I also had troubles with things:

These often resulted in the Relational or Navigation errors from above. They could sometimes be bypassed with x.Navigation.Select(t => t), although I’m unsure why.

Value converters DO still work in these queries. I believed they didn’t for some time, and went around exposing all the underlying columns.

The GroupBy Problem

One of the most frustrating issues for us was that GroupBy didn’t work anymore.
We used a common pattern of ordering and paging, and used GroupBy to do it.
The issue is that in 2.1, it was being done through some amalgamation of DB and client-side, thus we were allowed to group on whole entities, which we can’t do anymore.
The fix we went with was applying filtering and ordering, return the ID’s only, THEN fetch all that data and group by on the client.
Approaching it this way allowed us to keep the ordering, grouping, etc, without all the joins blowing out into massive cross mutliplication, while allowing us to do the ordering and paging on the database.

An example of the GroupBy problem:

If you have a type: {Guid Id, string Name, int Price}, and you want to have a result: {Id, Name, List<int> Prices}.

// Worked on 2.1 and brought the required rows back (or the whole table?) and functioned correctly.
// Fails to translate in EFCore 3 as you haven't told it how to aggregate the price.
  .GroupBy(x => new { x.Id, x.Name })

// One option for 3.1 to do client-side.  
  .GroupBy(x => new { x.Id, x.Name });  

// This can done to group by the ID and Name to get them and the MAX price, just not ALL the prices
  .GroupBy(x => new { x.Id, x.Name })
  .Select(x => new { x.Key.Id, x.Key.Name, MaxPrice = x.Max(t => t.Price) })

Database Sequences and generated/sourced Ids

There was a breaking change regarding Database sequences and DB generated ID’s. If you have an entity that has a DB generated or sourced Id as a PK AND you assign it one in C# anyway, EF now assumes the row exists and tracks it as Modified instead of Added.
This was a problem for me, and I needed to add .ValueGeneratedNever to the property in entity configuration. I couldn’t add it to the key definition, but I was allowed to also have a property definition I could set it on.

config.HasKey(x => x.Id);
config.Property(x => x.Id).ValueGeneratedNever();

Ordering Differences

Because EFCore 2 used to pull things into memory, behaviours were C# like. Database ordering isn’t always the same :(
For me, ordering of Includes wasn’t behaving consistently. Ok, I need to get around that another way, fair enough.
Another situation was that we had ordering on an early table join, which was working fine in EFCore 2 because it was clientside.
EFCore 3 disregarded that OR at least didn’t preserve the ordering indicated after further operations, like joins, which is fair enough.
It also seems that ordering on a nullable field wasn’t being done correctly in EFCore 2x, and resulted in nulls spread throughout the rest of the values. EFCore 3 has fixed that.

Unit Testing

Catch 22 when unit testing EF queries - I had a query that worked fine in EF but failed in the unit test (NSubstitute, etc) because one of the entites was null, and while SQL could handle that, C# + Linq couldn’t. For reference, this is issue in question:

.Select(x => x.NullableProperty.Id)

The tests mocked out the DB call, and it turns out C# doesn’t like that NullableProperty being null :)
SQL generates a null check in that case, and so the actual call works fine in Production.
In the end, there was no way I could make both of them happy, so I had to move to a subcutaneous test that hit a localdb instance.


The actual dotnetcore 2 –> 3 migration was quite nice and simple.
The EFCore migration was more difficult, but it was mostly due to our abuse of EFCore 2 (although we really shouldn’t have been allowed to)
I feel like some of our pain points could have been solved with entity configuration, but I wasn’t able to figure it out.
If you have more luck migrating or insight on any of this, I’d love to hear from you!

Happy dotnetcore 3.1!

Tags: dotnetcore Entity Framework