Using Include vs ThenInclude

Question

I have been experimenting a little with Entity Framework, and after facing the error below, I tried using ThenInclude to resolve it.

The expression '[x].ModelA.ModelB' passed to the Include operator could not be bound

But now it seems I lack some understanding of why it did solve the problem

What's the difference between this:

.Include(x => x.ModelA.ModelB)

And this:

.Include(x => x.ModelA).ThenInclude(x => x.ModelB)

Answer 1

"Include" works well with list of object, but if you need to get multi-level data, then "ThenInclude" is the best fit. Let me explain it with an example. Say we have two entities, Company and Client:

public class Company
{
    public string Name { get; set; }

    public string Location { get; set; }

    public List<Client> Clients {get;set;}
}

 public class Client
 {
    public string Name { get; set; }

    public string Domains { get; set; }

    public List<string> CountriesOfOperation { get; set; }
 }

Now if you want just companies and the entire client list of that company, you can just use "Include":

using (var context = new YourContext())
{
  var customers = context.Companies
    .Include(c => c.Clients)
    .ToList();
}

But if you want a Company with "CountriesOfOperation" as related data, you can use "ThenInclude" after including Clients like below:

using (var context = new MyContext())
{
   var customers = context.Companies
    .Include(i => i.Clients)
      .ThenInclude(a => a.CountriesOfOperation)
    .ToList();
}

Answer 2

The difference is that Include will reference the table you are originally querying on regardless of where it is placed in the chain, while ThenInclude will reference the last table included. This means that you would not be able to include anything from your second table if you only used Include.