c# - Only sources that implement IAsyncEnumerable can be used for Entity Framework asynchronous operations

Question

I'm implementing a Model using EF 6.1.3 and .NET Framework 4.6.1. This model is used by an ASPNET app and by an ASPNET CORE app, for that reason it uses System.Data.Entity and it is located in a separate assembly mymodel.dll.

This is the model

using System.Data.Entity;
public partial class MyDbContext : DbContext
{    
        public virtual DbSet<Athlete> Athletes{ get; set; }
}
public partial class Athlete
{
    public Athlete()
    {
    }
    //...
    public string Country { get; set; }
}

I'm developing the MVC app that is implemented in aspnet core with .NET Framework 4.6. It references EF 6.1.3 so that the model can be used.

public class MyViewModel
{
    public IList<Athlete> ItalianAthletes{ get; set; }
}

using Microsoft.EntityFrameworkCore;
//solution: comment the previous line and use instead System.Data.Entity;
public class MyController : Controller
{
    private readonly MyDbContext _context;
    //...
    public IActionResult Index()
    {
       MyViewModel myvm = new MyViewModel();
       var result = _context.Athletes.Where(a=>a.Country=="Italy").ToList();
       myvm.ItalianAthletes = result ;
       return View(myvm);
    }
}

... and it works as expected.

Now changing the Index method to async

public async Task<IActionResult> Index()
{
   MyViewModel myvm = new MyViewModel();
   var result = _context.Athletes.Where(a=>a.Country=="Italy").ToListAsync();
   await result; //at this point an exception is thrown
   //...
}

InvalidOperationException: The source IQueryable doesn't implement IAsyncEnumerable. Only sources that implement IAsyncEnumerable can be used for Entity Framework asynchronous operations.

Removing the Where() clause the problem persists, so the problem seems related to ToListAsync();

var result = _context.Users.ToListAsync();

Carefully reading the text of the exception I understand that "the IQueryable generated by ToList() doesnt implement IAsyncEnumerable " but this doesnt make sense to me because all that behavior is internal to ToListAsync();

Someone can help me to better understand what's happening here under the hood? and what can I do so that ToListAsync() works as expected ?

thank you in advance for any comment

Answer 1

If you are using Entity Framework Core then you have to use this namespace:

using Microsoft.EntityFrameworkCore;

instead of

using System.Data.Entity;