An example of how you can run into a InvalidOperationException stating that "There is already an open DataReader associated with this Connection which must be closed first." using Entity Framework Core.

Using the command line console

1. Navigate to the src\EFFUnkiness.Server folder
2. Execute dotnet run

Using the command line console

1. Execute dotnet test

The code below will normally throw an InvalidOperationException because while a the underlying DataReader the context is using is open, the Connection is in use exclusively by that DataReader. You cannot execute any commands for the Connection, including creating another DataReader, until the original DataReader is closed.

var clients = new List<ClientDto>();

var query = from client in _context.Clients
            select client;

foreach (var client in query)
{
    var user = _context.Users.Single(x => x.UserId == client.CreatedByUserId);

    clients.Add(new ClientDto(client.ClientId, client.Name, new UserDto(user.UserId, user.Name)));
}

If you enabled automatic retries (see snippet below), you will not experience any issues.

services.AddDbContext<EFFunkinessDbContext>(options =>
{
    options.UseSqlServer("your-connectionstring",
        builder => builder.EnableRetryOnFailure());
});

You could materialize the query and use ToList(). That would be synchronise and not good. Entity Framework gives you an extension called ToListAsync() that seems to be percisely for this situation. It gives you the streaming behaviour and manages the connections.

foreach (var client in await query.ToListAsync())
{
    var user = _context.Users.Single(x => x.UserId == client.CreatedByUserId);

    clients.Add(new ClientDto(client.ClientId, client.Name, new UserDto(user.UserId, user.Name)));
}