'Invalid attempt to read when no data is present' - exception happens "sometimes" in Entity Framework - sql-server

Question

I get the above error sometimes during the read. The exception originates from ASP.NET SqlDataReader whenever you try to read data before calling the Read() method. Since EF does all these internally, I am wondering what else can cause this error. could it be network (or) db connectivity?
thanks
Additional Bounty Info (GenericTypeTea):
I've got the same error after upgrading to EF Code First RC (4.1):
"Invalid attempt to read when no data
is present"
This is the code in question:
using (var context = GetContext())
{
var query = from item in context.Preferences
where item.UserName == userName
where item.PrefName == "TreeState"
select item;
// Error on this line
Preference entity = query.FirstOrDefault();
return entity == null ? null : entity.Value;
}
The table structure is as follows:
Preference
{
Username [varchar(50)]
PrefName [varchar(50)]
Value [varchar(max)] Nullable
}
The table is standalone and has no relationships. This is the DbModelBuilder code:
private void ConfigurePreference(DbModelBuilder builder)
{
builder.Entity<Preference>().HasKey(x => new { x.UserName, x.PrefName });
builder.Entity<Preference>().ToTable("RP_Preference");
}
Exactly the same code works perfectly in CTP5. I'm guessing this is an RC bug, but any ideas of how to fix it would be appreciated.

Answer 1

This error occurs when there is a large amount of data in the RC release. The difference between the RC and CTP5 is that you need to specify the [MaxLength] property that contains a large amount of data.

Answer 2

Are you re-using contexts? I would guess this is happening as a result of something you are doing within GetContext
If GetContext() provides a stale context, in which the DataReader is closed/corrupted, I could see the above happening.

Answer 3

I cannot reproduce your problem on EF4.1 RC1.
POCO:
public class Preference
{
public string UserName { get; set; }
public string PrefName { get; set; }
public string Value { get; set; }
}
Context:
public class PreferenceContext : DbContext
{
public DbSet<Preference> Preferences {get;set;}
public PreferenceContext()
: base("Data Source=localhost;Initial Catalog=_so_question_ef41_rc;Integrated Security=SSPI;") {
}
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
ConfigurePreference(modelBuilder);
base.OnModelCreating(modelBuilder);
}
private void ConfigurePreference(DbModelBuilder builder)
{
builder.Entity<Preference>().HasKey(x => new { x.UserName, x.PrefName });
builder.Entity<Preference>().ToTable("RP_Preference");
}
}
My little Console App:
class Program
{
static void Main(string[] args)
{
string userName = "Anon";
for (int i = 0; i < 10000; i++)
{
var p = GetPreference(userName);
}
}
private static string GetPreference(string userName)
{
using (var context = new PreferenceContext())
{
var query = from item in context.Preferences
where item.UserName == userName
where item.PrefName == "TreeState"
select item;
// Error on this line
Preference entity = query.FirstOrDefault();
return entity == null ? null : entity.Value;
}
}
}
I do 10,000 reads, and no error. You will need to post more complete code to continue.

Answer 4

Increase the CommandTimeout on the context.

Answer 5

I had the same issue with EF4 - In my case I was (trying to) return the list of entities within the using{} section. This is the same as you are doing in your question:
return entity == null ? null : entity.Value;
} // end using
I moved the return to after the } and it worked.
I think I had the problem because the code was in a function which had already queried the database in another using block, I suspect the table was locking but not reporting the error, ending the using block before the return released the database lock.
Steve