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

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 thes开发者_如何学JAVAe 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.

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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.

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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.

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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.

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Increase the CommandTimeout on the context.

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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