Question

I have the following code shape. It seems that I’m misunderstanding the C# method return values. How is it possible that a “full” enumerator gets returned as an empty one?

class ThingDoer
{
    public NpgsqlDataReader DoQuery()
    {
        NpgsqlCommand c = new NpgsqlCommand(...);
        NpgsqlDataReader dataread = c.ExecuteReader();
        return dataread;  // Debugger confirms that six data are enumerable here.
    }
}

...

class OtherThing
{
    public void higherLevelFunction()
    {
        NpgsqlDataReader result = myThingDoer.DoQuery();
        result.Read();  // No data! result's enumerable returns nothing!
    }
}

Answer 1

You don’t detail where your connection is coming from. Assuming it’s something like:

public NpgsqlDataReader DoQuery()
{
    using(NpgsqlConnection = GetConnectionCode())
    {
        NpgsqlCommand c = new NpgsqlCommand(...);
        NpgsqlDataReader dataread = c.ExecuteReader();
        return dataread;
    }//Connection closes at this using-scope being left because that triggers Dispose()
}

Then change it to:

public NpgsqlDataReader DoQuery()
{
    bool ownershipPassed = false;
    NpgsqlConnection conn = GetConnectionCode();
    try
    {
        NpgsqlCommand c = new NpgsqlCommand(...);
        NpgsqlDataReader dataread = c.ExecuteReader(CommandBehavior.CloseConnection);
        ownershipPassed = true;
        return dataread;
    }
    finally
    {
        if(!ownershipPassed)//only if we didn't create the reader than takes charge of the connection
          conn.Dispose();
    }
}

Then where you use the reader, you have to dispose it to in turn dispose the connection’s underlying connection to the database:

public void higherLevelFunction()
{
    using(NpgsqlDataReader result = myThingDoer.DoQuery())
      result.Read();
}