Question

Could someone explain to me why my IEnumerable is not lazy loaded on GetMessages().Take(5)? If I debug the foreach loop, it appears to lazy load one message at a time for the first 5, adding them to the listBox1, but then after those 5 it will continue to populate the rest of the list (which takes like a minute) before continuing executing after the loop.

    public void PopulateMessages()
    {
        foreach (string message in GetMessages().Take(5))
        {
            listBox1.Items.Add(message);
        }
    }

    private static IEnumerable<string> GetMessages()
    {
        using (var conn = new SqlConnection(connectionString))
        {
            conn.Open();

            // The Message table has thousands of rows
            SqlDataReader reader = new SqlCommand("SELECT * FROM Message", conn).ExecuteReader();

            while (reader.Read())
            {
                yield return reader.GetString(0);
            }
        }
    }

Thanks.

Answer 1

It is lazy loading them, however: the sql command is still running. One thing you might try is a bit more using:

using(var cmd = new SqlCommand("SELECT * FROM Message", conn))
using(var reader = cmd.ExecuteReader())
{
    while (reader.Read())
    {
        yield return reader.GetString(0);
    }
}

However, the optimal solution would be to generate the TSQL with a TOP 5 in it, or maybe a TOP (@count) if you want to parameterise.

This will then dispose the reader and command as soon as possible. Saying that: the connection is already correctly disposed.