Question

The following simple code retrieve data with SqlDataReader:

    SqlConnection conn = new SqlConnection("Data Source=.;Initial Catalog=myDB;Integrated Security=True");
    conn.Open();
    SqlCommand cmd = conn.CreateCommand();
    cmd.CommandText = "SELECT [t0].[ID], [t0].[Name] FROM [Persons] AS [t0]";
    SqlDataReader rdr = cmd.ExecuteReader();
    while (rdr.Read())
    {
        Console.WriteLine(rdr["id"] + " " + rdr["name"]);
    }

While the code is running, I use Sql Profiler to monitor the DB. And I saw only one select command.

SELECT [t0].[ID], [t0].[Name] FROM [Persons] AS [t0]

So it seems the SqlDataReader will retrieve ALL the data from server first, and then enumerate them.

Is that true? What if the data is too many?

Answer 1

SqlDataReader has a 8 kilobytes buffer.

Duplicate question: How DataReader works?