How does SqlDataReader receive the data?

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?

I tested where I started a reader and changed the next row but the reader did not get the revised value. So it seems to read ahead. — paparazzo
– paparazzo, Commented Sep 27, 2012 at 16:08

Community · Accepted Answer · 2017-05-23 11:43:30Z

2

SqlDataReader has a 8 kilobytes buffer.

Duplicate question: How DataReader works?

edited May 23, 2017 at 11:43

CommunityBot

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answered Sep 27, 2012 at 15:16

Stefan P.

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