public void Sample()
{
    string strConn = "Data Source=.;Initial Catalog=pubs;Integrated Security=SSPI;"; 
    StringBuilder sb = new StringBuilder();
    using (SqlConnection conn = new SqlConnection(strConn))
    {
        conn.Open();
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = conn;
        cmd.CommandText = "SELECT * FROM Product";
        
        // SqlDataReader 객체를 리턴
        SqlDataReader rdr = cmd.ExecuteReader();                

        // 다음 레코드 계속 가져와서 루핑
        while (rdr.Read())
        {
            // C# 인덱서를 사용하여
            // 필드 데이타 엑세스
            string s = rdr["Name"] as string;
            sb.Append(s);
        }
        // 사용후 닫음
        rdr.Close();

    }
    //... sb 데이타 사용
}

// SqlDataReader 객체를 리턴
public SqlDataReader GetDataReader()
{            
    SqlConnection conn = new SqlConnection(strConn);
    conn.Open();

    SqlCommand cmd = new SqlCommand();
    cmd.Connection = conn;
    cmd.CommandText = "SELECT * FROM Production.Product";
    SqlDataReader rdr = cmd.ExecuteReader();
    return rdr;
}

// SqlDataReader를 사용후 Close
private void button1_Click(object sender, EventArgs e)
{
    MySample sample = new MySample();
    SqlDataReader dr = sample.GetDataReader();
    dr.Read();
    data = dr[0].ToString();
    dr.Close();
}

// 문제점 해결 방법: 
// System.Data.CommandBehavior.CloseConnection
SqlDataReader rdr = cmd.ExecuteReader(CommandBehavior.CloseConnection);