Login ASP.Net C# with Session

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Configuration;
using System.Data.SqlClient;
using System.Data;

public partial class Login : System.Web.UI.Page {
    string username, password, namalengkap;
    SqlCommand sql_cmd;
    SqlDataAdapter data_adapter;
    DataTable data_tabel;
    int r;
    protected void Page_Load(object sender, EventArgs e) {

    }
    protected void btnLogin_Click(object sender, EventArgs e) {
        string koneksi_string = ConfigurationManager.ConnectionStrings["koneksi_material"].ConnectionString;
        SqlConnection koneksi = new SqlConnection(koneksi_string);
        koneksi.Open();
        sql_cmd = new SqlCommand("select * from t_users");
        data_adapter = new SqlDataAdapter(sql_cmd.CommandText, koneksi);
        data_tabel = new DataTable();
        data_adapter.Fill(data_tabel);
        r = data_tabel.Rows.Count;
        for (int i = 0; i < r; i++) {
            username = data_tabel.Rows[i]["username"].ToString();
            password = data_tabel.Rows[i]["password"].ToString();
            namalengkap = data_tabel.Rows[i]["namalengkap"].ToString();
            if (username == txtUsername.Text && password == txtPassword.Text) {
                Session["username"] = username;
                Session["namalengkap"] = namalengkap;
                if (data_tabel.Rows[i]["level"].ToString() == "Admin")
                    Response.Redirect("~/Home.aspx");
                //persiapan untuk role :)
                //else if (data_tabel.Rows[i]["level"].ToString() == "test1")
                //Response.Redirect("~/test/test1.aspx");
                //else if (data_tabel.Rows[i]["level"].ToString() == "test2")
                //Response.Redirect("~/test/test2.aspx");
            } else {
                Response.Write("<script type=\"text/javascript\">alert('Login gagal, periksa username dan password anda !');</script>");
            }
        }
    }
}