Session Sign in - Session string is null - asp.net

Question

This is not working at all. I have done it many times but I don't know what's going wrong. the textbox always shows "not found" whereas it should be showing username.
Note: Textbox is just an example.
Login Page:
protected void login_Click(object sender, EventArgs e)
{
Session.RemoveAll();
Session.Abandon();
Session.Clear();
string username = email.Text.ToLower().Trim();
SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["myConnectionString"].ToString());
con.Open();
string sql = "SELECT USERNAME, PASSWORD FROM MANAGER WHERE USERNAME = #USERNAME AND PASSWORD=#PASSWORD";
SqlCommand command = new SqlCommand(sql, con);
command.Parameters.AddWithValue("#USERNAME", username);
command.Parameters.AddWithValue("#PASSWORD", password.Text.Trim());
SqlDataAdapter da = new SqlDataAdapter(command);
DataTable dt = new DataTable();
da.Fill(dt);
command.ExecuteNonQuery();
command.Dispose();
con.Close();
if (dt.Rows.Count > 0)
{
Session["manager"] = username;
Response.Redirect("ManagerHomePage.aspx");
Session.RemoveAll();
}
else
{
Label1.Text = "Invalid Email or Password!";
}
}
ManagerHomePage.aspx :
protected void Page_Load(object sender, EventArgs e)
{
if(Session["manager"]!=null)
{
TextBox1.Text = Session["manager"].ToString();
}
else
{
TextBox1.Text = "not found";
}
}

Answer 1

Do not use ExecuteNonQuery for Select command. ExecuteNonQuery is used for Insert, Update, Delete command. Try using ExecuteReader
Bellow is the code
protected void login_Click(object sender, EventArgs e)
{
Session.RemoveAll();
Session.Abandon();
Session.Clear();
string username = email.Text.ToLower().Trim();
SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["myConnectionString"].ToString());
con.Open();
string sql = "SELECT USERNAME, PASSWORD FROM MANAGER WHERE USERNAME = #USERNAME AND PASSWORD=#PASSWORD";
SqlCommand command = new SqlCommand(sql, con);
command.Parameters.AddWithValue("#USERNAME", username);
command.Parameters.AddWithValue("#PASSWORD", password.Text.Trim());
SqlDataReader rdr=command.ExecuteReader();
if(rdr.HasRows())
{
Session["manager"] = username;
Response.Redirect("ManagerHomePage.aspx");
}
else
{
Label1.Text = "Invalid Email or Password!";
}
}