Can't insert in a table having composite primary key using LINQ

Question

Added that SqlConnection part as someone suggests on this website but still getting exception

Cannot insert explicit value for identity column when identity insert is OFF

on dv.SubmitChanges();

The composite primary key is composed of stid and bookid.

Also please tell me how I can make this query without concatenating textbox's value i.e bname.Text

using (SqlConnection conn = new SqlConnection(cs))
{
    conn.Open();

    using (DataClasses1DataContext dv = new DataClasses1DataContext())
    {
        var iq = (from b in dv.Books
                  where SqlMethods.Like(b.name, "%" + bname.Text + "%")
                  select b.Id).FirstOrDefault();

        IssueInfo info = new IssueInfo()
                             {
                                 stid = Convert.ToInt32(rollno.Text),
                                 due_date = Convert.ToDateTime(dateTimePicker1.Text),
                                 bookid = Convert.ToInt32(iq)
                             };

        dv.ExecuteCommand("SET IDENTITY_INSERT IssueInfo ON");
        dv.IssueInfos.InsertOnSubmit(info);
        dv.SubmitChanges();
        dv.ExecuteCommand("SET IDENTITY_INSERT IssueInfo OFF");
    }
}