While Inserting A Row Into Data Base I Am Getting An Exception

public partial class _Default : System.Web.UI.Page
{

    SqlConnection connectionString = new SqlConnection("Data Source=AZHAR-PC\\SQLEXPRESS;Initial Catalog=Emp_DB;Integrated Security=True");
    SqlCommand commandString;

    protected void Page_Load(object sender, EventArgs e)
    {
     connectionString.Open();
}
    protected void btnInsert_Click(object sender, EventArgs e)
    {
        String query;
        query = "insert into SalesEmpTable values(" +txtId.Text+ ",'" + txtName.Text + "' , " + txtOrder + ")";

        commandString = new SqlCommand(query, connectionString);
        int record = commandString.ExecuteNonQuery();

        lblStatus.Text = "Status: Successfully Inserted";
        commandString.Dispose();
    }
}