Kết nối SQL server - C#

public String con_str = "Data Source=.;Initial Catalog=LTFDatabase;Integrated Security=True";

    public DataSet Opendataset(String sql)

    {

        SqlConnection con = new SqlConnection(con_str);

        SqlDataAdapter da = new SqlDataAdapter(sql, con);

        DataSet ds = new DataSet();

        da.Fill(ds);

        return ds;

    }

    public DataTable Opendatatable(String sql)

    {

        SqlConnection con = new SqlConnection(con_str);

        SqlDataAdapter da = new SqlDataAdapter(sql, con);

        DataTable dtb = new DataTable();

        da.Fill(dtb);

        return dtb;

    }

    public string ExecuteScalar(string sql)

    {

        SqlConnection con = new SqlConnection(con_str);

        SqlCommand cmd = new SqlCommand(sql, con);

        con.Open();

        string kq = cmd.ExecuteScalar().ToString();

        con.Close();

        cmd.Dispose(); return kq;

    }

    public bool ExcSql(string sql)

    {

        SqlConnection con = new SqlConnection(con_str);

        SqlCommand cmd = new SqlCommand();

        cmd.CommandText = sql;

        cmd.Connection = con;

        try

        {

            con.Open();

            cmd.ExecuteNonQuery();

            return true;

        }

        catch

        {

            return false;

        }

        finally

        {

            con.Close();

        }

    }