practice.dataset does not contain a definition for Tables[0] and no extension method accepting a first argument of type practice.dataset could be found are you missing a user directive or...

string CS = ConfigurationManager.ConnectionStrings["SampleConnectionString"].ConnectionString;
        SqlDataAdapter da; Dataset ds; DataRow Rec; SqlCommandBuilder bldr;
        protected void Button1_Click(object sender, EventArgs e)
        {
            try
            {
                Rec = ds.Tables[0].NewRow();//here when i writing the ds.Tables[] here iam getting the red curling lines with above mentioned error.
                Rec[0] = TextBox1.Text;
                Rec[1] = TextBox2.Text;
                Rec[2] = TextBox3.Text;
                Rec[3] = TextBox4.Text;
                Rec[4] = TextBox5.Text;
                ds.Tables[0].Rows.Add(Rec);
                Console.WriteLine("Record Inserted");               
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex);
            }