Question

SqlConnection con1 = new SqlConnection(ConfigurationManager.ConnectionStrings["GVRAgencyConnectionString1"].ConnectionString);
            con1.Open();
            SqlDataAdapter da = new SqlDataAdapter("select      weight,mrp_pack,no_of_pack,mrp_master_pack from dbo.purchase_order where   product_ID=@product_ID",con1);
            da.SelectCommand.Parameters.AddWithValue("@product_ID", DropDownList3.Text);
            DataTable dt = new DataTable();
            da.Fill(dt);
            con1.Close();           
            DataRow row = dt.Rows[0];
            txtwgt.Text = row["weight"].ToString();
            txtmrpsinglepack.Text = row["mrp_pack"].ToString();
            txtnoofmasterpack.Text = row["no_of_pack"].ToString();
            txtmrpmaster.Text = row["mrp_master_pack"].ToString();

Answer 1

1) try this :

2)welcome to StackOverFlow.

3) you cant move to live with some girl if you dont have a girl.

Same here – you cant touch with records in datatable withindex 0 if you dont know that you already have rows in index 0.

SqlConnection con1 = new SqlConnection(ConfigurationManager.ConnectionStrings["GVRAgencyConnectionString1"].ConnectionString);
            con1.Open();
            SqlDataAdapter da = new SqlDataAdapter("select      weight,mrp_pack,no_of_pack,mrp_master_pack from dbo.purchase_order where   product_ID=@product_ID",con1);
            da.SelectCommand.Parameters.AddWithValue("@product_ID", DropDownList3.Text);
            DataTable dt = new DataTable();
            da.Fill(dt);
            con1.Close();           
             if (dt.Rows.Count==0) return;
            DataRow row = dt.Rows[0];
            txtwgt.Text = row["weight"].ToString();
            txtmrpsinglepack.Text = row["mrp_pack"].ToString();
            txtnoofmasterpack.Text = row["no_of_pack"].ToString();
            txtmrpmaster.Text = row["mrp_master_pack"].ToString();