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isPostBack not working correctly

Posted on 2009-07-11
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Last Modified: 2012-05-07
In the following code,  i generate dropdownlists dynamically  and then  try to add the selected list values to an arraylist and display them on a button(submit) click event.

I have a  if(page.isPostBack == false)  check inside the page_load event  that checks if the page is being loaded for the first time and if it is it generates the lists dynamically and fills option values from database tables(through tableadapter methods).

Otherwise, if the button(submit) was clicked(that is if this was a postback), the selected values from the list should get added to the arraylist and then be displayed.

But this is not happening. Instead it shows some == output and no arraylist values display.

When i tried it without ispostback, it works fine(that is arraylist values get displayed) but then the  form (dropdownlists  themselves with  all option values)  are also displayed along with the  output(arraylist values).

How to correct this behaviour so that it just displays the selected list values and not the form itself along with it?


Code Follows :


using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using QuesDSTableAdapters;
using AnsDSTableAdapters;
using System;
using System.Collections;



public partial class _Default : System.Web.UI.Page
{
    int i = 1;

    //int j;

    //ArrayList alist = new ArrayList();








    protected void Page_Load(object sender, EventArgs e)
    {


        if (Page.IsPostBack == false)
        {

            /* for (j = 0; j<= 1; j++)
             {


                 Response.Write(al[j].ToString());
             }

         }

         else
         {
         */

            QuesDSTableAdapters.QuestionTableAdapter questionsAdapter = new
     QuesDSTableAdapters.QuestionTableAdapter();
            QuesDS.QuestionDataTable questions;

            questions = questionsAdapter.GetQuestions();



            AnsDSTableAdapters.QueAnsTableAdapter queansAdapter = new
     AnsDSTableAdapters.QueAnsTableAdapter();

            AnsDS.QueAnsDataTable answer;



            // AnsDS.QueAnsDataTable queans;






            foreach (QuesDS.QuestionRow quesRow in questions)
            {


                //Response.Write("Question: " + quesRow.Que + "<br />");



                //answer = queansAdapter.GetAnswersByQuestionId(quesRow.QID);

                Form.Controls.Add(new LiteralControl(quesRow.Que));

                answer = queansAdapter.GetAnswersByQuestionId(quesRow.QID);

                DropDownList ddl = new DropDownList();

                ddl.ID = "ddl" + Convert.ToString(i);
                Form.Controls.Add(new LiteralControl("<br>"));

                i += 1;
                Form.Controls.Add(ddl);

                // ddl.DataSource = queansAdapter.GetAnswersByQuestionId(Convert.ToInt32(quesRow.QID));

                ddl.DataSource = answer;

                // DropDownList1.Items.Add(Convert.ToString(answer.Columns ["Answer"]));
                ddl.DataTextField = "Answer";
                ddl.DataBind();

                Form.Controls.Add(new LiteralControl("<br>"));

                Form.Controls.Add(new LiteralControl("<br>"));







            }


            Button btnSubmitResults = new Button();
            btnSubmitResults.ID = "btnSubmitResults";
            btnSubmitResults.Text = "Submit Results";
            btnSubmitResults.Click += SubmitResults;
            Form.Controls.Add(btnSubmitResults);


        }


    }










    protected void SubmitResults(object sender, EventArgs e)
    {
        ArrayList alist = new ArrayList();

        int j;


            foreach (Control ctl in form1.Controls)
            {
                if ((ctl) is DropDownList)
                {
                    alist.Add(((DropDownList)ctl).SelectedValue.ToString());


                }

            }

            for (j = 0; j <= 2; j++)
            {


                Response.Write(alist[j].ToString());
            }


        }
    }
0
Comment
Question by:ee-itpro
  • 2
3 Comments
 
LVL 3

Expert Comment

by:Joep_Killaars
ID: 24831162
Look at the attached code snippet.

You should use the page_init event to create the control. Save the control as a private member of your page and you can do whatever you want with it, even after postbacks. In this case, set visible to false or simply remove the object from the control collection of the form.
private DropDownList _ddl;
 
        protected void Page_Init()
        {
            _ddl = new DropDownList();
            _ddl.ID = "DropDownListId1";
 
            this.Form.Controls.Add(_ddl);
 
            if (!IsPostBack)
            {
                _ddl.Items.Add("A");
                _ddl.Items.Add("B");
                _ddl.Items.Add("C");
            }
            else
            {
                foreach (Control ctrl in this.Form.Controls)
                    if (ctrl.ID == "DropDownListId1")
                    {
                        _ddl = (DropDownList)ctrl;
                        break;
                    }
            }
 
        }
 
        protected void Button1_Click(object sender, EventArgs e)
        {
            Response.Write(_ddl.SelectedValue);
 
            _ddl.Visible = false;
        }

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0
 
LVL 3

Accepted Solution

by:
Joep_Killaars earned 500 total points
ID: 24831169
the else clause is not necessary, the correct snippet :
public partial class WebForm1 : System.Web.UI.Page
    {
        private DropDownList _ddl;
 
        protected void Page_Init()
        {
            _ddl = new DropDownList();
            _ddl.ID = "DropDownListId1";
 
            this.Form.Controls.Add(_ddl);
 
            if (!IsPostBack)
            {
                _ddl.Items.Add("A");
                _ddl.Items.Add("B");
                _ddl.Items.Add("C");
            }
        }
 
        protected void Button1_Click(object sender, EventArgs e)
        {
            Response.Write(_ddl.SelectedValue);
 
            _ddl.Visible = false;
        }
    }

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0
 

Author Closing Comment

by:ee-itpro
ID: 31602438
Hi,

  Thanks, that was helpful.  
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