dropdown selected item and repose.redirect

Question

I have two dropdownlist in my project and get the items from sql server. One of them show list of something (And I named it DropdownSoore) and another one show list of members of it (And I named it DropdownAye) that update when select an item from DropdownSoore. then when select an item from DropdownAye go to page of them by forwarding query string. I have no problem to updating DropdownAye but when I select item from it and redirect to page of it, the DropdownAye lose the selected item and show the first item (as default) and index to query string in url.

what should I do?

Excuse me about my grammer, I'm not English...

I set DropdownSoore:

<asp:DropDownList ID="DropDownListSoore" runat="server" class="nav-link btn btn-outline-secondary dropdown-toggle" aria-haspopup="true" aria-expanded="true" AutoPostBack="True"></asp:DropDownList>

for (int i = 0; i <= dt.Rows.Count - 1; i++) 
{ 
    string IdSoore = Convert.ToString(dt.Rows[i]["IdSoore"]); 
    string NameSoore = Convert.ToString(dt.Rows[i]["NameSoore"]); 
    DropDownListSoore.Items.Add(IdSoore + "." + NameSoore); 
}

string forwardedIdSoore = Request.QueryString["IdSoore"];

if (forwardedIdSoore != null)
{
    DropDownListSoore.Items.FindByValue(forwardedIdSoore);
}

And then set DropdownAye:

<asp:DropDownList ID="DropDownListAye" runat="server" class="nav-link btn btn-outline-secondary dropdown-toggle" aria-haspopup="true" aria-expanded="true" OnSelectedIndexChanged="DropDownListAye_SelectedIndexChanged" AutoPostBack="True"></asp:DropDownList>

DropDownListAye.Items.Clear();
for (int i = 0; i <= dt1.Rows.Count - 1; i++)
{
    DropDownListAye.Items.Add(Convert.ToString(dt1.Rows[i]["NumberAye"]));
}

int iSelectedAye = Convert.ToInt32(DropDownListAye.SelectedIndex) + 1;
string SelectedAye = Convert.ToString(dt1.Rows[iSelectedAye]["IdAye"]);
Session["SSelectedAye"] = SelectedAye;

string forwardedIdAye = Request.QueryString["IdAye"];
if (forwardedIdSoore != null)
{
    DropDownListSoore.Items.FindByValue(forwardedIdAye);
}

in page_Load. and dont have problem to update DopdownAye but when I used it to redirect always get IdAye=1 in url and show the first item of DropdownAye ...:

protected void DropDownListAye_SelectedIndexChanged(object sender, EventArgs e)
{
    int SelectedAye = Convert.ToInt32(Session["SSelectedAye"]);
    
    int SelectedSoore = Convert.ToInt32(DropDownListSoore.SelectedIndex) + 1;
    
    string forward = "~/contentAye.aspx?IdSoore=" + SelectedSoore + "&IdAye=" + SelectedAye;
    Response.Redirect(forward);
}

I try !IsPostBack but dont have utility too.

I do all thing I think and i dont know what shoild i do.

Answer 1

And where are you loading up this information?

Remember, page load ALWAYS fires and triggers for each post-back, for each button click, and any OTHER event on the page that calls code behind.

What does the above really mean?

Well, it means that you need to ONLY ONE TIME load up the drop downs, the gridview(s), or whatever else you have.

So, for the last 200+ web pages I have built, I ALWAYS have this code stub in the page load event:

so this markup:

        <h3>Select Hotel</h3>
        <asp:DropDownList ID="DropDownList1" runat="server"
            DataValueField="ID"
            DataTextField="HotelName" Width="356px">
        </asp:DropDownList>
        <br />
        <br />
        <asp:Button ID="Button1" runat="server"
            Text="Show/get/display drop selection" OnClick="Button1_Click"
            CssClass="btn" />
        <br />
        <h3>Selected result</h3>
        <asp:Label ID="Label1" runat="server" Text=""></asp:Label>

and this code behind:

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        // load up data, controls etc.
        string strSQL =
            "SELECT ID, HotelName FROM tblHotelsA ORDER BY HotelName";
        DataTable dt = General.MyRst(strSQL);
        DropDownList1.DataSource = dt;
        DropDownList1.DataBind();
        DropDownList1.Items.Insert(0, new ListItem("Please Select Hotel", ""));

    }
}

protected void Button1_Click(object sender, EventArgs e)
{
    string sResault =
        $@"dropdown list value (pkid) = {DropDownList1.SelectedItem.Value}
        <br/>
        Dropdown list Text (Hotel) = {DropDownList1.SelectedItem.Text}";

    Label1.Text = sResault;
}

and the result:

SUPER important:

If I leave out the !IsPostBack test in page load, then for EVERY button click, the dropdown list re-load code will trigger, and runs BEFORE your button stub, or post back. As a result, I will see/find/get no valid value from the drop down list, since page load ALWAYS runs before the given code stub, and thus if page load "every time" re-loads the data, then it will blow out the chosen value in the drop down list.

So, loading up of grids, data, dropdowns etc. can ONLY occur one time in page load, since page load ALWAYS runs before each button or event code stub you have behind.

If you thus only load up the controls and dropdowns on the first REAL page load, then the values the user selects in those dropdowns should work, but only will work if you ALWAYS include that all important if (!IsPostBack) code stub in your page load event.

Now, of course in place of that button to loadup + display the dropdown list into that label?

Well, of course that button click may well (like often) jump or navigate to another page. So, in place of that code to fill out the selected value(s) into that label, we could pass the selected value to the next page. And how to do that? Well, you can use session(), you can use parameters in the URL, or you can even use a post-back URL, and then the WHOLE previous page becomes available (Page.Previous), which then in theory allows you to grab ANY and ALL values from the previous page.

But, say we choose session() to pass the value, then this:

  Session["HotelPK"] = DropDownList1.SelectedItem.Value;

  Response.Redirect("GridFun3.aspx");