Question

I have a user control which contains the following code:

<form id="CurrencyForm" method="post" runat="server">    
    Display prices in&nbsp;
    <asp:DropDownList ID="currency" runat="server" AutoPostBack="true">
        <asp:ListItem Value="GBP">GBP (£)</asp:ListItem>
        <asp:ListItem Value="USD">USD ($)</asp:ListItem>
        <asp:ListItem Value="EUR">EUR (€)</asp:ListItem>
    </asp:DropDownList>
</form>

This user control is included in my Master page so it is available on every page of the site.

Here is the codebehind:

protected void page_load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        currency.SelectedValue = (string)Session["currency"];
    }
    else
    {
        session["currency"] = currency.SelectedValue;
        Response.Redirect(Request.RawUrl);
    }
}

When I change the selection of the drop down list, a postback is triggered. However, the value of IsPostBack seems to always be false. I can’t quite put my finger on what I am doing wrong here?

Solution

After looking at the advice from the majority of posts I understand I was going wrong by using code_behind code etc instead of the controllers. I figured out how to use an action to basically redirect back to the same page so the following is now what I have:

User control:

<% using (Html.BeginForm("ChangeCurrency", "Home", FormMethod.Post)) {%>    
    Display prices in&nbsp;
    <select id="currency" name="currency">
        <option value="GBP">GBP (£)</option>
        <option value="USD">USD ($)</option>
        <option value="GBP">EUR (€)</option>
    </select>
    <input type="submit" value="change" /> // this is temporary to test the submit
<%} %>

Action

public ActionResult ChangeCurrency(string currency)
{
    Session["currency"] = currency;
    return new RedirectResult(Request.UrlReferrer.AbsoluteUri);
}

Answer 1

MVC never does postbacks. Therefore, IsPostBack will always, correctly, be false. The thing you think is a postback is actually a POST, because MVC doesn’t do postbacks. You should not be using the ASP.NET server controls at all in MVC.