R – ASP.NET Dropdown – Works when MasterPage EnableViewState = true

There is no selected value because:

1) there are no items in the drop down list, because you are only binding the data to it the first time around. Normally this is good practice, but when you have ViewState turned off, you must explicitly re-create the data for the control every time.

2) You are binding the data after the point in the page lifecycle where the Request dictionary values are applied to controls (namely, during the restoration of ViewState to controls.) The posted value exists in the Request dictionary, but since there are no items in the dropdownlist, it can't really do much with it. Even though ViewState is turned off, your author is correct - the posted value will be applied, at the point in the lifecycle where ViewState would normally be applied.

If you were to re-create the data for the list in Init(), then the dropdown would be populated for the posted value to be applied to, it would be applied, and the selected value would be available by the time you got to Load(). Hopefully that's clear. Some working code is below:

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            Label1.Text = DropDownList1.SelectedValue;
        }
    }

    protected void Page_Init(object sender, EventArgs e)
    {
        string[] number = { "first", "second", "third" };
        DropDownList1.DataSource = number;
        DropDownList1.DataBind();
    }
}

As per the MSDN article on ViewStateMode, ViewStateMode only has meaning when EnableViewState="true". To achieve what you want, you'll have to leave ViewStateMode="Enabled" on the control, and then wrap the sub-controls of the main one in an asp:Placeholder that has ViewStateMode="Disabled". That way, you can still manipulate ViewState in the codebehind, but no child control will have ViewState (except the ones you explicitly set to have it via ViewStateMode="Enabled").