selectList.SelectedValue为null

Question

I keep getting this error when I implement my code for a dropdown list. Here is my code:

String sort = "";
String day_hire = "Daily hire rate";
String man_date = "Manufacturing date";
string man_n = "Manufacturer's name";

if (sl2.SelectedValue.ToString()== man_n)
{
    sort = "man.manufacturer_name";

}
else if (sl2.SelectedValue.ToString() == man_date)
{
    sort = "veh.manufacturing_date";
}
else if (sl2.SelectedValue.ToString() == day_hire) 
{
    sort = "veh.daily_hire_rate";
}
else
{
    sort = "veh.daily_hire_rate"; 
}

this is in constructor. In view I've got:

<%=Html.DropDownList ("sort",((SelectList)ViewData["selectOptions2"]))%>

it's a runtime error that says sl2.SelectedValue.ToString() is null. Please help

Answer 1

When you do a POST, the selected value will be stored in the "sort" value, which you had declared here:

<%=Html.DropDownList ("sort",((SelectList)ViewData["selectOptions2"]))%>

So you need to do:

[HttpPost]
public void Whatever()
{
    var selectedItem = Request.Form["sort"] // Or however you want to pick up the value.
}

This is a very good article which goes into more detail.