Here is another approach to use ListPicker(DropDown) in windows phone. its more flexible way to use listpicker in windows phone. Here is a sample code how to use listpicker, this may help you.
//In your xaml
<toolkit:ListPicker Height="60" Name="Dropdown" ExpansionMode="FullScreenOnly" Width="210" >
<toolkit:ListPicker.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding}"/>
</DataTemplate>
</toolkit:ListPicker.ItemTemplate>
<toolkit:ListPicker.FullModeItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding}" Margin="2,10,0,0" FontSize="31"/>
</DataTemplate>
</toolkit:ListPicker.FullModeItemTemplate>
</toolkit:ListPicker>
protected override void OnNavigatedTo(NavigationEventArgs e)
{
if (e.NavigationMode != NavigationMode.Back)
{
//Code behind On page load event.
List<string> dropDownList = new List<string>();
dropDownList.Add("item1");
dropDownList.Add("item2");
dropDownList.Add("item3");
dropDownList.Add("item4");
dropDownList.Add("item5");
dropDownList.Add("item6");
dropDownList.Add("item7");
dropDownList.Add("item8");
Dropdown.ItemsSource = dropDownList;
}
}