I am setting IsSorting property for unbound column in xaml. When accessing the property in code behind its always false even if I set the IsSortable property to true in xaml

Here what I am doing. In Xaml

                <Grids:UnboundColumn Key="FirstName" HeaderText="First Name" Width="200"                 IsSortable="True"   IsFilterable="true"  IsReadOnly="False">
                    <Grids:UnboundColumn.ItemTemplate>
                        <DataTemplate>
                            <TextBlock Text="{Binding RowData.FirstName}" />
                        </DataTemplate>
                    </Grids:UnboundColumn.ItemTemplate>

               </Grids:UnboundColumn>

In code behind :

foreach (var col in this.FilterGrid.Columns)
{
    if (col.GetType() == typeof(UnboundColumn))
    {
       var column = col as UnboundColumn;
 bool test= column.IsSortable
     }
}

IsSortable property returns always false.Whereas other properties like IsFilterable,IsReadOnly etc works fine.

I will appreciate if any one has a fix for  this