Subsonic ActiveRecord give error for type casting

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ASP.NET

Here is code which give error for type conversion....

ImageButton button = sender as ImageButton;
Query qry = new Query(Descriptor.Schema).WHERE(Descriptor.Columns.ProductId, Comparison.Equals, this.productId).ORDER_BY(Descriptor.Columns.SortOrder);
DescriptorCollection descriptors = new DescriptorController().FetchByQuery(qry);
if (descriptors != null)
{
Predicate<descriptor> match = null;
int descriptorId = 0;
int.TryParse(button.CommandArgument.ToString(), out descriptorId);
if (descriptorId > 0)
{
if (match == null)
{
match = descriptor => descriptor.DescriptorId == descriptorId;
}
Descriptor item = descriptors.Find(match);
int index = descriptors.IndexOf(item);

Error: Cannot implicitly convert type 'SubSonic.ActiveRecord<Etech.CMS.Content.ImageGallary>' to 'Etech.CMS.Content.ImageGallary'. An explicit conversion exists (are you missing a cast?)

What i have to do?