new GetListResponse Details();
Is not a valid constructor, but judging by your code it looks like the name of the orderDetails structure is Details so instead you should probably do:
new Details();
You do not need to specify the structure's name when you are initializing an object that is inside of a structure.
What you could do is change your GetListResponserDetailsOrder by making ItemDetails and ItemDetailsField into a list like so:
public partial class GetListResponserDetailsOrder
{
private string priceTypeField;
private List<GetListResponseDetailsItemDetails> ItemDetailsField;
public List<GetListResponseDetailsItemDetails> ItemDetails
{
get
{
return this.ItemDetailsField;
}
set
{
this.ItemDetailsField = value;
}
}
}
Then change your code so you can call the constructor like this:
GetListResponse newone = new GetListResponse ();
newone.marker = "";
newone.count = 1;
for (int x = 0; reader.Read(); x++)
{
newone.orderDetails[x] = new Details();
newone.orderDetails[x].order.orderId = Convert.ToInt32(reader[4]);
newone.orderDetails[x].order.orderPlacedTime = Convert.ToUInt64(reader[0]);
newone.orderDetails[x].order.orderValue = Convert.ToInt32(reader[3]) * Convert.ToDecimal(reader[5]);
newone.orderDetails[x].order.ItemDetails = new List<GetListResponseDetailsItemDetails>();
newone.orderDetails[x].order.ItemDetails.Add(new GetListResponseDetailsItemDetails());
newone.orderDetails[x].order.ItemDetails[0].Price = Convert.ToDecimal(reader[5]);
newone.orderDetails[x].order.ItemDetails[0].filledQuantity = Convert.ToInt32(reader[3]);
newone.orderDetails[x].order.ItemDetails[0].ItemNumber = 0;
newone.orderDetails[x].order.ItemDetails[0].orderedQuantity = Convert.ToInt32(reader[3]);
}
You will still be able to use the rest of the code as usual. You can call .Add(...) if you want to add more elements to the list later on, or .Remove(...) if you want to remove any elements.