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Unable to cast object of type 'System.Data.Linq.DataQuery`1[System.Int32]' to type 'System.IConvertible'

var db = new ProductOrder();
var idNum = from p in db.Product
            where p.IDNumber == 200900110 
            select p.ProductID; 
var order = new Order();
            order.productID = Convert.ToInt32(idNum);
            order.Date = DateTime.Now;
db.Order.InsertOnSubmit(nTime);
db.SubmitChanges();
After I run it gives me the error like this: 
  
Unable to cast object of type
  'System.Data.Linq.DataQuery`1[System.Int32]'
  to type 'System.IConvertible'
                I hope no-one minds - I've fixed the indentation to show where the statements are more clearly.
– Jon Skeet
                Apr 27, 2009 at 6:27
does not return a single result but a list of results. In your case this will be a list containing a single product id. You should modify it to this:
(from p in db.Product
 where p.IDNumber == 200900110
 select p.ProductID).Single()
If you run your code in the debugger and you hover your mouse over the idNum variable, you'll see that it is a DataQuery instance.
        
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