Question

I have a rather ugly service job that runs through a legacy database and compares it to our production database:

if (vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number) != null)                        {
 var oldDbContractItem = vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number).First();
                            // check to see if there were changes
                            if (oldDbContractItem.DateStamp != vendorContractItem.Date_Stamp)
                            {
                                oldDbContractItem.Update(vendorContractItem);
                            }
                        }

I will get an error on var oldDbContratItem, “Sequence contains no elements”, yet I just did a != null check. This must be simple, what’s going on?

Answer 1

This is the test against null you are doing:

vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number) != null

That will always be true; That will always return at least an empty sequence… never a null.

You might be meaning to test that its length is greater than 0?

There’s an easier way, though, IMO. Call FirstOrDefault() instead of First() and leave out the pre-test completely. Then instead, test if the result of FirstOrDefault() is null.

var oldDbContractItem = vendorContract.Item
  .Where(x => x.ItemNumber == contractItem.Item_Number).FirstOrDefault();
if(oldDbContractItem != null) //would be null if there are no items
{
  // check to see if there were changes
  if (oldDbContractItem.DateStamp != vendorContractItem.Date_Stamp)
  {
    oldDbContractItem.Update(vendorContractItem);
  }
}
}