C# linq nullable object

Question

In a C# 2008 application I am using linq to sql to connect to a sql server 2008 r2 database. One of the linq statements returns the value to a datetime field called startdate. My first if test looks like if (startdate != null). When there is nothing returned from the linq query and the code is on the statement (startdate != null) the program then lands on the catch statement of a try catch block. Thus can you tell me what I can do so the the application does not land on the catch block code with an exception. I would think there is a way to obtain the datetime value when nothing is returned. Can you show me what code solves the problem and explain how it solves the problem?

Vulpes · Answer

No I don't think that line could be causing the error. If there are no records returned by the query then complete_date will be equal to default(DateTime) - it can't be null because it's not a nullable DateTime. It therefore follows that if isn't null and it isn't default(DateTime) then it must be some other DateTime value - namely the value of the a.Complete_Date column for the first (or only) record returned. You should use the debugger to track down errors by seeing what the values of variables actually are when the program runs (just hover your mouse over them). If you're not sure how to use the debugger then the insertion of a MessageBox.Show Console.WriteLine or Debug.WriteLine can be just as informative.

dc · Answer

It is my mistake. I accidently copied the catch statement twice. Could the following statement be causing my error: if (complete_date != null && complete_date != default(DateTime)) If so can you tell me why it could be causing the error?

Vulpes · Answer

I don't know how that's even compiling as your inner catch doesn't have a corresponding try statement. Also the clause: complete_date != null is unnecessary as complete_date is a non-nullable DateTime and so can't be null. That reduces your code to the following: try { DateTime complete_date = (DateTime)(from a in rData.eStag where a.PID == pId select a.Complete_Date).FirstOrDefault(); if (complete_date != default(DateTime)) { // code } } catch (Exception e) { throw new Exception(e.Message); } If you're still getting an exception thrown then it's probably something you're doing inside the 'if' statement - it can't be the 'if' statement itself and the LINQ query looks OK to me.

dc · Answer

Here is the code I have:try { DateTime complete_date = (DateTime)(from a in rData.eStag where a.PID == pId select a.Complete_Date).FirstOrDefault(); if (complete_date != null && complete_date != default(DateTime)) { } catch { catch (Exception e) { throw new Exception(e.Message); } } Can you tell me what you think is wrong with it when no records are selected from the database?

Vulpes · Answer

I imagine you're using FirstOrDefault() - or a similar LINQ extension method - to get a single DateTime value if there is one and assigning it to a DateTime? variable startdate otherwise the code wouldn't compile. startdate should only be null if the underlying query returns no values. If there are any values then startdate should contain the first one. So I'd have thought that your code should be written on these lines - it shouldn't be necessary to throw an exception: if (startdate != null) { // do something with startdate } else { // either give startdate some default value or take some other appropriate action }