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.NET Forum / Languages / C# / February 2008Nullable types giving error
I have the following in c# VS 2005. fn.ProjectTableID = dbReader["ProjectTableID"] == DBNull.Value ? null : (int?)dbReader["ProjectTableID"]; This works fine as long as dbReader["ProjectTableID"] is null. If it is a value I get an error: Specified cast is not valid. Why is that? How do I assign the value? I tried: fn.ProjectTableID.Value = dbReader["ProjectTableID"] == DBNull.Value ? null : (int?)dbReader["ProjectTableID"]; but that didn't work either. Thanks, Tom > I have the following in c# VS 2005. > [quoted text clipped - 17 lines] > > but that didn't work either. Have you tried casting to "int" instead of "int?" ?  SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk >> I have the following in c# VS 2005. >> [quoted text clipped - 20 lines] > > Have you tried casting to "int" instead of "int?" ? Yes. If I change it to: fn.ProcessingType = dbReader["ProcessingType"] == DBNull.Value ? null : (int)dbReader["ProcessingType"]; I get: Type of conditional expression cannot be determined because there is no implicit conversion between '<null>' and 'int' Thanks, Tom tshad, I am guessing that fn.ProcessingType is of type nullable int. See if this works fn.ProcessingType = dbReader["ProcessingType"] == DBNull.Value ? (int?)0 : (int)dbReader["ProcessingType"]; Does that work? Kalpesh My bad. I am not too good with the concept of nullable type Either above or this should work. fn.ProcessingType = dbReader["ProcessingType"] == DBNull.Value ? (int?)0 : (int?)dbReader["ProcessingType"]; > I have the following in c# VS 2005. > [quoted text clipped - 17 lines] > > but that didn't work either. Ignore my previous response. Find out what dbReader["ProjectTableID"].GetType() shows. I suspect it won't be System.Int32. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk >> I have the following in c# VS 2005. >> [quoted text clipped - 23 lines] > Find out what dbReader["ProjectTableID"].GetType() shows. I suspect it > won't be System.Int32. It isn't. It depends. The type in SQL is tinyint. In my record fn.ProjectTableID is an "int?". The value will either be null or a number from 1-25. If it is null, which is the case for the first 12 records, I get the following in the debugger watch window {Name = "DBNull" FullName = "System.DBNull"} System.Type {System.RuntimeType} :If it is a value like 10, I get: {Name = "Byte" FullName = "System.Byte"} System.Type {System.RuntimeType} I would assume that I would be casting it to either int? or int since I am putting it in an int? field. I can't change the fn.ProjectTableID to int (or byte) as it can be null and I need to keep track of that. Thanks, Tom > It depends. The type in SQL is tinyint. In my record fn.ProjectTableID is > an "int?". The value will either be null or a number from 1-25. [quoted text clipped - 8 lines] > > {Name = "Byte" FullName = "System.Byte"} System.Type {System.RuntimeType} ^^^^^^^^^^^ > I would assume that I would be casting it to either int? or int since I am > putting it in an int? field. I can't change the fn.ProjectTableID to int > (or byte) as it can be null and I need to keep track of that. Try cast it to byte. Arne >> It depends. The type in SQL is tinyint. In my record fn.ProjectTableID >> is an "int?". The value will either be null or a number from 1-25. [quoted text clipped - 15 lines] > > Try cast it to byte. That works fine. Since a tinyint ranges from 0 to 255, I assume that is why it is assumed to be a byte. fn.ProcessingType = dbReader["ProcessingType"] == DBNull.Value ? null : (byte?)dbReader["ProcessingType"]; This works ok I would have assumed that int would also work but I guess not. Thanks, Tom > Arne <snip> > That works fine. > [quoted text clipped - 7 lines] > > I would have assumed that int would also work but I guess not. No, because you're unboxing. When you unbox, you have to specify the correct type, not just one which the correct type can be implicitly converted to. (You can also specify the nullable version of the correct type.) SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk >>> It depends. The type in SQL is tinyint. In my record fn.ProjectTableID >>> is an "int?". The value will either be null or a number from 1-25. [quoted text clipped - 25 lines] > > I would have assumed that int would also work but I guess not. Maybe you will find this code snippet illustrative: using System; namespace E { public class Program { public static void Main(string[] args) { byte b = 123; object o = b; Console.WriteLine("Cast to byte works: " + (byte)o); try{ Console.WriteLine("Cast to int works: " + (int)o); } catch { Console.WriteLine("Cast to int did not work"); } Console.WriteLine("Cast to byte cast to int works: " + (int)(byte)o); Console.ReadKey(); } } } Arne >>>> It depends. The type in SQL is tinyint. In my record >>>> fn.ProjectTableID is an "int?". The value will either be null or a [quoted text clipped - 53 lines] > } > } (int)(byte)o So what you did here was unbox the object and then implicitly convert it to int? And why is dbReader["ProcessingType"] considered boxed? Thanks, Tom > Arne <snip> > (int)(byte)o > > So what you did here was unbox the object and then implicitly convert it to > int? Well, explicitly convert it to int - but yes. > And why is dbReader["ProcessingType"] considered boxed? Because its return type is "object" - i.e. it will return a reference, so the only way of returning value type data is to return a boxed form. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk <snip> > It isn't. > [quoted text clipped - 10 lines] > > {Name = "Byte" FullName = "System.Byte"} System.Type {System.RuntimeType} Right - that's the important part. > I would assume that I would be casting it to either int? or int since I am > putting it in an int? field. The important thing is what you're casting. You're trying to unbox, which means the type has to be right. You can cast it to byte? (or indeed byte) and it'll be fine. Cast the null to int? so that compiler knows what to make the overall expression type. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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