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.NET Forum / Languages / C# / February 2008Cannot access HasValue and Value Property of Nullable class
Hi I am unable to access the HasValue and Value properties in an Nullable object. int? i = 9; object value = i; if ((value as Nullable).HasValue) { ... } It does not compile. VS2005 says "System.Nullable does not contain a definition of HasValue". I want to achieve this to make just one function which can handle various nullables like int?, decimal?, long? etc. However, following code compile and runs OK for me. int? i = 9; if (i.HasValue) { ... } MSDN Says: Fundamental Properties: The two fundamental members of the Nullable structure are the HasValue and Value properties. If the HasValue property for a Nullable object is true, the value of the object can be accessed with the Value property. Any ideas? Thanks. AM > Hi > [quoted text clipped - 8 lines] > It does not compile. VS2005 says "System.Nullable does not contain a > definition of HasValue". And, if you look at the Nullable class, it doesn't. Nullable<int>, a struct, does, but Nullable doesn't. > I want to achieve this to make just one function which can handle various > nullables like int?, decimal?, long? etc. I'm not an expert on nullable types, but I think this may be non-trivial. The Nullable<T> type can give you the HasValue property, but you need to somehow get that structure, and doing so at run-time doesn't look straightforward to me. The most obvious route I see uses reflection, starting with the Nullable.GetUnderylingType() method, somehow creating an instance of the correct Nullable<T> type, and using that to get the value of the HasValue property (again, via reflection seems most direct). Hopefully I'm wrong, and if so I hope someone will correct me. Especially since I'm not really clear on how that middle part would work (i.e. "creating an instance of the correct Nullable<T> type" based on the underlying type you get from GetUnderylingType()). > However, following code compile and runs OK for me. > > int? i = 9; > if (i.HasValue) { > ... > } Right, because there you're using Nullable<int>, not Nullable. You could also just cast the object to int? and test the HasValue property, but based on what you wrote that doesn't seem like that would solve your problem. > MSDN Says: > Fundamental Properties: > The two fundamental members of the Nullable structure are the HasValue > and > Value properties. If the HasValue property for a Nullable object is true, > the value of the object can be accessed with the Value property. Right. That's the docus for the generic Nullable<T> structure. But that's not what you get when you use the Nullable class. Pete I'd like to see you type on a keyboard. It must be of iron and go red hot. Anyway, my reply was more effecient than yours, while we say pretty much the same thing. =) Mine might have been somewhat less helpful.. maybe... - Michael Starberg Ok, then what is the best way to do what I want to do? Or is there any way at all (other than the reflection thing)? If not Nullable class, is there any other abstract class or interface from which all these structures derive the HasValue and Value members. IMO, I have a very genuine reason to have something like this. > I'd like to see you type on a keyboard. It must be of iron and go red hot. > [quoted text clipped - 3 lines] > > - Michael Starberg > Ok, then what is the best way to do what I want to do? > Or is there any way at all (other than the reflection thing)? Well, no one else has offered an alternative suggestion. I'd give it another 12 hours or so to let some of the people in other timezones catch up, but I'd say that it may be going through reflection is the only way. But even there, it's not entirely clear on what you're trying to do. To use reflection, you'd need an instance of the Nullable<T> class itself, but that's not what you get when you assign a nullable value to object. > If not Nullable class, is there any other abstract class or interface > from > which all these structures derive the HasValue and Value members. Nullable types are actually structs. So, no...obviously they have no other class from which they derive. And if you look at the Nullable<T> docs, you'll note that it doesn't implement any other interface. > IMO, I have a very genuine reason to have something like this. Why? In the code you posted, you could just check to see whether the "value" variable is null or not. What is it about the HasValue property specifically that you need it for? Pete Nullable<T> has special boxing rules that return an empty Nullable<T> as a null reference, rather than a boxed empty sruct. This means that everything you need is as simple as testing your object variable for null, and casting - i.e. if(value != null) { // ... } Marc > Hi > [quoted text clipped - 7 lines] > } > It does not compile. Of cource it won't. You are way over your head.. .. and have a lot of reading to do. - Michael Starberg > I am unable to access the HasValue and Value properties in an Nullable > object. [quoted text clipped - 8 lines] > I want to achieve this to make just one function which can handle various > nullables like int?, decimal?, long? etc. Sure - it just needs to be a generic method: public void DoSomething<T>(Nullable<T> potentialValue) { if (potentialValue.HasValue) { ... } } As others have pointed out, Nullable isn't the same as Nullable<T>.  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 Minor addition for the OP; you need a constraint for that generic method to compile: public void DoSomething<T>(Nullable<T> potentialValue) where T : struct {...} Of course, this won't help if *all* you know is that you have an "object value", since the compiler won't allow this usage... Marc > Hi > [quoted text clipped - 30 lines] > > AM Why don't you just cast it back into an int? int? i = 9; object o = i; int? j = (int?)i; Nullable is a class for finding information about a Nullable<T> type. Nullable<T> is a struct that makes a value type, in a sense, null- able. I am not sure what the difficulty is here. ??? 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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