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.NET Forum / Languages / C# / March 2008How to return null from c# class constructor after a validation?
Hi, This may be a silly question. I would like to return null while creating a new instance of a class and the validation inside the class constructor fails. For example: class employee { public employee(int employeeid) { if(isemployeeexist(employeeid)) { // validation success and proceed normally } else { // validation fails and needs to return null ????? } } } void main() { employee abc = new employee(1234); if(abc == null) { .... } } How do i return null from a class constructor? Any help will be much appreciated. Thanks, Saravanan. > This may be a silly question. I would like to return null while creating a > new instance of a class and the validation inside the class constructor > fails. You can't. Instead, write a static method which performs the validation and then either returns the new instance or null. An alternative is to make your constructor throw an exception.  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 Thanks for your response. I got your alternatives by searching the net already. Though the alternatives does the job, it doesn't look great atleast for me. I guess i have no other choice than to stick with one of the alternatives. cheers. > > This may be a silly question. I would like to return null while creating a > > new instance of a class and the validation inside the class constructor [quoted text clipped - 4 lines] > > An alternative is to make your constructor throw an exception. > Thanks for your response. > > I got your alternatives by searching the net already. Though the > alternatives does the job, it doesn't look great atleast for me. > > I guess i have no other choice than to stick with one of the alternatives. Look at it from the other point of view - everywhere in the code where you call a constructor, you can guarantee that if the constructor returns, it *won't* return a null reference. Personally I like that a lot :) 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 >> Thanks for your response. >> [quoted text clipped - 7 lines] > returns, it *won't* return a null reference. Personally I like that a > lot :) I agree with that, the explicit guarantee that if it doesn't throw an exception, you got an instance reference, is a lot better than having to check for null. The original poster might argue that this would only be used with this special class, but if the guarantee goes out the window (ie. the C# compiler or .NET runtime would allow it), then it really is out the window, special class or not. But then I'm a proponent of the exception system as opposed to the check-for-error-returncode system :) SignatureLasse Vågsæther Karlsen mailto:lasse@vkarlsen.no http://presentationmode.blogspot.com/ PGP KeyID: 0xBCDEA2E3 Getting a null reference from calling a class constructor is not (I don't think) a logical or best-practices approach. Better to throw an exception when you have a lot of business logic going on in the constructor. After all the whole idea of having a constructor is to get back an instance of the class, and getting a null reference gives us no information of any value at all. -- Peter Site: http://www.eggheadcafe.com UnBlog: http://petesbloggerama.blogspot.com Short Urls & more: http://ittyurl.net > Hi, > [quoted text clipped - 35 lines] > Thanks, > Saravanan. > Getting a null reference from calling a class constructor is not (I don't > think) a logical or best-practices approach. Better to throw an exception > when you have a lot of business logic going on in the constructor. After all > the whole idea of having a constructor is to get back an instance of the > class, and getting a null reference gives us no information of any value at > all. And in general there should not be so much bussines logic in the constructor - it can easily become inflexible. Arne > This may be a silly question. I would like to return null while creating a > new instance of a class and the validation inside the class constructor [quoted text clipped - 28 lines] > > How do i return null from a class constructor? As already stated then you don't. The C# paradigm is to use exceptions. If you really want to do it this way then: - make the employee constructor do very little - create a factory method that constructs an instance, set properties and either return the instance or return null depending on status Arne you could do it in this way: public class Employee { private static Dictionary<int, Employee> _employeesTable = new Dictionary<int, Employee>(); private readonly int _id; public Employee(int id) { this._id = id; } public int Id { get { return (this._id); } } public static Employee GetInstance(int id) { Employee returnInstance = null; Employee._employeesTable.TryGetValue(id, out returnInstance); //better would be //if(!Employee._employeesTable.TryGetValue(id, out returnInstance)) //{ // returnInstance = new Employee(id); // Employee._employeesTable.Add(returnInstance._id, returnInstance); //} return (returnInstance); } } cheers vo1d >> This may be a silly question. I would like to return null while creating >> a new instance of a class and the validation inside the class constructor [quoted text clipped - 37 lines] > > Arne vo1d... The "better method" looks like it needs a lock since it would access a static dictionary which makes me nervous. Perhaps I can entice John to chime in here about concurrent access to the static dictionary. Regards, Jeff > vo1d... The "better method" looks like it needs a lock since it would > access a static dictionary which makes me nervous. Perhaps I can > entice John to chime in here about concurrent access to the static > dictionary. Yes, it should indeed use a lock. Still a perfectly good approach, but it does need some care on the thread-safety aspects :) 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 > vo1d... The "better method" looks like it needs a lock since it would > access a > static dictionary which makes me nervous. Perhaps I can entice John to > chime in > here about concurrent access to the static dictionary. Unless the dictionary is totally immutable, both methods need a lock to be thread-safe. > Regards, > Jeff > > *** Sent via Developersdex http://www.developersdex.com *** Saravanan... One approach is to use a static class factory to return a reference of type employee by id. If an object of class Employee with the given id already exist then the factory would return a reference to the existing instance of class Employee. If an object of class Employee with the given id did not exist, then factory could create a new instance of Employee with the given id and return a reference to the new instance OR the factory could throw an exception, etc. Regards, Jeff
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