Factory methods Private abstract/interface methods.

Question

Ok i think my head is about to explode. I'm trying to create a framework for a couple of applications that i'm rewriting from vb6. I've read a little bit about factory design patterns and decided i'd try and implement it. I have created a business entity and given it a private constructor (So no myObject m = new myObject()) i've also given it a static CreateObject method to create new instances. What i would like to do is build as much functionality (Such as logging and security) into a base class and have my other business objects automatically inherit that functionality without the need to program it into my derived classes. The only way i could think to do it was :- employee emp = employee.CreateEmployee(); emp.setfieldvalues..... emp.Update. Upon calling emp.Update (Which incidently is a public method in the base class) a log file will be updated saying that the record was being updated then call a private method called UpdateMe (using reflection) on the employee which would then actually write the record to the database. Now i'm not sure that any of that even makes sense and it wouldn't supprise me if it was bad programming practice. If it is acceptible to do this is there a way a forcing my derrived classes to implement the private method UpdateMe ? Anyone?? Anyone?? Thanks.

Satya Dev · Answer

Correct!!!

Satya Dev · Answer

Wedon't have to call UpDateDerived explicietely as it is being called from the Update() method of the base Employee class automatically. The client program just calls Update on the HourlyEmployee and every thing will be taken care automatically. Here is how things will work: in the client code (may be a Main method) it may be like this: Employee emp = new HourlyEmployee(); emp.Update(); The Update method is only defined in the Employee base class only. If the Update is called on the derived class object of Employee in our example HourlyEmployee then base Employee class's Update is called. Not only that as the base class's Update method calls into UpdateDerived method which is just declared (abstract with no code) in the base class but actuall code is there in the derived class the derived class gets the oppertunity to do its own extra Updates() in its UpdateDerived method that it implements. So it is like whne Update method on the derived class object is called then base classes's Update is called it finishes it job and hands control over to the derived class to do the extra stuff. I hope I explained well

Matthew Barnett · Answer

One other question. Could i force my derived classes to implement the new 'Protected' method when building them? EDIT: Sorry scrub that question. I missed the line - protected abstract void UpdateDerived();. Thanks Matt.

Matthew Barnett · Answer

Thanks for the quick reply. Sorry i forgot to mention i was developing in C#. So if i'm understanding this correctly. When i create an instance of HourlyEmployee using say a static method called HourlyEmployee.CreateNewEmployee() i can then call Update (Based in Employee) this would then do it's stuff then finally call UpdateDerived on my HourlyEmployee almost like returning full circle and that the 'Protected' access modifier should stop the method from showing up in intellisense on my winform ?

Satya Dev · Answer

I think you need to use template method pattern. I don't know VB so I am giving it in C# but concept should be the same Base Class: public abstract class Employee { public void Update() { Log(Updating employee); WriteToDatabase(); // Do updations Now: Update1(); Update2(); // Following is the Template Method Call. We could call this either before // updations (above ones) or here i.e. after the base class updation is done // It depends on the requirement UpdateDerived(); // Hook (Template Method) into the Derived class } // This template method is made abstract so that the derived classes must // implement it if they don't want to be abstract. However a default body with no code // could also be given. In that case this Employee class is nolonger abstract protected abstract void UpdateDerived(); } public class HourlyEmployee(): Employee { protected override void UpdateDerived() { // Do your derived class updations here } }

Factory methods, Private abstract/interface methods.