Design pattern suggestion needed

Question

I need a programming pattern but I couldn't figure out what it will be. I am not even sure if what I want is possible or not. Lets say I have a Class A and 10 class inherited from A. What I want is call the same method Method Print() implemented differently on each inherited class, but all of them need to repeat a same method of Class A.

Class A
{
    public virtual void Print()
    { 
    }
    protected void StartingProcedure()
    {
        /// something 
    }
    protected void EndingProcedure()
    {
        /// something 
    }
}

Class A_1 : A
{
    public override void Print()
    {
        StartingProcedure();
        /// class specific print operation
        EndingProcedure();
    }
}

Class A_2 : A
{
    public override void Print()
    {
        StartingProcedure();
        /// class specific print operation
        EndingProcedure();
    }
}

As you see I dont want to keep writing StartingProcedure(); on each overridden Print method. I want to automate that procedure, when I inherit a new class from A I don't want to mind the Starting and Ending procedures, i want to just write the class specific print operation. Is there any pattern that will provide me this kind of a class behaviour?

BTW I work with C#

base.Print() has no implementation in it. It is a virtual method. Even if it did how would I be able to call inherited classes specific operations? — Tolga Evcimen
– Tolga Evcimen, Commented Nov 13, 2013 at 14:19
In addition, your "derived" classes are not inheriting from A. — Yair Nevet
– Yair Nevet, Commented Nov 13, 2013 at 14:19

Rik · Accepted Answer · 2013-11-13 14:26:22Z

9

Class A
{
    public void Print()
    { 
        StartingProcedure();
        SpecificPrint();
        EndingProcedure();
    }
    protected void StartingProcedure()
    {
        /// something 
    }
    protected void EndingProcedure()
    {
        /// something 
    }
    protected virtual SpecificPrint() // could possibly be abstract
    {
    }
}

Class A_1 : A
{
    public override void SpecificPrint()
    {
        /// class specific print operation
    }
}

Class A_2 : A
{
    public override void SpecificPrint()
    {
        /// class specific print operation
    }
}

edited Nov 13, 2013 at 14:26

answered Nov 13, 2013 at 14:19

Rik

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4 Comments

gunr2171 Over a year ago

I personally would make SpecificPrint virtual, so that it does not need to be used.

Yair Nevet Over a year ago

override what? the classes are not inheriting anything.

Tolga Evcimen Over a year ago

Yep, that's what I was looking for :) I knew it was an easy thing but just couldn't think through it. Thanks. After 4 minute block I will accept this ;)

Param Over a year ago

You will want to make the Print method "final" so that this behaviour isn't overridden in subclasses.

O.O · Accepted Answer · 2013-11-13 14:37:22Z

3

The pattern you are looking for in this case is the Template Pattern by Gang of Four (GoF).

Class A
{
    public virtual void Print()
    { 
    }
    protected void StartingProcedure()
    {
        /// something 
        Print();
    }
    protected void EndingProcedure()
    {
        /// something 
    }
}

usually you'll add another method called the Template Method:

/// Template Method
protected void Run()
{
    StartingProcedure();
    Print();
    EndingProcedure();
}

class A_1 : A
{
    public override void Print()
    {
        /// class specific print operation
    }
    public A_1()
    {
       base.Run();
    }
}

edited Nov 13, 2013 at 14:37

answered Nov 13, 2013 at 14:18

O.O

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2 Comments

Tolga Evcimen Over a year ago

yes I am looking for something like that but, don't know how to call the virtual method. could you explain?

Tolga Evcimen Over a year ago

I knew it :) I knew the problem had something to do with Template Pattern but since I wasn't sure I didn't ment that. Thanks though.

mao47 · Accepted Answer · 2013-11-13 14:39:49Z

1

As an alternative to Rik's answer, if you can change the class hierarchy, and you really don't want to call the base methods in the derived methods, you can use composition instead of inheritance to accomplish this. By using the decorator pattern, you can specify a print method that wraps the derived functions.

class PrintDecorator : A
{
    private A wrappedA;
    public PrintDecorator(A a)
    {
        wrappedA = a;
    }
    public virtual void Print()
    {
        //wrap the derived logic with the pre- and post- processing
        StartingProcedure();
        wrappedA.Print();
        EndingProcedure();
    }
}
class A
{
    public virtual void Print()
    { 
    }
    protected void StartingProcedure()
    {
        /// something 
    }
    protected void EndingProcedure()
    {
        /// something 
    }
}

class A_1 : A
{
    public override void Print()
    {
        /// class specific print operation
    }
}

class A_2 : A
{
    public override void Print()
    {
        /// class specific print operation
    }
}

Example usage:

A dec = new PrintDecorator(new A_1());
dec.Print();

edited Nov 13, 2013 at 14:39

answered Nov 13, 2013 at 14:25

mao47

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4 Comments

Tolga Evcimen Over a year ago

Liked this approach too, but could not figure out where will I use the PrintDecorator class.

mao47 Over a year ago

@TolgaEvcimen: You'd construct this in place of your A_1, A_2, etc classes. When you construct it, you inject the A_1/A_2 instance into the constructor. If you are using a dependency injection framework, you can configure this to automagically happen when you use these objects.

Tolga Evcimen Over a year ago

I get it now ;) thanks a lot. I will keep this in mind. Actually I liked this solution better then the other one since it is more generic.But for now I used the other solution, since it is easier to implement ;) Thanks for the usage example too.

mao47 Over a year ago

@TolgaEvcimen: I've edited to include a simple example, but based on how it fits in your situation you may need to modify it slightly or go with one of the other solutions.

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