Abstract Method in Non Abstract Class

Question

I want to know the reason behind the design of restricting Abstract Methods in Non Abstract Class (in C#).

I understand that the class instance won't have the definition and thus they wont be callable, but when static methods are defined,they are excluded from the instance too. Why abstract methods are not handled that way, any specific reason for the same?

They could be allowed in concrete class and the deriving class can be forced to implement methods, basically that is what, is done in case of abstract methods in an abstract class.

Solution 1

First, I think that what you're asking doesn't logically make sense. If you have an abstract method, it basically means that the method is unfinished (as @ChrisSinclair pointed out). But that also means the whole class is unfinished, so it also has to be abstract.

Or another way to put it: if you had an abstract method on a class that wasn't abstract, that would mean you had a method that cannot be called. But that means the method is not useful, you could remove it and it would all work the same.

Now, I'll try to be more concrete by using an example: imagine the following code:

Animal[] zoo = new Animal[] { new Monkey(), new Fish(), new Animal() };

foreach (Animal animal in zoo)
    animal.MakeSound();

Here, Animal is the non-abstract base class (which is why I can put it directly into the array), Monkey and Fish are derived from Animal and MakeSound() is the abstract method. What should this code do? You didn't state that clearly, but I can imagine few options:

1. You can't call MakeSound() on a variable typed as Animal, you can call it only using a variable typed as one of the derived classes, so this is a compile error.

   This is not a good solution, because the whole point of abstract is to be able to treat instances of derived classes as the base class, and still get behaviour that's specific to the derived class. If you want this, just put a normal (no abstract, virtual or override) method into each derived class and don't do anything with the base class.

2. You can't call MakeSound() on an object whose runtime type is actually Animal, so this is a runtime error (an exception).

   This is also not a good solution. C# is a statically typed language and so it tries to catch errors like “you can't call this method” at compile time (with obvious exceptions like reflection and dynamic), so making this into a runtime error wouldn't fit with the rest of the language. Besides, you can do this easily by creating a virtual method in the base class that throws an exception.

To sum up, you want something that doesn't make much sense, and smells of bad design (a base class that behaves differently than its derived classes) and can be worked around quite easily. These are all signs of a feature that should not be implemented.

Solution 2

So, you want to allow

class C { abstract void M(); }

to compile. Suppose it did. What do you then want to happen when someone does

new C().M();

? You want an execution-time error? Well, in general C# prefers compile-time errors to execution-time errors. If you don't like that philosophy, there are other languages available...

Solution 3

I think you've answered your own question, an abstract method isn't defined initially. Therefore the class cannot be instanciated. You're saying it should ignore it, but by definition when adding an abstract method you're saying "every class created from this must implement this {abstract method}" hence the class where you define the abstract class must also be abstract because the abstract method is still undefined at that point.

Solution 4

You can achieve what you want using "virtual" methods but using virtual methods can lead to more runtime business logic errors as a developer is not "forced" to implement the logic in the child class.

I think there's a valid point here. An abstract method is the perfect solution as it would "enforce" the requirement of defining the method body in children.

I have come across many many situations where the parent class had to (or it would be more efficient to) implement some logic but "Only" children could implement rest of the logic"

So if the opportunity was there I would happily mix abstract methods with complete methods.

@AakashM, I appreciate C# prefers compile time errors. So do I. And so does anybody. This is about thinking out-of-the-box.

And supporting this will not affect that.

Let's think out of the box here, rather than saying "hurrah" to big boy decisions.

C# compiler can detect and deny someone of using an abstract class directly because it uses the "abstract" keyword.

C# also knows to force any child class to implement any abstract methods. How? because of the use of the "abstract" keyword.

This is pretty simple to understand to anyone who has studied the internals of a programming language.

So, why can't C# detect an "abstract" keyword next to a method in a normal class and handle it at the COMPILE TIME.

The reason is it takes "reworking" and the effort is not worth supporting the small demand.

Specially in an industry that lacks people who think out of the boxes that big boys have given them.

Solution 5

The abstract class may contain abstract member. There is the only method declaration if any method has an abstract keyword we can't implement in the same class. So the abstract class is incompleted. That is why the object is not created for an abstract class.

Non-abstract class can't contain abstract member.

Example:

namespace InterviewPreparation
{
   public abstract class baseclass
    {
        public abstract void method1(); //abstract method
        public abstract void method2(); //abstract method
        public void method3() { }  //Non- abstract method----->It is necessary to implement here.
    }
    class childclass : baseclass
    {
        public override void method1() { }
        public override void method2() { }
    }
    public class Program    //Non Abstract Class
    {
        public static void Main()
        {
            baseclass b = new childclass(); //create instance
            b.method1();
            b.method2();
            b.method3();
        }
    }

}

View more solutions

Comments

[Ashish Jain]

I stated that issue and given example of static methods being handled by compiler. My question was on compiler design for abstract methods, basically why they chose abstract class specifically for these methods?

[Chris Sinclair]

@AshishJain Because an abstract class cannot be directly instantiated. It is "unfinished", just like an interface cannot be directly instantiated because what the empty methods/properties do is undefined. It forces programmers who wish to use them to inherit and implement all abstract methods before being able to instantiate. EDIT: Static methods are a different concept. They belong to the Type and not the object instance. Abstract methods are on the instance level and require to be implemented (also note that static methods require to be implemented -- no "abstract static" exists)

[svick]

If you want execution-time error, you can do that already: create a virtual method that throws an exception.

[AakashM]

Static methods are a different case, since they aren't on an instance in any case. I don't know what you mean by "why they chose abstract class specifically for these methods?". I've shown you the potential problem if a concrete class were allowed to have abstract methods. What would you rather happened?

[Chris Sinclair]

@AakashM Even more fun if the abstract method had a return type instead of void. :) At least a void method can do nothing, but what does a return type do? default value? Yeah, I don't know what else would happen. Abstract is there for a reason, anything else can be handled by virtual as we suggested.

[Ashish Jain]

@AakashM I state it this way: Problem Statement was to provide abstract method functionality (if there was some other intention too, then its fine to provide a new concept "abstract class"). Now this problem could be solved by compiler designers in "n" ways (I am terming it as "n" because there could be other ways too), one of them is the current implementation. I am reasoning on alternate way adoption, why JAVA/C# (may be more languages) adopted to provide an additional type "abstract at class level i.e. abstract classes" and not chose to handle it at method level?

[AakashM]

You still haven't suggested what 'handle at method level' might actually mean. What do you want to happen, in the above situation?

[Ashish Jain]

Meaning there is no concept of "abstract class", rather there would be abstract keyword applicable to methods/properties/indexers etc only. And for providing a type blueprint there is interface. I am sure there would be a reason "virtual" is not applicable at class level while "abstract" is.

[Ashish Jain]

Its fine to accept that alternatives are not best or even have severe drawbacks. But why the current implementation of abstract clases (why not keep abstract methods/properties/indexers etc like virtual) is best and was the concept of "abstract classes" inevitable (due to its advantages over other ways), keeping in mind interfaces are already there?

[Ashish Jain]

Make sense to me and point 1 gives me the answer why there is abstract at "class level" and why abstract classes should be restricted making instances. Thanks vick.

[Geert Bellekens]

So you replace a compiler error with a runtime exception? If your class is not abstract you risk at one point or another that your not implemented method is called on an instance of your class and throws an exception. If you mean abstract then simply use abstract and don't try to work around it.

[Jim K]

A runtime error is a reasonable tradeoff given that the framework/language provides for no other mechanism to achieve what the OP is asking for. In my case, I am confident that if I hit the runtime error, I will find it very early in the development process. Either the child class is never used or it's used all the time (core utility). I am just offering a alternate solution to the need to define a method in a concrete class that is required to be overridden in any child class.

[Jim K]

Ed it's not clear to me how my post is not directly addressing the question in the OP by offering a workaround to the limitation of the framework/language??? Please contact me directly if you feel my post is so inappropriate.

[Noob]

same situation here, like why did they have to make it so that the class has to be abstract if a methods is abstract. The class is clearly not unfinished, as the unfinished methods are intended, thus the class is finished, having unfinished methods. If that makes any sense??