Is it good to have a constructor in abstract class?
is it a good programming practice to create constructor of abstract class? since abstract classes can not be initialized, their child classes are initialized.
Following is my class structure.
public abstract class Scheduler
{
private Storyboard timer;
protected Scheduler()
{
// initialize the timer here.
timer = new Storyboard();
this.PollInterval = 60;
}
}
public class TaskScheduler : Scheduler
{
public TaskScheduler()
: base()
{
}
}
Yes, it's absolutely fine. Just because the constructor can only be called by derived classes doesn't mean it won't be useful. For example, you might have an abstract class which represents a named entity of some kind - it would make sense to take the name as a constructor parameter.
It would probably be worth making the constructor protected, to make it even more obvious that you can't just call it from elsewhere.
Note that there being a constructor (or multiple constructors) in an abstract class does force derived class constructors to go through it, but it doesn't force the derived classes to have the same constructor signatures. For example:
public abstract class NamedFoo
{
private readonly string name;
public string Name { get { return name; } }
protected NamedFoo(string name)
{
this.name = name;
}
}
public class DerivedFooWithConstantName
{
public DerivedFooWithConstantName() : base("constant name")
{
}
}
In this case the derived class constructor is "removing" a parameter (by providing a constant value as the argument to the abstract class constructor) but in other cases it could "add" parameters that it required, or have a mixture.
There is absolutely no reason not to have a constructor in an abstract base class.
The abstract class is initialized and works just like any other class. The abstract keywords only do the following:
It prevents the class itself to be instantiated directly. It can only be instantiated by instantiating an inherited class. This does not change the behavior of initialization compared to a not abstract base class;
It allows you to have abstract methods, properties and events in the class.
If you e.g. do not have abstract methods, properties or events, exactly the same result can be accomplished by making the constructor of a class protected (like you did). This also prevents the class to be instantiated directly. The behavior does not change however compared to an abstract class.
The primary difference then becomes the ability to declare methods, properties and events as abstract which you can only do when the class is marked abstract.
Having constructor in abstract class can be useful at times. This question is a duplicate, and dealt great deal in an related post. Even though it specifically reference JAVA, conceptually it applies to C# also.
Can an abstract class have a constructor?
Constructors on abstract types can be called only by derived types. Because public constructors create instances of a type, and you cannot create instances of an abstract type, an abstract type that has a public constructor is incorrectly designed. CA1012: Abstract types should not have constructors
Fix the violation by changing the accessibility of the constructor from public to protected.
Example:
namespace Sample
{
public abstract class Book
{
protected Book()
{
}
}
}
Related
This question already has answers here:
If derived class inherits the private members of a base class, then why not constructors?
(4 answers)
Closed 5 years ago.
I can't find the reason that why inheriting from a base class the a private parameterless constructor is not allowed in C#. Here's the simplest example:
public class Base
{
private Base()
{
}
public string Name { get; set; }
}
public class Derived : Base
{
public Derived(string name)
{
base.Name = name;
}
}
Compiler doesn't compile this code. Why is it so? The class is not sealed of course, and while the base class is meant not to be instantiated many times (as in Singleton), I think the derived class has every right to be instantiated with a parametered constructor many times.
Why inheritance is disallowed in this case?
Because you need a constructor to instantiate a class, and the constructor is held private by the class you want to derive. No one but the class itself is allowed access to it. For all it knows it doesn't exist! (You might want to use a private constructor for the Singleton pattern for example)
If you want to allow inheritance, but keep the constructor from called outside, you need the protected keyword.
You can't access something from another class if the private modifier is used; even from a derived class.
Have a look at this: Access Modifiers
private
The type or member can be accessed only by code in the same class or
struct.
And because the Base constructor has to be called when an instance of Derived will be created you can't use private in your case. You should use protected instead.
I'm just after a little advice on how to best organise the inheritance here. I have a class (below) that needs to use most of, and overide some of, the classes in the base ExportData class:
public class ExtendedExportData : ExportData
{
private tData _Data;
private ResStrings _diaStrings;
public ExtendedExportData(tData Data, ResStrings diaStrings)
: base(tData, diaStrings)
{
_tData = Data;
_diaStrings = diaStrings;
}
}
What isn't neccesary in this case is having to call the base constructor as none of the base class methods used in the class above require that initialisation, the constructor is only neccesary when creating an instance of the base class directly for other (verbose) purposes.
If i remove the base constructor i get an error on the sub-class constructor saying the base constructor doesn't take 0 arguments. How can i avoid this?
Here's the base class:
public class ExportData
{
private tData _Data;
private ResStrings _diaStrings;
public ExportLines(tData Data, ResStrings diaStrings)
{
_Data = Data;
_diaStrings = diaStrings;
}
}
Thanks in advance.
A constructor of the base class is always used - but when the base class has no explicitly defined constructor, C# will generate a public parameterless constructor automatically.
Similarly, if you don't explicitly mention : base(...) in your derived class, C# assumes you want to use the parameterless constructor of the base class.
As soon as you define a constructor, the parameterless constructor is not automatically generated anymore, and so you have to call : base(...).
You can easily change this by adding such a constructor to the base class:
public class ExportData
{
protected ExportData()
{
}
}
By making this constructor protected, only classes that inherit from ExportData can use this constructor, and they no longer have to include : base(...).
You can always have a parameterless constructor in the base class that does all the initialising it needs to i.e.:
public ExportData()
{
//Initialising here
}
And then in your example just call base() i.e.
public ExtendedExportData(tData Data, ResStrings diaStrings)
: base()
Couldn't you just create an empty constructor (public ExportData(){}) in ExportData class ?
Else you could do something like here.
I know that this is old question but for future similar problem solution finders I would write something else.
If I undersand this question properly there is more elegant and common solution. Just make your base class abstract. You can still call base constructor during inheritance.
More information can be found here:
Constructor of an abstract class in C#
The error is because the base class always has to be instantiated and your base class doesn't have a default constructor.
If you have functionality in your base class that's not needed in the base class review your inheritance architecture.
In c# we can't create an obeject of a abstact class or interface it means abstract class do not have any constructor, is it true ?
or if it have then what is it's purpose there?
As others have said, abstract classes usually have constructors (either explicitly or the default one created by the compiler) - and any derived class constructor will have to chain through the abstract class's constructor in the normal way. That's the important bit... suppose you have an abstract class which stores the name associated with an instance - because you always want a name, and you don't want to write the Name property in each concrete derived class. You might provide a constructor which takes that name and assigns it to a field... and then every subclass constructor would have to go through that constructor, so that you still knew you'd always have a name. If you want to know more about constructor chaining, read my article on it.
Here's an example of that:
public abstract class DemoBase
{
private readonly string name;
public string Name { get { return name; } }
protected DemoBase(string name)
{
this.name = name;
}
// Abstract members here, probably
}
public class FixedNameDemo : DemoBase
{
public FixedNameDemo()
: base ("Always the same name")
{
}
// Other stuff here
}
public class VariableNameDemo : DemoBase
{
public VariableNameDemo(string name)
: base(name)
{
}
// Other stuff here
}
To further answer your comment on BoltClock's answer, asbtract classes can't have private abstract methods, but they can have private constructors. Indeed, it's sometimes useful to have only private constructors in an abstract class, because it means the class can only be derived from within the program text of the same class. This allows you to create pseudo-enums:
public abstract class ArithmeticOperator
{
public static readonly ArithmeticOperator Plus = new PlusOperator();
public static readonly ArithmeticOperator Minus = new MinusOperator();
public abstract int Apply(int x, int y);
private ArithmeticOperator() {}
private class PlusOperator : ArithmeticOperator
{
public override int Apply(int x, int y)
{
return x + y;
}
}
private class MinusOperator : ArithmeticOperator
{
public override int Apply(int x, int y)
{
return x - y;
}
}
}
In this respect, an abstract private method/property could make sense - it could be accessed by the base class but provided by the derived classes within the same class's program text. However, it's prohibited by the specification. Usually, protected abstract members would solve the same problem - but not quite always.
Good question. Here's why Abstract classes need constructors even though they cannot be instantited.
In any Object oriented language like C#, object construction is an hierarchical process. Look at the code below. When you instantiate any object of type DerivedClass, it must construct the base object first before creating the object of typeof DerivedClass. Here the base class may or may not be an Abstract class. But even when you instantiate an object of a concrete type derived from an abstract class it will still need to call the constructor of the Base class before the object of DerivedClass type is created, hence you always need a constructor for Abstract class. If you have not added any constructor, C# compiler will automatically add a public parameterless constructor to the class in the generated MSIL.
public class BaseClass
{
public BaseClass()
{
Console.WriteLine("BaseClass constructor called..");
}
}
public class DerivedClass : BaseClass
{
public DerivedClass()
{
Console.WriteLine("DerivedClass constructor called..");
}
}
DerivedClass obj = new DerivedClass();
//Output
//BaseClass constructor called..
//DerivedClass constructor called..
PS: Assuming, If Abstract base classes
are not allowed to have constructors
because they need not be instantiated,
the whole fundamentals of the object
oriented programming will go on toss.
The idea behind Abstract types are to
represent objects that have some
features and behaviours but not
complete as whole to allow independant
existence.
No. it means that operator new is not allowed to create object from this type of class.
The purpose might be that are allocated/initialized some properties of class.
abstract usually leave some methods to implement.
Regarding the interface, this structure holds only the signatures of method, delegates or events. That may be implemented in class that use interface. You cant create a object.
Read about new
EDIT:
What is the purpose of constructor in abstract class ?
When one class inherit another class, the parent class of it had to be created first while object is crated. In class do not implement some special constructor always is used default one [className()]. When you override some method then the implementation of functionality is taken form class which override the method. This is why method used in constructor should never be virtual. Same logic for abstract class, such class can have a lot of functionality, and only one method that should be implemented by child class.
Abstract classes have constructors but you can't call them directly as you can't directly instantiate abstract classes.
To answer your comment, the concept of a private abstract method or property makes no sense, because private prevents anybody else from accessing it, and abstract prevents itself from accessing it. So there would essentially be no possible way to call it.
EDIT: see Jon Skeet's answer on private constructors. Private members of other kinds cannot exist in abstract classes, though.
Abstract classes do have constructors. When you create an instance of a derived class, its parent class' constructors are called. This applies to classes derived from abstract classes as well.
As an abstract class cannot be instantiated, why is a constructor still allowed inside the abstract class?
public abstract class SomeClass
{
private string _label;
public SomeClass(string label)
{
_label=label;
}
}
Constructors of any derived class still have to call a constructor in the abstract class. If you don't specify any constructors at all, all derived classes will just have to use the default parameterless one supplied by the compiler.
It absolutely makes sense to have a constructor - but "public" is really equivalent to "protected" in this case.
Because you can still do the following:
public class SomeChildClass : SomeClass
{
public SomeChildClass(string label) : base(label){ }
public string GetLabel() { return _label; }
}
As you can see, the child class can call the base contructor (on the abstract class) to create an instance of itself.
Like Jon said though, public really isn't necessary. It's effectively the same as protected.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Why can’t I create an abstract constructor on an abstract C# class?
Why I can't declare abstract an constructor of my class like this:
public abstract class MyClass {
public abstract MyClass(int param);
}
Constructors are only applicable to the class in which they are defined, that is, they are not inherited. Base class constructors are used (you have to call one of them, even if only calling the default one automatically) but not overridden by deriving classes. You can define a constructor on an abstract base class -- it can't be used directly, but can be invoked by deriving classes. What you can't do is force a derived class to implement a specific constructor signature.
It is perfectly reasonable to have a constructor defined, typically as protected, in order to define some common set up code for all derived classes. This is especially true, perhaps, when the abstract class provides some other default behavior which relies on this set up. For example:
public abstract class Foo
{
public string Name { get; private set; }
protected Foo( string name )
{
this.Name = name;
}
}
public class Bar : Foo
{
public Bar() : base("bar")
{
...
}
}
You can't declare it abstract, but you can have a constructor on your abstract class; just remove the word abstract and provide a body for it.
Constructors are closer to static methods rather than "regular" methods. Like static methods, they can be overloaded, but not overriden. That is, they are not inherited but can be redefined.
public BaseClass
{
public BaseClass( String s ) { ... }
public static void doIt ( String s ) { ... }
}
public SubClass extends BaseClass
{
public SubClass( String s ) { ... }
public static void doIt ( String s ) { ... }
}
public SubClass2 extends BaseClass
{
}
new SubClass( "hello" );
SubClass.doIt( "hello" );
new SubClass2( "hello" ); // NOK
SubClass2.doIt( "hello" ); // NOK
Constructors and static methods are never dispatched dynamically (virtually) -- You always know the concrete type you instantiate or the concrete class of the static method. That's why it makes no sense to have abstract constructor and abstract static method. That's why you can also not specify constructor and static method in interfaces.
You can even think of constructor as static factory method (and see the corresponding pattern):
MyClass obj = new MyClass(); // the way it is
MyClass obj = MyClass.new(); // think of it like this
The only case I see where it would make sense to define abstract constructor or abstract static method would be if reflection is used. In this case, you could ensure that all subclass would redefine the corresponding static method or constructor. But reflection is another topic...
Note: in languages such as Smalltalk where classes are regular objects, you can override static method and have abstract constructor. But it doesn't apply to Java because classes are not "regular" objects even if you can get them with reflection.
Abstract implies virtual. A non-default constructor can never be called polymorphically, so virtual and abstract are not allowed on constructors.
IF in a future version of C#, generics are enhanced to allow calling non-default constructors through a generic type parameter, then polymorphic calls to constructors would be possible and virtual and abstract constructors might be added as well.
What wrong with this:
public abstract class MyClass {
protected MyClass(int param)
{
}
}
In this case you oblige all derived classes to call base class constructor.
Because abstract constructors are not supported.
But a abstract class can have a constructor.
A constructor is not an ordinary method. It has a special purpose, and so is restricted to language features that make sense for that purpose. See also: Why do constructors not return values?
By definition, the class can't be instantiated directly, so in a sense, it already is abstract.