Is there any possibility in inheritance in c# so that we can have subclass constructor call only without calling base class constructor as a default base class constructor calls firstly
A subclass constructor will always call a superclass constructor: either the default superclass constructor or a (maybe parametrized) superclass constructor, which can be chosen by you, when you declare your subclass constructor.
Related
The title explains it all I believe.
In C#, I am now aware that regardless, constructors in derived classes will call a base class constructor whether it is an explicit call or an implicit default constructor. My question is why? I think it's because the derived class needs to create an object of the base class but why?
I think it's because the derived class needs to create an object of the base class but why?
An instance of a derived class is an instance of the base class. If you have a rule for what must happen when you construct an Animal, and you're constructing a Giraffe, then somehow you have to execute the rule for constructing an Animal. In C# that mechanism is "call a base class constructor".
A derived class is the base class plus extra things the derived class adds.
You still need some code that initializes the base class portion so you can add your extra parts on top. The call to the base constructor is where that initialization happens.
let's say the car is your abstract class and bmw,mazda,jeep are your driven classes.
you have fields: Name , Model,..
you have constroctor: car(),car(string model)
in car class, then when compiler doing memory allocation for object need to read abstract constructor to be aware of Name, Model.
this will call the default constructor. but you can use base("z4") to force calling overridden constructor in the abstract class.
As Scott Chamberlain said the derived class is something (plus some more features).
Of course an object can have behaviors in the constructor.
Anyway if you don't need to inherit them, you should structure your code in the right way,
you should leave base constructor empty, depends on your specific needs
I don't have any idea about this error please how I make that work?
The error appears because the base class does not have a default constructor. You need to define a constructor in the derived class, which gets the same parameters as the constructor in the base class, and invokes that base class constructor using the base keyword as explained here
I could not open the screenshot of CenterExamen class. The error happens becauase your base class CenterExamen does not have a default constructor. It does not mean like you need to have a default constructor in the base class. In such cases, when inheriting a class, the inherited class should instantiate the base class by passing the required parameters.
I have a base class Character which has several classes deriving from it. The base class has various fields and methods.
All of my derived classes use the same base class constructor, but if I don't redefine the constructor in my derived classes I get the error:
Error: Class "child class" doesn't contain a constructor which takes this number of arguments
I don't want to redefine the constructor in every derived class because if the constructor changes, I have to change it in every single class which, forgive any misunderstanding, goes against the idea of only writing code once?
You can use the following syntax to call the base class constructor from the classes that derive from it:
public DerivedClass() : base() {
// Do additional work here otherwise you can leave it empty
}
This will call the base constructor first, then it will perform any additional statements, if any, in this derived constructor.
Note that if the base constructor takes arguments you can do this:
public DerivedClass(int parameter1, string parameter2)
: base(parameter1, parameter2) {
// DerivedClass parameter types have to match base class types
// Do additional work here otherwise you can leave it empty
}
You can find more information about constructors in the following page:
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/using-constructors
In a derived class, if a base-class constructor is not called explicitly by using the base keyword, the default constructor, if there is one, is called implicitly.
You do have to redeclare constructors, because they're effectively not inherited. It makes sense if you think of constructors as being a bit like static methods in some respects.
In particular, you wouldn't want all constructors to be automatically inherited - after all, that would mean that every class would have a parameterless constructor, as object itself does.
If you just want to call the base class constructor though, you don't need to write any code in the body of the constructor - just pass the arguments up to the base class as per Waleed's post.
If your base class starts requiring more information, it's natural that you should have to change all derived classes - and indeed anything calling the constructors of those classes - because they have to provide the information. I know it can seem like a pain, but it's just a natural consequence of what constructors do.
I had the same problem, and I solved it by replacing my constructor with a factory method like this:
A is the parent class.
public static T getChild<T>(int number) where T:A, new()
{
T child = new T();
T._number = number;
return child;
}
You can create a Child class with
Child b = A.getChild<Child>(2);
A kind of alternative could be to rely on a Dependency Injection container to initialize your objects, that way the that reference to the base class (could be the call to the base constructor or another initializer method) would "externalized" to the DI container.
I don't know if it makes sense to your case or not
A default constructor has no parameters. And nor does a constructor that you write with no parameters. So what is the ultimate difference in c#?
Added to this when you inherit a default constructor and a parameterless constructor are they exposed on the inheritting type exactly the same? Because my IOC container doesn't seem to think so. (Unity).
Cheers,
Pete
The "default" constructor is added by the C# compiler if your class does not contain an explicit instance constructor. It is a public, parameterless constructor. If you have created an explicit non-public, parameterless constructor, Unity will not be able to use it in the same way it would have used a public constructor (regardless of whether this was generated by the compiler, and regardless of whether it had parameters).
You can override the behaviour of the default constructor by creating a parameterless constructor. A common use of this is when you have a custom object as a member of your class and you need to initialize it with a default value when an instance of your class created.
A default constructor is a synonym for a parameterless constructor.
Adding to #Sachin Kainth and #Fredrik Leijon answers, in .NET (also in Java) constructors are not inherited.
You either explicitly call a base constructor or, by default, the base parameterless constructor is invoked.
If you don't add a constructor then a default constructor with no parameters is inserted for you. If you add a constructor (with or without parameters) no default constructor is generated
I'll let the code speak for itself:
public class ThisHasADefaultConstructor
{
}
public class ThisHasAParameterlessConstructor
{
public ThisHasAParameterlessConstructor()
{
}
}
Note that a default constructor is effectively a parameterless constructor that is automatically generated when you don't specify a constructor (with or without parameters).
Maybe you need the the parameterless constructor if you want to have more constructors. Let's say one without parameters and one with parameters.
if your class has only a constructor with parameters, you're are not allowed to use the parameterless constructor, unless you define one, in other words, no default parameterless constructor is provided if you define a custom constructor in your class.
MSDN states:
A constructor that takes no parameters is called a default constructor. Default constructors are invoked whenever an object is instantiated using the new operator and no arguments are provided to new.
So the default constructor does not have to be automatically generated. A hand-written, parameterless constructor is also the default constructor.
This actually makes sense when you look at derived classes:
class A
{
public A() { ... } // do something
public A(object o) { ... } // do something else
}
class B
{
public B() { ... }
}
Which A constructor is called while creating B? The default one.
Default constructor refers to a constructor without parameters, wheter created automatically or created by you.
A defualt constructor is created automatically if you don't declare any other instance constructor, with or without paramters. (declaring a static constructor won't avoid the defautl constructor to be automatically created).
The constructors of a hierarchy of classes are not inherited, but all the constructor on the inheriting chain are invoked when using the constructor of a derived class, starting from the root of the hierarchy. Unless you specify which constructor to invoke, the default constructor of each class is invoked.
public class Base
{
public Base()
{
}
}
public class Derived : Base
{
public Derived()
{
}
}
// invokes, the Base() construtor, and then the Derived() constructor
var d = new Derived();
You can also explicitly invoke a base class constructor, with or without parameters like so:
// Derived class explicitly invoking base class constructor.
public Derived() :base()
{
}
If you want, you can invoke a base class constructor with parameters like so:
public Derived(int j) :base(j)
or so
public Derived() :base(23)
In this last code sample, if you create a Derived object with the default constructor, the constructor invoked in the base class isn't the default one, but a constructor with an integer parameter, invoked with the parameter 23.
(of course, this constructor with parameter must exist in the Base class).
You can only invoke explicitly the constructor of the inmediate base class.
A default contructor is called implicitly by the comppiler, except if you have explicitly created a new one (with no parameters).
I have just seen following code but I do not understand the derivation of base class right in the constructor declaration. What is this and is this possible with ordinal methods?
public SplashAppContext(Form mainForm, Form splashForm) : base(splashForm)
{
this.mainForm = mainForm;
splashTimer.Tick += new EventHandler(SplashTimeUp);
splashTimer.Interval = 2000;
splashTimer.Enabled = true;
}
It's calling a base class constructor, passing the argument splashForm of the type Form to it.
You can call base class methods as well. If you for example have overridden a method which behaviour you want to modify slightly, you do your modifications and call the base class method with base.TheMethod(). This would look like this:
public override void FireMissiles()
{
PrimeMissiles();
base.FireMissiles();
}
The syntax for calling a base class constructor and a base class method differs as you can see.
The syntax you've listed means that this constructor calls the superclass's constructor with the parameter splashForm before continuing with the rest of the construction of this object. When no superclass constructor is listed, the no-arg constructor is assumed, i.e:
public SplashAppContext(Form mainForm, Form splashForm)
{ ... }
is equivalent to
public SplashAppContext(Form mainForm, Form splashForm) : base()
{ ... }
What is this...
This line is called a constructor initializer. It means "call the base class's constructor with the parameter splashForm, then start this constructor's body".
When no constructor initializer is explicitly specified for a constructor, it's assumed that you want base(), the parameterless constructor.
...is this possible with ordinary methods?
For regular methods, you can of course call base class methods (with base.SomeMethod(...)), but there's no "initializer" style available.
: base(splashForm)
Is calling the parent constructor and passing splashForm to it.
This is actually not really a case of inheritance, what this syntax specifies is that before SplashAppContext's constructor is called, its base constructor should be called with splashForm passed as a parameter.
That is the syntax for calling the constructor of the immediate parent class. This is the C# syntax, whereas VB.NET requires that you call MyBase.New(splashForm) as the first line of code in your constructor.
This is expressed this way because a parent object is guaranteed to be fully constructed and initialized by the time your child constructor code begins to execute. Because of this, both compilers (VB.NET and C#) specify that you may not execute any of your own code before the parent constructor is called (if you don't specify a parent constructor and a parameterless constructor exists, it will be called automatically).
This syntax is not possible (nor necessary) for other method calls, as it's possible to invoke the base implementation of any other function by calling base.FunctionName() in your code. Constructors are just special cases because they have to execute before any other code.