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
Related
I have the simple context class definition:
namespace PowerSupply.Persistance.Facade
{
class PowerSupplyDBContext : DbContext
{
public PowerSupplyDBContext() : base("PowerSupplyDatabase")
{
}
}
}
There is nothing inside the body of
public PowerSupplyDBContext() : base("PowerSupplyDatabase")
{
what is the purpose of this empty constructor. How does this empty constructor works?
It calls the constructor of the base class. So it is not empty. It just doesnt add something new.
There are many reasons you could use that "empty" constructor.
You may not have any new properties, or you instantiate those properties somewhere else (i dont recommend)
You may just want to add functionality (new methods) so you extend the class, and implement new methods using the properties and methods of base class. So you dont need the constructor to do something new, opposed to the base class.
Or it has many constructors, and you use the empty constructor in circumstances you need to use limited functionality of the class, or default values.
It's best practice to include an empty constructor in any class that might be used by a third party / other project / anywhere that the compiler can't catch a change in the class.
If you omit the empty constructor, and that class is used by a third party or in another one of your projects/applications, then you can't later change that class to include a constructor with arguments. Otherwise, all the uses of this class that previously did not rely on passing constructor arguments will break, because they will have no empty-constructor to reference.
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
public class ParentBaseClass
{
public ParentBaseClass()
{
// Parameter less constructor
}
public ParentBaseClass(string type)
{
// single parameter constructor
}
public ParentBaseClass(Entity model)
{
// entity type constructor
}
public ParentBaseClass(string type, bool IsNewEntity)
{
// two parameter constructor
}
public ParentBaseClass(string type, bool IsNewEntity, Entity model)
{
// three parameter constructor
}
}
public class ChildClassFirst : ParentBaseClass
{
public ChildClassFirst() : base("Customer", false)
{
// implementation of 4th constructor having string, bool parameters.
}
}
in child class I implemented only one constructor of parent class having two parameters. why there no need to implement other constructor? can anyone explain me? please consider other constructor may have MVC Model class or other entity class of dbContext.
Every child class calls it's base class constructor, if there is a parameter-less constructor and we haven't implemented any parameter based constructor in it, the parameter-less is automatically called, but like in your case you are specifying which constructor to be used when ChildClassFirst is instantiated, so now the calling code has only access to this parameter-less constructor to instantiate an object of ChildClassFirst which in turn will instantiate the parent class object using the parameter based constructor which is called using base().
In short, Child class only needs to specify one base class construction which would be called during instantiation of child class, and when there is none specified the default would be parameter-less constructor which would get called, but remember the parameter-less only exists if we have no parameterized constructor specified in the class.
Abstract classes are designed for inheritance by other classes, usually more than one. Different derived classes may need to construct their abstract base class in slightly different ways. Hence, an abstract class can supply different constructors for different derived classes.
Non-abstract derived classes, on the other hand, are designed to be instantiated by the class users. User construction needs may be completely different from the needs of the class itself, so derived classes provide their own constructors.
Therefore, the two sets of constructors are completely independent of each other: they serve different needs. Derived classes can provide more or fewer constructors, and the signatures of their constructors may be similar or completely different. This is exactly the case in your code: ChildClassFirst provides a parameterless constructor, while its abstract base class provides two constructors with different sets of parameters. As long as you provide any constructor in the derived class, C# compiler is fine with your code.
In C#...
I'm designing a base class (can NOT use constructors for this), and I want to force any inheriting classes to implement a method to initialize their respective objects. I can expect that their initialization method may take an arbitrary number and type of parameters.
how would I declare this abstract method in the base, so that the children can implement any parametered variation of it? If it is not possible, could you suggest an alternative? Thank you.
updated details : the base class should be abstract, i need a method to act as a constructor (but I can't use a constructor to do it). The child class needs a way to initialize itself, and I was hoping to force an implementation with an (abstract?) method of my own
I think it is possible try this code in your base class
public abstract void Test(params object[] list);
now it will force to declare method in child class.
Hope this help.
In addition, you can use new Modifier to hide explicitly the same name methods, fields, ... of derived class
.NET framework 4.0 provide optional parameters and name arguments to deal with a various type of parameters Named and Optional Arguements
I want to do something like this
public class Class1
{
public Class1()
{
}
public Class1(int a)
{
}
}
public class Class2 :Class1
{
public Class2(int a)
{
}
public Class2(): base(2)
{
this(2); // new Class2(2);
}
}
I know this can't be achieved in Java (can use one between (super or this) in the first line)
But somehow I am in need of this kind of work how to achieve that? Means calling the base class's parameterised and derived class's parameterised constructor in default constructor of derived class.
MSDN article on constructors is pretty good. Here are some relevant bits:
A constructor can use the base keyword
to call the constructor of a base
class.
....
A constructor can invoke another
constructor in the same object using
the this keyword. Like base, this can
be used with or without parameters,
and any parameters in the constructor
are available as parameters to this,
or as part of an expression.
This should work:
public class Class1
{
public Class1()
{
}
public Class1(int a)
{
}
}
public class Class2 :Class1
{
public Class2(int a) : base(a)
{
}
public Class2(): this(2)
{
}
}
Igor's answer is a fine example of how you should write the constructors in this situation. To address the more general case of your final sentence: you can't chain to more than one constructor. You can't call a base constructor and another constructor in the current class.
There are two typical patterns for overloaded constructor. In the first pattern, the set of overloads of the derived class roughly matches the set of overloads for the base class - you try to make the derived class feel like it's inherited the constructors, effectively. (Constructors themselves aren't inherited, but if you provide the same signatures then it feels like it to the caller.) This is typically the case when your derived class doesn't need additional information. Of course each constructor can have extra parameters, and only pass a subset up to the base constructor, but that can start to get complicated.
In the second pattern, you have several constructors in the derived class each of which calls a "master" constructor in the same (derived) class. This master constructor has the most parameters, as it needs to be able to handle everything specified by any of the other constructors. Sometimes the master constructor should be private, if some combinations wouldn't make sense, but are convenient to specify in one place when you know you can only reach the code via a sensible public constructor. In this case, only that "master" constructor chains directly to the base class constructor. This is typically used when the derived class has several additional pieces of information beyond what the base class needs.
There are hybrids of this pattern where you have multiple masters with "groups" of overloads calling the masters... but I'd advise you to try to keep it simple where possible. Also consider the possibility of providing static factory methods instead of constructors - that can end up making for more readable code as you can name the methods by their purpose/parameters - see TimeSpan.FromMinutes for example.
Thats impossible in both languages (for a good reason). Otherwise you would call the base-constructors multiple times due to the implicit base call if theres no explicit base or this. This could lead to unwanted behaviour.
Write multiple constructors, and one Init() method
Within each constructor, you can write whatever code you need to execute before passing it to the Init method.