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c# Is it mandatory to write child class constructor? [duplicate]

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

Force Attribute Declaration in Derived Classes

I recently read about attributes and reflection and I thought it would be a good method to include metadata in my program. I have this abstract class and I wanted all classes inheriting from it to declare with the class some attribute, since I wanted custom components(those derived classes) to be created for my program and wanted to read the metadata of these classes on runtime. However, the derived classes all have to explicitly declare the attribute in which I store metadata. So how to I force an attribute declaration in the derived classes? Thanks.

Define your attribute class to itself have an AttributeUsageAttribute attribute where the Inherited property is true.
Or don't, since that's the default...
Derived targets (that is, classes if the attribute is on a class, methods if it is on a method, etc.) will then inherit the attribute without explicit declaration.

If by "force", you mean "compile time enforcement": You can't.

As Daniel said, you cannot enforce attributes at compile time.
But if you want to read the data at runtime, why bother with attributes and reflection at all? You can create an abstract method in your abstract class:
abstract class Base
{
public abstract string Metadata();
}
class Derived1 : Base
{
public override string Metadata()
{
return "Metadata for Derived1";
}
}
class Derived2 : Base // won't compile, since Metadata has not been provided
{
}
The behaviour is slightly different, of course. With this option, you need a reference to an instance of the derived class instead of just the type information itself. On the other hand, it avoids reflection.

As Daniel says you can't force at compile time.
You could add the attribute(s) to the abstract parent and pick them up.
Another option is to add a method to check for the existence of the attrribute in the parent class and throw an exception if not present. Call that from suitable methods.

Came across this old question due to a similar use case. One way to enforce Attribute usage at compile time is by writing an analyzer, similar to what the xunit framework does. here's an example:
https://github.com/xunit/xunit.analyzers/blob/main/src/xunit.analyzers/TheoryMethodMustHaveTestData.cs
Involves a little more effort but does the work.

Can a class be aliased in C#?

In a namespace, is it possible to provide an alias for a class? And if not, why not?
By example, if I had several libraries of things that were derived from a contained, but named base class, but wanted to alias that as "BaseClass", while retaining its actual class name (i.e. "HtmlControl").
Then consumers could always come along and extend from HtmlControls.BaseClass, without having to figure out which class it really comes from.

using SomeClass = Large.Namespace.Other.FunkyClass;
class Foo : SomeClass
{
}

There really isn't an ideal way to do this in C#/.NET. What you can do is have a public BaseClass that inherits from an internal class. You can change this inheritance internally without breaking your consumers as long as the interface to the class remains intact.
public class PublicBaseClass : SomeInternalClass {
}
Consumers inherit from PublicBaseClass, and as long as you are careful, you can change what SomeInternalClass is as you wish.

You could create a dummy class that just inherits HtmlControl without adding any other functionality:
public class BaseClass : HtmlControl {}

The closest I know of is to customize your using statement:
using BaseClass = HtmlControls.BaseClass;
This is normally used to avoid ambiguity between classes with the same name in different used namespaces, without having to fully qualify one or the other. Your devs would have to include it in every code file, so probably not a good solution for what you're doing.
As far as deriving from BaseClass without knowing what you are actually deriving from, not possible. The compiler must, at some level, know what and where the parent class is, meaning it must be statically defined somewhere in code.

Interface and base class mix, the right way to implement this

I have some user controls which I want to specify properties and methods for.
They inherit from a base class, because they all have properties such as "Foo" and "Bar", and the reason I used a base class is so that I dont have to manually implement all of these properties in each derived class.
However, I want to have a method that is only in the derived classes, not in the base class, as the base class doesn't know how to "do" the method, so I am thinking of using an interface for this. If i put it in the base class, I have to define some body to return a value (which would be invalid), and always make sure that the overriding method is not calling the base. method
Is the right way to go about this to use both the base class and an interface to expose the method? It seems very round-about, but every way i think about doing it seems wrong...
Let me know if the question is not clear, it's probably a dumb question but I want to do this right.
EDIT : Thanks to all the people with your excellent abstract suggestions, but this breaks the designer. If abstract was not a selectable option, what would you do?

Alternatively you could define the method as 'abstract' in the base class, which will not require the class to implement it. For example:
abstract class A
{
public abstract void B();
}
Of course this will force your base class to be abstract as well, but it sounds like this would work just fine for you.
See Abstract methods on MSDN.
Update
Since abstract is not an option for you due to designer issues, you could just define the method as part of your base class, and have it throw a NotImplementedException if it is called directly from the base class:
void DerivMethod()
{
// Must be implemented by derived class
throw new NotImplementedException();
}
Otherwise, using an interface would be fine, especially if the above leaves a bad taste in your mouth...

You should make your base class an Abstract class. Then the base class can implement the Interface by marking the method abstract.
http://msdn.microsoft.com/en-us/library/aa664435(VS.71).aspx

Mark the method as abstract in your base class. You'll be forced to implement it in the derived classes, but the base class will not need to have a method definition.

I agree with with others, but making your user control abstract has some issues for the designer. The designer will often not display the abstract user control.
I would implement the interface methods in the base class. You can throw a NotImplemented exception or Assert.Fail in the methods if you want to make sure the inheritors are overriding these methods properly.

Declare the function signature in the base class and use the "abstract" modifier.

is it good design practice to only have a parameterless base class constructor?

In base class constructors I always see a parameterless constructor, like so:
public abstract BaseClass {...
protected BaseClass() { }
...}
but is it acceptable design to include a parameter in the base class constructor?
public abstract BaseClass {...
protected BaseClass(string initObj) { }
...}

Yes, it is acceptable for a base class to require a parameterized constructor. This simply imposes a requirement that any classes which inherit must provide a value to the base constructor.

In most cases the derived classes have some form of parameterized constructors.
So when those constructors are called they can still call the parameterless base constructor:
public employee(int age) : base(this)
The answer is if you need one just add one, there is nothing wrong with that. Think of a business object base class that requires some validations to say a phone number or email address. You want to ensure the derived classes get these business rules loaded into them. If you did not have the base class constructor you could not add these rules to your derived class objects.

It is a good practice if the object cannot be used or has dependencies on every method on this class. For instance, if you have a class that have the same parameters in all the functions, it would be better to set that in the constructor, so the function signature is smaller.

What the writer of the base class is doing in your first example is just making sure that no public constructors are exposed. Probably the base class needs nothing special in the constructor but, if you don't write any, the compiler will add the default (parameter-less) constructor for you.
Not that I think this is specially useful. You cannot instantiate an abstract class anyway.

It is certaibly acceptable. Whether is is needed or useful depends entirely on the (design of) the classes.

Yes, its a perfectly acceptable design decision, only it must make sense for the base class - presumably to initialise its own members from the parameter. Also, it imposes a restriction on derived classes: either they must pass in a literal, or impose a similar restriction on their clients or further derived classes.