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Why do derived classes need to use a base class constructor

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

Roslyn - Find implementation of abstract method

I have several classes that inherit from the same base class. In the base class there is an abstract method that is implemented in the derived classes.
How can I find all implementations of the abstract method in the derived classes? I cannot use SymbolFinder.FindImplementationsAsync, because I don't have a Solution object (only models in a compilation). I prefer to not create a Solution object if not strictly needed.

Identical constructor in child and parent c# [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

Tell if a child class ctor called the base class one

How do you programmatically find out if the ctor of a class invoked its base class ctor and with what arguments?

A derived class constructor always calls a base class constructor. This is required by the C# language specification.
To find out what arguments are passed, you might be able to ask the base class, if it saved them, but there is no universal way to do it.
Finding out which base class constructor got called is impossible for all code I've ever seen. The base class would have to save that information explicitly, but typically different constructors just use default values for some internal data.

Abstract vs Normal class inheritance performance

When you derive from a class and instance the subclass, the runtime also instances the super class, right?
Since abstract classes can't be instanced, are they not created by the runtime when a subclass is instanced?
If so, then abstract class inheritance would be faster than normal class instance?

The runtime never creates separate instances of the base class and the derived class - it's just that the derived class instance also has all the variables etc of the base class, and runs the base class constructor as part of initialization. There's no difference here between "normal" base classes and abstract base classes.

I think you have some details confused.
When you construct an object, where the class of that object inherits from another class, it's not like you get two objects in memory.
You only get one, but it has space set aside for fields that comes from both.
Put another way, if the original class needs 10 bytes to hold its fields, and your inherited class needs 5 bytes to hold its specific fields, when you construct an object from your inherited class, it would occupy 15 bytes, where the first 10 corresponds to the fields from the base class. (note, this is a very simplified explanation, there's a lot more going on that dictates the actual size of objects).