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Executing constructor from Parent class to Child class

In what situations should I execute a constructor of the parent class for a child class? And why use the "base" reserved word?
Exemple:

The constructor of the base class is always called. Or more exact, one constructor of the base class. If you don't specify which one (by using the base reserved word), the default constructor of the base class is called (that means the one with no arguments). If the base class has no constructor without arguments, a compiler error occurs. With the base keyword, you specify that you want to call a certain constructor. Here are some simple examples:
public class Base
{
private int _a;
public Base() // default ctor
{_a = 0;}
public Base(int a) // a ctor with an argument
{_a = a;}
}
public class Derived : Base
{
public Derived() : base(2) // call the ctor with argument "2"
{}
public Derived(bool b) // uses the ctor without argument of the base class (_a will stay 0)
{
}
}
The keyword base here is used as a generic placeholder for the name of the base class. The only other word syntactically allowed there would be this (if you want to call a constructor of the same class)

If you not use :base word for child constructor.
Default constructor of parent will be executed.
If you have some basic construnctor in base class and you have some fields in it and want to fill it using basic constructor you should use base keyword.

Forcing partial base Initialization (and methods) in Inherited Class Constructor C# (like abstract does to methods) - work around to do it

I am having a C# abstract class which have some methods to be implemented by its children.
Though it is so, the initialization values for those children consist of two parts: one which is the same as the parent, and another one which is unique to the children.
public abstract class parentClass {
public abstract bool IsInputValid(string input); //children must implement this
public parentClass () {
//Some shared initialization
}
}
If the class is not abstract we could do something like this to implement that
public class parentClass {
public parentClass (string input) {
//Some shared initialization
}
}
public class childClass : parentClass {
public childClass (string input) : base (input) {
//Some unique initialization
}
}
But that cannot be done using abstract class and some more, the method not need not to be implemented (since it is not abstract).
So I am in a dilemma here. On one hand, I want to have some base initialization called and on the other, I also want to have some methods enforced.
So my question is, how do we normally implement such case? On one hand it is enforcing some base initialization, and on another some methods.
Note: I am new to abstract class so I would be glad to receive any inputs regarding it.
Where do I declare wrongly (if any)? If we cannot do so, is there a way to get around to produce the same result (that is, to enforce the child class to use certain signature for constructor)?

There should be no need to enforce this. You say that the base class has some common initialization and the child classes have their own specialized initialization as well.
This is enforced already, if you have this:
public abstract class Base
{
protected Base(int value) { ... }
}
Then you have a couple of guarantees:
Nobody can construct an object of the type Base since it is abstract
Nobody can construct an object that inherits from Base without indirectly calling the only existing constructor of Base, that takes an int value parameter.
The last part there is important.
A child class can deal with this type of base constructor in at least three ways:
It can provide a constructor that looks identical save the name of it, just passing the value down to the base constructor:
public class Child : Base
{
public Child(int value) : base(value) { ... }
}
It can provide a constructor that has this parameter but has additional parameters to the child class constructor as well:
public class Child : Base
{
public Child(int value, string other) : base(value) { ... }
}
It can provide a constructor that doesn't have the parameter to the base class, but manages to compute this parameter:
public class Child : Base
{
public Child(string other) : base(other.Length) { ... }
}
The last part also handles the case where the child constructor has no parameters at all:
public class Child : Base
{
public Child() : base(new Random().Next(100)) { ... }
}
Regardless of which approach you use, it is impossible to call the base class constructor without passing a value for that parameter, hence you have enforce the following:
Child classes has to be aware of the base class constructor and its parameter
But you cannot, and should not, try to enforce the presence of a particular constructor with a specific signature.
Now, having said that, what if you want to create some sort of common way to construct two distinct child classes, that has such different constructors, in such a way that code that uses them doesn't need to know the specifics of either constructor?
Enter the factory pattern (Wikipedia):
In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. This is done by creating objects by calling a factory method—either specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling a constructor.
(quoted text copied from entry paragraph in the Wikipedia-article)
Here's a way to abstract away the presence and knowledge of such different constructors and child classes:
void Main()
{
Test(new Child1Factory());
Test(new Child2Factory());
}
public void Test(IBaseFactory baseFactory)
{
Console.WriteLine("In Test(...");
var b = baseFactory.Create();
}
public class Base
{
public Base(int value)
{
Console.WriteLine($"Base.ctor({value})");
}
}
public interface IBaseFactory
{
Base Create();
}
public class Child1 : Base
{
public Child1(int value) : base(value)
{
Console.WriteLine($"Child1.ctor({value})");
}
}
public class Child1Factory : IBaseFactory
{
public Base Create() => new Child1(42);
}
public class Child2 : Base
{
public Child2(string name) : base(name.Length)
{
Console.WriteLine($"Child2.ctor({name})");
}
}
public class Child2Factory : IBaseFactory
{
public Base Create() => new Child2("Meaning of life");
}
Pay special attention to the Test(...) method, as this has no knowledge of which Base child it will get, nor how to construct such an object. If you later on add new child types from Base, you will have to provide new factories as well but existing code that uses these factories should not need to be changed.
If you want a simpler factory pattern all you have to do is replace the interface and factory classes with a delegate:
void Main()
{
Test(() => new Child1(42));
Test(() => new Child2("Meaning of life"));
}
public void Test(Func<Base> baseFactory)
{
Console.WriteLine("In Test(...");
var b = baseFactory();
}
Final note here. Since the factory pattern means you will have to create a different type that does the actual construction of the object you can enforce the signature of that other type, either by
Adding parameters to the Create method on the factory interface
Specifying a delegate that has parameters to the factory delegate
This means you can enforce the signature of "the creation process". Still, you cannot enforce the presence or signature of a particular constructor, but the constructor is just a means to an end, create an object, and with the factory pattern you can actually formalize this pattern in your code and thus you should get what you want.

You cannot enforce the signature or even existence of constructors of your derived classes. (or any class for that matter)
I'm afraid that's the end of the story. You aren't doing anything wrong, it's just not possible.

Since you can't override constructors in c#, you cannot enforce the existence of a certain constructor in the derived class .
This means:
a constructor cannot be abstract, virtual etc
constructors aren't polymorphically

You cannot have an abstract constructor, but neither is there any need to.
All you need to do is remove the "abstract" keyword from your parentClass and you should be good to go.

Why can't we use a constructor with parameter in derived classes

Why is this not possible?
I get the following compiler-error when instantiating "DerivedClass" with a constructor-parameter:
'GenericParameterizedConstructor.DerivedClass' does not contain a constructor that takes 1 argument
But calling a very similar method works.
Why?
class Program
{
static void Main(string[] args)
{
// This one produces a compile error
// DerivedClass cls = new DerivedClass("Some value");
// This one works;
DerivedClass cls2 = new DerivedClass();
cls2.SomeMethod("Some value");
}
}
public class BaseClass<T>
{
internal T Value;
public BaseClass()
{
}
public BaseClass(T value)
{
this.Value = value;
}
public void SomeMethod(T value)
{
this.Value = value;
}
}
public class DerivedClass : BaseClass<String>
{
}

Constructors aren't inherited - it's as simple as that. DerivedClass contains a single constructor - the public parameterless constructor provided by default by the compiler, because you haven't specified any constructors.
Note that this has nothing to do with generics. You'd see the same thing if BaseClass weren't generic.
It's easy to provide constructors for DerivedClass though:
public class DerivedClass : BaseClass<String>
{
public DerivedClass() : base()
{
}
public DerivedClass(string value) : base(value)
{
}
}

The deriving class needs to expose the constructor
public class DerivedClass : BaseClass<String>
{
public DerivedClass(string str) :base(str) {}
}

It would sometimes be helpful if there a way of instructing the compiler to automatically generate for a particular derived class constructors which precisely mimic and wrap all those of the base class. Having such behavior occur by default, however, would be problematic. Many derived classes expect that some of their code will be called whenever an instance of their type is created. Suppose a parent type had two constructors:
parentType(int foo) {...}
parentType(string foo) {...}
and a derived type had one:
derivedType(string foo) {...}
What should be the effect of new derivedType(7);? The compiler would know how to create a new baseType(7);, but if it created a new "blank" derivedType object and then simply called the parent-type constructor, the result would be a derivedType object which had never run any of derivedType's construction code. While some classes wouldn't have any problem with that (and for such classes, the earlier-mentioned hypothetical feature would be helpful), a lot of classes would.
Incidentally, a somewhat-related issue occurs with protected constructors. In some .net languages including at least the current version of C#, if non-abstract type Foo defines a protected constructor, that constructor may only be used to create instances of derived types. In other languages, including the current vb.net, it's possible for code within a derived type to call a protected constructor of the base type to create a new base-type instance.

Parameterized abstract class constructor

I have a class like below
public abstract class ABC
{
int _a;
public ABC(int a)
{
_a = a;
}
public abstract void computeA();
};
Is it mandatory for the derived class to supply the parameters for the base/abstract class constructor? Is there any way to initialize the derived class without supplying the parameters?
Thanks in advance.

Yes, you have to supply an argument to the base class constructor.
Of course, the derived class may have a parameterless constructor - it can call the base class constructor any way it wants. For example:
public class Foo : ABC
{
// Always pass 123 to the base class constructor
public Foo() : base(123)
{
}
}
So you don't necessarily need to pass any information to the derived class constructor, but the derived class constructor must pass information to the base class constructor, if that only exposes a parameterized constructor.
(Note that in real code Foo would also have to override computeA(), but that's irrelevant to the constructor part of the question, so I left it out of the sample.)

You can create a default constructor in a derived class that does not need parameters and the derived class will supply default values, but you cannot remove the requirement entirely. It is a manadatory condition of the base class to have some sort of value.
public MyDerivedClass : ABC
{
public MyDerivedClass()
: base(123) // hard wired default value for the base class
{
// Other things the constructor needs to do.
}
public override void computeA()
{
// Concrete definition for this method.
}
}

Add a default constructor to the base class and call the other constructor providing an initial values for its parameters:
public abstract class ABC
{
int _a;
public ABC(int a)
{
_a = a;
}
public ABC() : this(0) {}
}

It doesn't have much to do with the fact that the base class is abstract.
Because you've declared a public constructor that has 1 parameter, the compiler removes the basic empty constructor.
So, if you want to create an instance of that class, you have to pass a parameter.
When you derive from such a class, a parameter must be passed to the base class for it o construct.

How can I prevent a base constructor from being called by an inheritor in C#?

I've got a (poorly written) base class that I want to wrap in a proxy object. The base class resembles the following:
public class BaseClass : SomeOtherBase
{
public BaseClass() {}
public BaseClass(int someValue) {}
//...more code, not important here
}
and, my proxy resembles:
public BaseClassProxy : BaseClass
{
public BaseClassProxy(bool fakeOut){}
}
Without the "fakeOut" constructor, the base constructor is expected to be called. However, with it, I expected it to not be called. Either way, I either need a way to not call any base class constructors, or some other way to effectively proxy this (evil) class.

There is a way to create an object without calling any instance constructors.
Before you proceed, be very sure you want to do it this way. 99% of the time this is the wrong solution.
This is how you do it:
FormatterServices.GetUninitializedObject(typeof(MyClass));
Call it in place of the object's constructor. It will create and return you an instance without calling any constructors or field initializers.
When you deserialize an object in WCF, it uses this method to create the object. When this happens, constructors and even field initializers are not run.

If you do not explicitly call any constructor in the base class, the parameterless constructor will be called implicitly. There's no way around it, you cannot instantiate a class without a constructor being called.

At least 1 ctor has to be called. The only way around it I see is containment. Have the class inside or referencing the other class.

I don't believe you can get around calling the constructor. But you could do something like this:
public class BaseClass : SomeOtherBase
{
public BaseClass() {}
protected virtual void Setup()
{
}
}
public BaseClassProxy : BaseClass
{
bool _fakeOut;
protected BaseClassProxy(bool fakeOut)
{
_fakeOut = fakeOut;
Setup();
}
public override void Setup()
{
if(_fakeOut)
{
base.Setup();
}
//Your other constructor code
}
}

If what you want is to not call either of the two base class constructors, this cannot be done.
C# class constructors must call base class constructors. If you don't call one explicitly, base( ) is implied. In your example, if you do not specify which base class constructor to call, it is the same as:
public BaseClassProxy : BaseClass
{
public BaseClassProxy() : base() { }
}
If you prefer to use the other base class constructor, you can use:
public BaseClassProxy : BaseClass
{
public BaseClassProxy() : base(someIntValue) { }
}
Either way, one of the two will be called, explicitly or implicitly.

When you create a BaseClassProxy object it NEEDS to create a instance of it's base class, so you need to call the base class constructor, what you can doo is choose wich one to call, like:
public BaseClassProxy (bool fakeOut) : base (10) {}
To call the second constructor instead of the first one

I am affraid that not base calling constructor isn't option.

I ended up doing something like this:
public class BaseClassProxy : BaseClass
{
public BaseClass BaseClass { get; private set; }
public virtual int MethodINeedToOverride(){}
public virtual string PropertyINeedToOverride() { get; protected set; }
}
This got me around some of the bad practices of the base class.

constructors are public by nature. do not use a constructor and use another for construction and make it private.so you would create an instance with no paramtersand call that function for constructing your object instance.

All right, here is an ugly solution to the problem of one class inheriting the constructors of another class that I didn't want to allow some of them to work. I was hoping to avoid using this in my class but here it is:
Here is my class constructor:
public MyClass();
{
throw new Exception("Error: Must call constructor with parameters.");
}
OK now you were warned that it was ugly. No complaints please!
I wanted to force at least the minimal parameters from my main constructor without it allowing the inherited base constructor with no parameters.
I also believe that if you create a constructor and do not put the : base() after it that it will not call the base class constructor. And if you create constructors for all of the ones in the base class and provide the same exact parameters for them in the main class, that it will not pass through. But this can be tedious if you have a lot of constructors in the base class!

It is possible to create an object without calling the parameterless constructor (see answer above). But I use code like this to create a base class and an inherited class, in which I can choose whether to execute the base class's init.
public class MyClass_Base
{
public MyClass_Base()
{
/// Don't call the InitClass() when the object is inherited
/// !!! CAUTION: The inherited constructor must call InitClass() itself when init is needed !!!
if (this.GetType().IsSubclassOf(typeof(MyClass_Base)) == false)
{
this.InitClass();
}
}
protected void InitClass()
{
// The init stuff
}
}
public class MyClass : MyClass_Base
{
public MyClass(bool callBaseClassInit)
{
if(callBaseClassInit == true)
base.InitClass();
}
}

Here is my solution to the problem
using System;
public class Program
{
public static void Main()
{
Console.WriteLine(new Child().Test);
}
public class Child : Parent {
public Child() : base(false) {
//No Parent Constructor called
}
}
public class Parent {
public int Test {get;set;}
public Parent()
{
Test = 5;
}
public Parent(bool NoBase){
//Don't do anything
}
}
}
A simple elegant solution. You can change it according to your need.

Another simple solution from me:
class parent
{
public parent()
{
//code for all children
if (this.GetType() == typeof(child1))
{
//code only for objects of class "child1"
}
else
{
//code for objects of other child classes
}
}
}
class child1 : parent
{
public child1()
{}
}
// class child2: parent ... child3 : parent ... e.t.c