Why Abstract Constructor ?

Question

Why we cant use the abstract constructor ? What it is the reasons ? If we will use what will be happen ? Please clarify me ? give some examples ?

Vulpes · Answer

Sure here's an example which as well as illustrating the constructor point also shows how to override an abstract method. It also illustrates that it is not necessary for all members of an abstract class to be themselves abstract: using System; abstract class Animal { protected string _name; protected int _age; public string Name { get { return _name; } set { _name = value; } } public int Age { get { return _age; } set { _age = value; } } public abstract void MakeNoise(); protected Animal(string name int age) { _name = name; _age = age; } } class Dog : Animal { public override void MakeNoise() { Console.WriteLine(Wuff!); } public Dog(string name int age) : base(name age) { } } class Cat : Animal { public override void MakeNoise() { Console.WriteLine(Meow!); } public Cat(string name int age) : base(name age) { } } class Program { static void Main() { Animal d = new Dog(Rover 5); Console.WriteLine(The dog is called {0} and is {1} years old d.Name d.Age); d.MakeNoise(); Console.WriteLine(); Animal c = new Cat(Tiddles 2); Console.WriteLine(The cat is called {0} and is {1} years old c.Name c.Age); c.MakeNoise(); Console.ReadKey(); } } The output is: The dog is called Rover and is 5 years old Wuff! The cat is called Tiddles and is 2 years old Meow!

Jignesh Trivedi · Answer

hi Please refer below link it might help to understand abstract class. http://msdn.microsoft.com/en-us/library/k535acbf(v=vs.71).aspx http://msdn.microsoft.com/en-us/library/vstudio/ms229047(v=vs.100).aspx http://msdn.microsoft.com/en-us/library/aa645615(v=vs.71).aspx

Vijay · Answer

Please give abstract class examples....

Vulpes · Answer

That code compiles fine but you're not allowed to call the constructor directly. You couldn't do it here anyway because the constructor is protected but even if you made it public the compiler still wouldn't allow you to call it directly. It would generate the following error: error CS0144: Cannot create an instance of the abstract class or interface 'Abstract' However calling it via a derived class constructor is no problem. When you do this you're not creating an instance of the abstract class as such. You're just laying the groundwork for creating an object of the 'concrete' Derived class.

Vijay · Answer

I am asking like abstract class Abstract { protected abstract Abstract(string message) { Console.WriteLine(message); } } we can create like this?

Vulpes · Answer

Well you can't create an instance of an abstract class because it's only intended to be a base class for other 'concrete' classes. However this doesn't mean that an abstract class can't have a constructor - it can - and you can call it from a derived class constructor using the ' : base ' syntax. For example: using System; abstract class Abstract { protected Abstract(string message) { Console.WriteLine(message); } } class Derived : Abstract { public Derived(string message) : base(message) { } } class Program { static void Main() { Abstract ab = new Derived(Hello); Console.ReadKey(); } } If you don't give an abstract class a constructor then the system provides it with a protected parameterless constructor which does nothing except call its base class's parameterless constructor.