[c#] Constructor of an abstract class in C#

Why is it possible to write constructor for an abstract class in C#?
As far as I know we can't instantiate an abstract class.. so what is it for?
You can't instantiate the class, right?

There are two following important features that prevent to inherit Abstract class

1. Abstract class must have an abstract method otherwise it is not a abstract class

2. Abstract class must be inherited by derived class,So if a class inherited by other class than what is use of to create object of that class

Defining a constructor with public or internal storage class in an inheritable concrete class Thing effectively defines two methods:

• A method (which I'll call InitializeThing) which acts upon this, has no return value, and can only be called from Thing's CreateThing and InitializeThing methods, and subclasses' InitializeXXX methods.

• A method (which I'll call CreateThing) which returns an object of the constructor's designated type, and essentially behaves as:

  Thing CreateThing(int whatever)
  {
      Thing result = AllocateObject<Thing>();
      Thing.initializeThing(whatever);
  }
  

Abstract classes effectively create methods of only the first form. Conceptually, there's no reason why the two "methods" described above should need to have the same access specifiers; in practice, however, there's no way to specify their accessibility differently. Note that in terms of actual implementation, at least in .NET, CreateThing isn't really implemented as a callable method, but instead represents a code sequence which gets inserted at a newThing = new Thing(23); statement.

I too want to make some shine on abstract surface All answer has covered almost all the things. Still my 2 cents

abstract classes are normal classes with A few exceptions

1. You any client/Consumer of that class can't create object of that class, It never means that It's constructor will never call. Its derived class can choose which constructor to call.(as depicted in some answer)
2. It may has abstract function.

It's there to enforce some initialization logic required by all implementations of your abstract class, or any methods you have implemented on your abstract class (not all the methods on your abstract class have to be abstract, some can be implemented).

Any class which inherits from your abstract base class will be obliged to call the base constructor.

You can instantiate it after you implemented all the methods. Then the constructor will be called.

Normally constructors involve initializing the members of an object being created. In concept of inheritance, typically each class constructor in the inheritance hierarchy, is responsible for instantiating its own member variables. This makes sense because instantiation has to be done where the variables are defined.

Since an abstract class is not completely abstract (unlike interfaces), it is mix of both abstract and concrete members, and the members which are not abstract are needed to be initialized, which is done in abstract class's constructors, it is necessary to have constructors in the abstract class. Off course the abstract class's constructors can only be called from the constructors of derived class.

It's a way to enforce a set of invariants of the abstract class. That is, no matter what the subclass does, you want to make sure some things are always true of the base class... example:

abstract class Foo
{
    public DateTime TimeCreated {get; private set;}

    protected Foo()
    {
         this.TimeCreated = DateTime.Now;
    }
}

abstract class Bar : Foo
{
    public Bar() : base() //Bar's constructor's must call Foo's parameterless constructor.
    { }
}

Don't think of a constructor as the dual of the new operator. The constructor's only purpose is to ensure that you have an object in a valid state before you start using it. It just happens to be that we usually call it through a new operator.

From https://msdn.microsoft.com/en-us/library/ms182126.aspx

Constructors on abstract types can be called only by derived types. Because public constructors create instances of a type, and you cannot create instances of an abstract type, an abstract type that has a public constructor is incorrectly designed.

Since only derived classes can use an abstract class constructor then an abstract class constructor, if needed, must be declared as protected.

However, that said VS compiler will not complain (with default rules) when declaring public constructors in abstract classes however it will not allow creating a new instance.

an abstract class can have member variables that needs to be initialized,so they can be initialized in the abstract class constructor and this constructor is called when derived class object is initialized.

Key Points About Abstract Class

1. An abstract class cannot be instantiated.
2. An abstract class can have constructor and destructor.
3. An abstract class cannot be a sealed class because the sealed modifier prevents a class from being inherited.
4. An abstract class contains abstract as well as non-abstract members.
5. An abstract class members can be private, protected and internal.
6. Abstract members cannot have a private access modifier.
7. Abstract members are implicitly virtual and must be implemented by a non-abstract derived class.

Far as I know we can't instantiate an abstract class

There's your error right there. Of course you can instantiate an abstract class.

abstract class Animal {}
class Giraffe : Animal {}
...
Animal animal = new Giraffe();

There's an instance of Animal right there. You instantiate an abstract class by making a concrete class derived from it, and instantiating that. Remember, an instance of a derived concrete class is also an instance of its abstract base class. An instance of Giraffe is also an instance of Animal even if Animal is abstract.

Given that you can instantiate an abstract class, it needs to have a constructor like any other class, to ensure that its invariants are met.

Now, a static class is a class you actually cannot instantiate, and you'll notice that it is not legal to make an instance constructor in a static class.

You are absolutely correct. We cannot instantiate an abstract class because abstract methods don't have any body i.e. implementation is not possible for abstract methods. But there may be some scenarios where you want to initialize some variables of base class. You can do that by using base keyword as suggested by @Rodrick. In such cases, we need to use constructors in our abstract class.