Abstract Class Designer in C #

Question

In C # we cannot create an obeject of an abstact class or an interface, this means that the abstract class has no constructor, is this true? or if he has what it is?

Answer 1

Good question. This is why abstract classes need constructors, even if they cannot be created.

In any object-oriented language, such as C #, building an object is a hierarchical process. Take a look at the code below. When you instantiate any object of type DerivedClass, it must first create a base object before creating an object of type DerivedClass. Here, the base class may or may not be an abstract class. But even when creating an object of a specific type obtained from an abstract class, you still need to call the constructor of the base class before creating an object of type DerivedClass, so you always need a constructor for the abstract class. If you have not added any constructor, the C # compiler will automatically add an open constructor without parameters to the class in the generated MSIL.

public class BaseClass
{
    public BaseClass() 
    {
        Console.WriteLine("BaseClass constructor called..");
    }
}

public class DerivedClass : BaseClass
{
    public DerivedClass()
    {
        Console.WriteLine("DerivedClass constructor called..");
    }
}

DerivedClass obj = new DerivedClass();

//Output
//BaseClass constructor called..
//DerivedClass constructor called..

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Answer 2

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public abstract class DemoBase
{
    private readonly string name;    
    public string Name { get { return name; } }

    protected DemoBase(string name)
    {
        this.name = name;
    }

    // Abstract members here, probably
}

public class FixedNameDemo : DemoBase
{
    public FixedNameDemo()
        : base ("Always the same name")
    {
    }

    // Other stuff here
}

public class VariableNameDemo : DemoBase
{
    public VariableNameDemo(string name)
        : base(name)
    {
    }

    // Other stuff here
}

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public abstract class ArithmeticOperator
{
    public static readonly ArithmeticOperator Plus = new PlusOperator();
    public static readonly ArithmeticOperator Minus = new MinusOperator();

    public abstract int Apply(int x, int y);

    private ArithmeticOperator() {}

    private class PlusOperator : ArithmeticOperator
    {
        public override int Apply(int x, int y)
        {
            return x + y;
        }
    }

    private class MinusOperator : ArithmeticOperator
    {
        public override int Apply(int x, int y)
        {
            return x - y;
        }
    }
}

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Answer 3

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Answer 4

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Answer 5

Abstract classes have constructors. When you instantiate a derived class, its parent class constructors are called. This also applies to classes derived from abstract classes.