Constructor calling hierarchy during inheritance-b

问题:

First of all this is not a duplicate question.

I know the facts that:

• During inheritance, the order of execution of constructors will be in the same order as their derivation and order of execution of destructors will be in reverse order of their derivation.
• Also, we can call base class constructor explicitly using base keyword in constructor definition.

My question is : For my below code

class ClassA
{
    public ClassA(int a)
    {
        Console.WriteLine("Parameterized Constructor of ClassA");
    }
}
class ClassB : ClassA
{
    public ClassB(int a)
    {
        Console.WriteLine("Parameterized Constructor of ClassB");
    }
}
class ClassC : ClassB
{
    public ClassC(int a)
    {
        Console.WriteLine("Parameterized Constructor of ClassC");
    }
}

class Program
{
    static void Main(string[] args)
    {
        ClassC classc = new ClassC(1);
        Console.ReadLine();
    }
}

I am getting the complie time error:

'ClassA' does not contain a constructor that takes 0 arguments
'ClassB' does not contain a constructor that takes 0 arguments

But when I Add the default constructors to both the classes as :

class ClassA
{
    public ClassA()
    {
        Console.WriteLine("Default Constructor of ClassA");
    }
    public ClassA(int a)
    {
        Console.WriteLine("Parameterized Constructor of ClassA");
    }
}
class ClassB : ClassA
{
    public ClassB()
    {
        Console.WriteLine("Default Constructor of ClassB");
    }
    public ClassB(int a)
    {
        Console.WriteLine("Parameterized Constructor of ClassB");
    }
}
class ClassC : ClassB
{
    public ClassC()
    {
        Console.WriteLine("Default Constructor of ClassC");
    }
    public ClassC(int a)
    {
        Console.WriteLine("Parameterized Constructor of ClassC");
    }
}

class Program
{
    static void Main(string[] args)
    {
        ClassC classc = new ClassC(1);
        Console.ReadLine();
    }
}

I got the output as expected.

Default Constructor of ClassA
Default Constructor of ClassB
Parameterized Constructor of ClassC

SO does this means, During inheritance all base classes needs a parameterless constructor.

I didn't get any clear explanation for this yet. Please dont make this as duplicate and any good explanation is appreciated.

回答1:

No

class ClassB : ClassA
{
    public ClassB(int a)
    {
        Console.WriteLine("Parameterized Constructor of ClassB");
    }
}

is same as

class ClassB : ClassA
{
    public ClassB(int a):base()
    {
        Console.WriteLine("Parameterized Constructor of ClassB");
    }
}

Compiler by default tries to call parameterless constructor of base class. If the base class doesn't have a parameterless constructor, you have to explicitly call it yourself:

class ClassB : ClassA
{
    public ClassB(int a):base(a)
    {
        Console.WriteLine("Parameterized Constructor of ClassB");
    }
}

Also, this has nothing to do with order of calling the constructors. That happens in runtime. You are having compile-time issues.