CODEWITHSUNDEEP
c#inheritanceconstructor
Frank
Frank

Reputation: 385

Constructor Parameters and Inheritance

New to OOP and I'm confused by how derived-class constructors work when inheriting from a base class in C#.

First the base class:

class BaseClass
{
    private string BaseOutput = null;

    public BaseClass(string BaseString)
    {
        BaseOutput = BaseString;
    }

    public virtual void PrintLine()
    {
        Console.WriteLine(BaseOutput);
    }
}

Here is the derived class:

class SubClass : BaseClass
{
    private string SubOutput = null;

    public SubClass(string BaseString, string SubString) : base(BaseString)
    {
        SubOutput = SubString;
    }

    public override void PrintLine()
    {
        Console.WriteLine(SubOutput);
    }
}

Finally, the main part of the program:

class Program
{
    static void Main(string[] args)
    {
        BaseClass theBase = new BaseClass("Text for BaseClass");
        SubClass theSub = new SubClass("2nd param", "Text for SubClass");

        theBase.PrintLine();
        theSub.PrintLine();

        Console.ReadKey();
    }
}

What I don't get is why, when calling the constructor for the derived class, I have to also pass the parameter that the base class needs. Shouldn't the BaseOutput field in the derived class just stay set to null if no value is assigned to it? Why can't something like this work:

public SubClass(string SubString) : base(BaseString)

Furthermore, when calling the constructor in this derived class, the first parameter has to be named the same as the one in the base class or else it throws an error. If I were to define a new string variable called AnotherString in the derived class, why wouldn't this work:

public SubClass(string AnotherString, string SubString) : base(BaseString)

Lastly, when you do this the right way and type out this...

public SubClass(string BaseString, string SubString) : base(BaseString)

...what is the first parameter in the SubClass constructor used for? It's not being assigned or used in any methods for my derived class. Why do I have to even put it there at all?

Upvotes: 37

Views: 83995

Answers (4)

ose
ose

Reputation: 4085

If you want to provide the opportunity for the derived class NOT to set the BaseString, then you need to provide a default constructor in the base class like this:

public BaseClass()
{

}

Now in the derived class, you can call the constructor with no arguments in the base class like this:

public SubClass(string AnotherString)
     : base()
{
    // base() => explicit call to default construct in the base class.
    // Do something else
}

This provides good software engineering practice: if you want to provide the opportunity not to set the base class's string, then do so in the base class. Other 'hacks' like passing null as an argument to the base constructor only serve to tinker with the base class's internals in ways that should not be done from subclasses.

Upvotes: 6

Servy
Servy

Reputation: 203847

As to why you can't do:

public SubClass(string SubString) : base(BaseString)

What would BaseString be?

You could do:

public SubClass(string SubString) : base("SomeFixedString")

or

public SubClass(string SubString) : base(SubString)

But if you want to pass one string to the base class constructor's parameter and have an additional one, you'll need to accept two parameters.

As to keeping the same name, you don't. You could do:

public SubClass(string AnotherString, string SubString) : base(AnotherString)

As to the last question, the first parameter isn't doing nothing, it's being passed to the base class constructor. You could use it for something else if you wanted to.

Upvotes: 34

Ayush
Ayush

Reputation: 42450

public SubClass(string BaseString, string SubString) : base(BaseString)

This constructor in the derived class says that when you receive two arguments BaseString and SubString, call the base class's constructor with BaseString.

Thus, doing 

public SubClass(string a, string b) : base(BaseString)

doesn't work, because you are telling it to call the base class's constructor with BaseString but there is no argument called BaseString.

If you want to instantiate the derived class without passing a String to the base class constructor, the base class needs a default constructor:

public BaseClass() { }

Upvotes: 2

Sergey Berezovskiy
Sergey Berezovskiy

Reputation: 236308

If you want to initialize BaseString with default value, then pass that value to base class constructor (thus you don't have to pass that parameter to derived class constructor)

public SubClass(string SubString) : base(null)

Or define parameterless constructor in base class.

Also about parameter naming - it does not matter what name has parameter, which you are passing to derived constructor. The only thing which matter is a value you are passing to base constructor.

public SubClass(string AnotherString, string SubString) : base(AnotherString)

Upvotes: 1

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