C# – Base Class Doesn’t Contain Parameterless Constructor

cconstructor

I'm making my constructors a bit more strict by removing some of my empty constructors. I'm pretty new to inheritance, and was perplexed with the error that I got: Base Class Doesn't Contain Parameterless Constructor. How can I make A2 inherit from A without there being an empty constructor in A. Also, for my own personal understanding, why is A2 requiring an empty constructor for A?

Class A{
    //No empty constructor for A
    //Blah blah blah...
}

Class A2 : A{
    //The error appears here
}

Best Answer

In class A2, you need to make sure that all your constructors call the base class constructor with parameters.

Otherwise, the compiler will assume you want to use the parameterless base class constructor to construct the A object on which your A2 object is based.

Example:

class A
{
    public A(int x, int y)
    {
        // do something
    }
}

class A2 : A
{
    public A2() : base(1, 5)
    {
        // do something
    }

    public A2(int x, int y) : base(x, y)
    {
        // do something
    }

    // This would not compile:
    public A2(int x, int y)
    {
        // the compiler will look for a constructor A(), which doesn't exist
    }
}

Related Solutions

C# – Calling the base constructor in C#

Modify your constructor to the following so that it calls the base class constructor properly:

public class MyExceptionClass : Exception
{
    public MyExceptionClass(string message, string extrainfo) : base(message)
    {
        //other stuff here
    }
}

Note that a constructor is not something that you can call anytime within a method. That's the reason you're getting errors in your call in the constructor body.

C# – Virtual member call in a constructor

When an object written in C# is constructed, what happens is that the initializers run in order from the most derived class to the base class, and then constructors run in order from the base class to the most derived class (see Eric Lippert's blog for details as to why this is).

Also in .NET objects do not change type as they are constructed, but start out as the most derived type, with the method table being for the most derived type. This means that virtual method calls always run on the most derived type.

When you combine these two facts you are left with the problem that if you make a virtual method call in a constructor, and it is not the most derived type in its inheritance hierarchy, that it will be called on a class whose constructor has not been run, and therefore may not be in a suitable state to have that method called.

This problem is, of course, mitigated if you mark your class as sealed to ensure that it is the most derived type in the inheritance hierarchy - in which case it is perfectly safe to call the virtual method.

Best Answer

Related Solutions

C# – Calling the base constructor in C#

C# – Virtual member call in a constructor

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