C# – Making all derived classes call the base class constructor

cconstructorinheritance

I have a base class Character which has several classes deriving from it. The base class has various fields and methods.

All of my derived classes use the same base class constructor, but if I don't redefine the constructor in my derived classes I get the error:

Error: Class "child class" doesn't contain a constructor which takes this number of arguments

I don't want to redefine the constructor in every derived class because if the constructor changes, I have to change it in every single class which, forgive any misunderstanding, goes against the idea of only writing code once?

Best Answer

You can use the following syntax to call the base class constructor from the classes that derive from it:

public DerivedClass() : base() {
    // Do additional work here otherwise you can leave it empty
}

This will call the base constructor first, then it will perform any additional statements, if any, in this derived constructor.

Note that if the base constructor takes arguments you can do this:

public DerivedClass(int parameter1, string parameter2) 
    : base(parameter1, parameter2) {
    // DerivedClass parameter types have to match base class types
    // Do additional work here otherwise you can leave it empty
}

You can find more information about constructors in the following page:

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/using-constructors

In a derived class, if a base-class constructor is not called explicitly by using the base keyword, the default constructor, if there is one, is called implicitly.

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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