Base Class Doesn't Contain Parameterless Constructor?

问题

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
}

回答1:

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
    }
}

回答2:

Example:

class A2 : A
{
   A2() : base(0)
   {
   }
}

class A
{
    A(int something)
    {
        ...
    }
}

回答3:

If your base class doesn't have a parameterless constructor, you need to call one from your derived class using base keyword:

class A
{
    public A(Foo bar)
    {
    }
}

class A2 : A
{
    public A2()
        : base(new Foo())
    {
    }
}

回答4:

It has to call some constructor. The default is a call to base().

You can also use static methods, literals, and any parameters to the current constructor in calls to base().

  public static class MyStaticClass
    {
        public static int DoIntWork(string i)
        {
            //for example only
            return 0;
        }
    }

    public class A
    {
        public A(int i)
        {
        }
    }

    public class B : A
    {
        public B(string x) : base(MyStaticClass.DoIntWork(x))
        {
        }
    }

回答5:

Because if A has no default constructor then the constructor of A2 needs to call base() with the arguments to the constructor of A. See this question: Calling the base constructor in C#

回答6:

when you create the object of your derived class,your base class constructor gets called automatically.So at the time you create your derived class object,and your derived class object has not constructor taking one or more arguments, there will be nothing to pass to the base class constructor that wants one argument. so to do that, you need to pass something to the base class constructor as follows:

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

Class A2 : A{
    public A2():base(some parameter)
}

来源：https://stackoverflow.com/questions/7689742/base-class-doesnt-contain-parameterless-constructor

标签

constructor