C# base() constructor order [duplicate]

问题

Possible Duplicate:
C# constructor execution order

class Foo
{
    public int abc;
    Foo()
    {
       abc = 3;
    }

}

class Bar : Foo
{
    Bar() : base()
    {
       abc = 2;
    }
}

In the example above, when an object of Bar is created, what will be the value of BarObject.abc? Is the base constructor called first, or is Bar() run, /then/ the base() constructor?

回答1:

It'll be 2. Constructors run in order from base class first to inherited class last.

Note that initialisers (both static and instance variables) run in the opposite direction.

The full sequence is here: http://www.csharp411.com/c-object-initialization/

回答2:

First base class constructor is called followed by the derived class constructor. The result is 2. You should explicitly state the accessibility of that class variable. Is it protected, private or public?

I see you changed it to public now, so it will be 2.

This link will further help you understand constructors, how they are used, when they are called, and order of constructor call when you use inheritance:

http://www.yoda.arachsys.com/csharp/constructors.html

Also you may want to actually try this out yourself, you will learn more by practicing and writing code then just reading it.

Try to declare Bar and output its value. Use some properties:

 class Foo
    {
        public int abc;
        public Foo()
        {
            abc = 3;
        }

        public int ABC
        {
            get { return abc; }
            set { abc = value; }
        }

    }

    class Bar : Foo
    {
        public Bar() : base()
        {
            abc = 2;
        }
    } 


    class Program
    {
        static void Main(string[] args)
        {
            Bar b = new Bar();
            Console.WriteLine(b.ABC);
            Console.ReadLine();

        }
    }

A simple printout would yield the result you are looking for. Here is the output:

Don't you just love my namespace :-). By the way you could also use automatic properties so that the property is simply public int ABC {get;set;}.

回答3:

Assuming you make abc protected so that this compiles, it will be 2; however, base() is called first.

For stuff like this, write a simple test application and setup some breakpoints to find the answer.

回答4:

The variable abc will be set to be 3 and then changed to be 2 (the base constructor is called first).

回答5:

The base constructor will be called first, but this code does not compile. Private fields are not accesable from sub-classes. At the very least a field must be protected to be used in a sub-class.

But even knowing this, the behaviour you are attempting is confusing because it is surprising. Just the fact you had to ask which order things go in implies that it will get messed up when the order is forgotten.

回答6:

The base constuctor is called first, and you would have a value of 2 for abc

来源：https://stackoverflow.com/questions/2021373/c-sharp-base-constructor-order

标签

constructor

base