C# 构造函数中调用虚函数

using System;
using System.Diagnostics;
using System.Text;
using System.Collections;
using System.Collections.Generic;

class Test
{
    public class Base
    {
        public Base()
        {
            System.Console.WriteLine("Base.Base");
            ABitDangerousCall();
        }

        public virtual void ABitDangerousCall()
        {
            System.Console.WriteLine("Base.ABitDangerousCall");
        }

        private class Inner
        {
            public Inner()
            {
                System.Console.WriteLine("Base.Inner.Inner");
            }
        }
        private Inner inner = new Inner();
    }

    class Derived : Base
    {
        public Derived()
        {
            System.Console.WriteLine("Derived.Derived");
            ctorInitializedMember = 5;
        }

        // ctorInitializedMember is default initialized to zero before the constructor initializes it.
        private int ctorInitializedMember;
        private int derivedInt;

        public override void ABitDangerousCall()
        {
            System.Console.WriteLine(String.Format("Derived.ABitDangerousCallctorInitializedMember={0} derivedInt={1}", ctorInitializedMember, derivedInt));
        }

        private class Inner
        {
            public Inner()
            {
                System.Console.WriteLine("Derived.Inner.Inner");
            }
        }
        private Inner inner = new Inner();
    }


    class Program
    {
        static void Main(string[] args)
        {
            Derived d = new Derived();
            Console.Read();
        }
    }

}

输出结果：

Derived.Inner.Inner
Base.Inner.Inner
Base.Base
Derived.ABitDangerousCallctorInitializedMember=0 derivedInt=0
Derived.Derived

由于实例构造函数的调用顺序是先基类构造函数再子类构造函数，如上面例子，在基类中调用被子类重写了的虚函数ABitDangerousCall时，实际会调用子类的ABitDangerousCall，

该函数中使用了变量ctorInitializedMember，但此时子类的构造函数还没调用，故ctorInitializedMember未被赋值，这样就出现了不想看到的结果。我们本来是想想看到ctorInitializedMember=5的结果，结果却是0。

因此在基类的构造函数中调用虚函数是很危险的，项目大了代码复杂了就很容易造成极隐蔽的BUG。同时在子类的构造函数中调用虚函数也是要小心的，尽量避免，原因同上。

如果非要调用，那就要细心察看虚函数中的变量是否已经全部在前面处理妥当了