Be cautious when calling Virtual Methods in the base class constructor.

Be cautious when calling Virtual Methods in the base class constructor.

Recently, a problem was found when calling a virtual method in the base class constructor. First, reproduce the problem as follows:

    class Program
    {
        static void Main(string[] args)
        {
            var c = new Chinese(18);
            Console.ReadKey();
        }
    }
    public class People
    {
        public int Age { get; protected set; }
        protected People()
        {
            this.Say();
        }
        public virtual void Say()
        {
Console. WriteLine (string. Format ("the base class says I am {0} years old", Age ));
        }
    }
    public class Chinese : People
    {
        public Chinese(int age)
        {
            this.Age = age;
        }
        public override void Say()
        {
Console. WriteLine (string. Format ("subclass says I am {0} years old", Age ));
        }
    }

 

If you run the above Code, will the result be one of the following two situations?
Base Class: I'm 18 years old.
Subclass: I am 18 years old.

 

None! But:

 

Why?
→ Call subclass Constructornew Chinese(18)
→ Call the non-argument constructor of the parent class and call the virtual MethodSay
→ Virtual Methods discoveredSayIf there is an overload, The subclass is called.ChineseMediumSayBut the subclassChineseNoAgeAttribute initialization.AgeThe default value is 0.

How can we avoid it?
-- Remove the code that calls the virtual method in the base class constructor.
-- Use a subclass instance to call the overload method

 

    class Program
    {
        static void Main(string[] args)
        {
            var c = new Chinese(18);
            c.Say();
            Console.ReadKey();
        }
    }
    public class People
    {
        public int Age { get; protected set; }
        protected People()
        {
        }
        public virtual void Say()
        {
Console. WriteLine (string. Format ("the base class says I am {0} years old", Age ));
        }
    }
    public class Chinese : People
    {
        public Chinese(int age)
        {
            this.Age = age;
        }
        public override void Say()
        {
Console. WriteLine (string. Format ("subclass says I am {0} years old", Age ));
        }
    }

 

 
Conclusion: Be cautious when calling Virtual Methods in the base class constructor, because when the parent class calls a virtual method overload, The subclass has not been initialized, some members may not be initialized yet.