This is another good question I got while teaching the C# Advanced course at SoftUni. Here is a detailed answer.
Why shouldn’t we call virtual methods within the constructor of a base class?
This is actually one of the Code Quality warnings under the Usage section. Please see the link here: https://docs.microsoft.com/en-us/visualstudio/code-quality/ca2214-do-not-call-overridable-methods-in-constructors?view=vs-2017
While the article is correct, I’m not sure the explanation is very understandable.
It all comes down to the following execution path:
- You instantiate the derived class
- The base class constructor is called
- It calls virtual methods overridden in the derived class
- But our derived class’s constructor has not been called yet
Here is my example:
public abstract class BaseClass
Console.WriteLine(“BaseClass constructor called.”);
public virtual void VirtualMethod()
Console.WriteLine(“Base virtual method called”);
public abstract void AbstractMethod();
public class DerivedClass : BaseClass
Console.WriteLine(“Derived class constructor executed VERY late.”);
public override void VirtualMethod()
Console.WriteLine(“Derived Virtual Method”);
public override void AbstractMethod()
Console.WriteLine(“Derived Abstract Method”);
BaseClass constructor called.
Base virtual method called
Derived Virtual Method
Derived Abstract Method
Derived class constructor executed VERY late.
So we just ran code in a class which might very well be in an invalid state. Objects should go through construction logic first before we execute their methods.