Virtual member call in a constructor

Question

I\'m getting a warning from ReSharper about a call to a virtual member from my objects constructor.

Why would this be something not to do?

Answer 1

The rules of C# are very different from that of Java and C++.

When you are in the constructor for some object in C#, that object exists in a fully initialized (just not "constructed") form, as its fully derived type.

namespace Demo
{
    class A 
    {
      public A()
      {
        System.Console.WriteLine("This is a {0},", this.GetType());
      }
    }

    class B : A
    {      
    }

    // . . .

    B b = new B(); // Output: "This is a Demo.B"
}

This means that if you call a virtual function from the constructor of A, it will resolve to any override in B, if one is provided.

Even if you intentionally set up A and B like this, fully understanding the behavior of the system, you could be in for a shock later. Say you called virtual functions in B's constructor, "knowing" they would be handled by B or A as appropriate. Then time passes, and someone else decides they need to define C, and override some of the virtual functions there. All of a sudden B's constructor ends up calling code in C, which could lead to quite surprising behavior.

It is probably a good idea to avoid virtual functions in constructors anyway, since the rules are so different between C#, C++, and Java. Your programmers may not know what to expect!