You made me… now tell me what to do

The question came up at work as to why a warning was issued against the call to DoSomething in the following code.

public class A
    public A()
    protected virtual void DoSomething()

The answer is, of course, that the virtual call can end up calling back into a derived class before it has fully finished initialising. For example, the following derived class

public class B : A
    int x;
    public B()
        x = 33;
    protected override void DoSomething()

when called as

  var b = new B();

ends up printing 0, as the virtual class runs one of the methods inside B before the constructor has finished running.

This brings up the interesting point that generally constructors should prepare a class for use, but should probably leave complicated actions such as subscribing to events or complicated setup to a subsequent initialization method call. Indeed, if a constructor throws an exception, then it is easy for a finaliser action to see a partially constructed object causing the finaliser to fail in interesting ways.

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