Can I set the state of an object to the object itself in the constructor call?

Question

I am using a pre-built third party class library (think .NET Framework...) that contains a class Foo with the following members:

public sealed class Foo {
  private object _state;
  public Foo(object state) { _state = state; }
}

Foo does not contain a public _state setter.

In a specific scenario, I want to set the state of a Foo object to the object itself.

This will NOT compile:

Foo f;
f = new Foo(f);   // Compilation error: Use of unassigned local variable 'f'

Given the above prerequisites, is there any way that I can set the state of a Foo object to the object itself?

---

Rationale The Timer class in Windows 8.1 does not contain the Timer(AsyncCallback) constructor which assigned the state of the Timer object with the object itself. I am trying to port .NET code that contains this Timer constructor to a Windows 8.1 class library, so my concern is How do I pass the object itself to its state member? This issue is further outlined here, but I thought it would be more efficient to also pose the principal question above.

Answer 1

Workaround:

var foo = new Foo(/*params*/);
var fieldInfo = foo.GetType().GetField("_state", BindingFlags.NonPublic | BindingFlags.Instance);
fieldInfo.SetValue(foo , foo);

Answer 2

If I understand your intention correctly you want a timer whose callback references the timer itself.

Timer timer = null;
timer = new Timer(() => {
 timer.Stop(); //sample
});

Creating an object is done through the newobj instruction which atomically allocates and invokes the constructor. Without cooperation from the ctor you cannot get a reference to the unconstructed object. So there's no other way that either this approach, or reflection.

You can extract the above code into a helper method, make timer a local variable and then every timer callback will close over its own private and unchanging variable.

Timer CreateTimer(Action<Timer> callback) {
    Timer timer = null;
    timer = new Timer(() => {
     callback(timer);
    });
}