I'm not sure what callback you're talking about, but an IDisposable class is just one that implements the IDisposable interface and has a public Dispose method:

public class MyClass : IDisposable
{
    public void Dispose()
    {
    }
}

Edit:

IDisposable isn't what you're looking for at all, you're wanting the finalizer syntax. Try this instead:

function Finalize()
{
}

• If you mean the finalizer (destructor) by "C# ~ callback", it is called at an seemingly arbitrary time after references to the object have been lost. So it gets called but maybe you expect them to behave differently.

• There is a IDisposable interface in .NET. (System.IDisposable) that you can use in case you need to release unmanaged resources.

• If you just want to destroy a script or a game object, use Destroy(scriptInstance); or Destroy(gameObject);

Points to remember:

• Destructors are invoked automatically, and cannot be invoked explicitly.
• Destructors cannot be overloaded. Thus, a class can have, at most, one destructor.
• Destructors are not inherited. Thus, a class has no destructors other than the one, which may be declared in it.
• Destructors cannot be used with structs. They are only used with classes.
• An instance becomes eligible for destruction when it is no longer possible for any code to use the instance.
• Execution of the destructor for the instance may occur at any time after the instance becomes eligible for destruction.
• When an instance is destructed, the destructors in its inheritance chain are called, in order, from most derived to least derived.