Shared an non-shared

[Begah via Digitalmars-d-learn]

I seem to be missing something.
It seems that if you want to create a shared object of a 
structure ( or class ), then I have to copy every functions and 
add "shared" to it. This seems way more work than it should.
For example why can't this simply work :
  class Image {
        string name;
        int contents;
        
        this(string name, int contents) {
                this.name = name;
                this.contents = contents;
        }
        
        void printContents() {
                writeln(name, " : ", contents);
        }
        
        void changeContents(int newContents) {
                this.contents = newContents;
        }
  }

  void main()
  {
        Image im = new Image("Test", 15);
        im.printContents();
        im.changeContents(45);
        im.printContents();
        
        shared Image imShared = new shared Image("Test2", 80);
        imShared.printContents();
        imShared.changeContents(6);
        imShared.printContents();
  }

How can I make a method that accepts being called by both a 
shared and non-shared object, to prevent having to copy methods 
and adding a "shared"?

I am not looking for an explanation for how to handle 
multi-threading ( synchronization and so on ). I am looking to 
use pre-coded classes and structures ( without using __gshared ) 
with non-shared and shared objects.