Main

public partial class Main{

  public class State{
    public State();
    public void Action();
  }
  
  public State doState;
  
  //どこかから呼ばれる
  void Update(){
    CheckState();
    doState.Action();
  }
  
  CheckState(){
    if(何か){
      doState = new State1();
    }
    if(何か){
      doState = new State2();
    }
  }
}

State1

public partial class Main{
  
  public class State1 : State{
    public State1();
    public override void Action();
  }

}

State2

public partial class Main{
  
  public class State2 : State{
    public State2();
    public override void Action();
  }

}