interface IStrategyDistance
{
     void Calculate(int a, int b)
}

class StrategyDistanceByCar : IStrategyDistance
{
     public virtual double Calculate(a, b) { ...     }
}

class StrategyDistanceByTrain : IStrategyDistance
{
     public virtual double Calculate(a, b) { ... }
}

class Navigator
{
     private IStrategyDistance _strategy;

     void SetStrategyDistance(IStrategyDistance strategy)
     {
          this._strategy = strategy
     }

     void ExecuteStrategyDistance(int a, int b)
     {
          return _strategy.Calculate(a, b)
     }
}


class Application
{
     void Main()
     {
          Navigator navigator = new Navigator();

          context.setStrategyDistance(new StrategyDistanceByCar());
          result = navigator.ExecuteStrategyDistance(5, 9);

          context.setStrategyDistance(new StrategyDistanceByPlain());
          result = context.ExecuteStrategyDistance(6, 4);
     }
}