C# 事件斗争

Question

I was trying to write a simple code, which contains events whenever the train stops or drives My application keeps crashing while saying that im trying to use a null Event. Program Class:

public delegate void trainHandler();

class Program
{
    static void Main(string[] args)
    {
        Train e = new Train();
        e.Boxcar += new trainHandler (Message);
        Console.WriteLine("Welcome to the train, next stop London!");
        string choice = "";
        do
        {
            Console.WriteLine("D-Drive\nS-Stop\nE-Exit ");
            choice = Console.ReadLine().ToUpper();
            switch (choice)
            {
                case "D":
                    Train q = new Train();
                    q.Driving();
                    break;
                case "S":
                    Train q1 = new Train();
                    q1.Stopping();
                    break; 

            }
        } while (choice != "E");
    }
    static void Message()
    {
        Console.WriteLine("Thanks for riding our train!");
    }
}

New Class:

class Train
{
    public event trainHandler Boxcar;

    public void Driving()
    {
        Console.WriteLine("The train took off!");
        if (Boxcar != null)
        {
            Boxcar(); 
        }

    }
    public void Stopping()
    {
        Console.WriteLine("Train stoped, get down!");
        if (Boxcar != null)
        {
            Boxcar(); 
        }

    }
}

Answer 1

You are creating a new train in each of the case statements, neither of which have an event handler attached. change your switch to trigger the event on th train that was created at the start of the program:

switch (choice)
{
    case "D":
        e.Driving();
        break;
    case "S":
        e.Stopping();
        break; 
}

Answer 2

The problem is that your Train e = new Train(); where you assign the event to a handler, is a different Train instance to the ones on which you call Drive or Stopping

If you want your event to be called, you need to either call Drive or Stopping on your e instance, or assign an event handler to your new instances you create inside your switch