接口-引发null异常

Question

im both new to this site and new to programming. I've been recently trying to learn new skills to help better organise/manage my code and make it both more efficient, readable and contained.

Okay well i wont go on too much about that, the problem i'm having is in XNA 3.1, I'm using C# express 08.

I have a self contained Game conponent called InputHandler, the update loops after the base loop (Game1) which so far just checks for keyboard input and stores the result into an instance of KeyboardState - which has a Get property, the only other code really is it exits Game1 if Escape key is pressed, which it checks for after storing the input.

Code:

        private KeyboardState keyboardstate;
        public KeyboardState Keyboard_State
        {
            get { return (keyboardstate); }
        }

        public override void Update(GameTime gameTime)
        {
            keyboardstate = Keyboard.GetState();
            if (keyboardstate.IsKeyDown(Keys.Escape))
                Game.Exit();

            base.Update(gameTime);
        }

moving onto the problem, another game conponent called Camera tries accessing the Keyboard_State property of the InputHandler via an instance of the IInputHandler (this is an interface btw)

    public interface IInputHandler
    {
        KeyboardState Keyboard_State { get; }
    }

it goes without saying that this interface is implemented within the InputHandler component. moving onto the error, well I have in my update loop within the Camera component some logic code, which tries to access the Keyboard_State property through the interface, check against some conditions, then alter the camera apropriatly.

            private IInputHandler input;

following code is within the void update loop.. within the Camera component.

            if (input.Keyboard_State !=null)
            {
                if (input.Keyboard_State.IsKeyDown(Keys.Left))
                    cameraYaw += spinRate;
                if (input.Keyboard_State.IsKeyDown(Keys.Right))
                    cameraYaw -= spinRate;

                if (cameraYaw > 360)
                    cameraYaw -= 360;
                else if (cameraYaw < 360)
                    cameraYaw += 360;
            }

I get a Null reference exception at the *if (input.Keyboard_State !=null)* line, complaining that it's not an instance.

I'm new with Interfaces, I've never seen much of a use for them in the past until i started to try learn XNA, and began learning about conponents, ultimately i want to create the basic components to manage a 3D game (nothing fancy, just organised and manageable).

any help would be appreciated. thanks :)

* Other info *

my camera constructer is :

        public Camera(Game game)
            : base(game)
        {
            graphics = (GraphicsDeviceManager)Game.Services.GetService(typeof(IGraphicsDeviceManager));
            input = (IInputHandler)game.Services.GetService(typeof(IInputHandler));
        }

and my InputHandler constructer is empty, my Game1 constructer is:

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";

            camera = new Camera(this);
            Components.Add(camera);

            input = new InputHandler(this);
            Components.Add(input);

            input.UpdateOrder = 0;
            camera.UpdateOrder = 1;

            // this component alows Asyncroniously save/load game.
            Components.Add(new GamerServicesComponent(this));

#if DEBUG
            fps = new FPS(this);
            Components.Add(fps);
            fps.UpdateOrder = 1;
            camera.UpdateOrder = 2;
#endif

        }

input is an instance of the Input handler game component.

         private InputHandler input;

hope this helps :)

Answer 1

It seems to me that you are not initializing the "input"-variable in the camera anywhere ( = input is null ).

Because of that, if (input.Keyboard_State !=null) -line throws NullReferenceException ( and by the way KeyboardState is a struct thus it can't be null). You said that both InputHandler and Camera is a game-component? Try doing something like this then:

In the InputHandler constructor:

public InputHandler(...)
{
    // Your initialization code here

    this.Game.Services.AddService(typeof(IInputHandler), this);
}

And in the Camera constructor:

public Camera(...)
{
    // Your initialization code here

    input = this.Game.Services.GetService(typeof(IInputHandler)) as IInputHandler;
}

EDIT, Updated code:

Change your Game-constructor to this:

  public Game1()
    {
        graphics = new GraphicsDeviceManager(this);
        Content.RootDirectory = "Content";

        input = new InputHandler(this);
        Components.Add(input);
        Services.AddService(typeof(IInputHandler), input);

        camera = new Camera(this);
        Components.Add(camera);



        input.UpdateOrder = 0;
        camera.UpdateOrder = 1;

        // this component alows Asyncroniously save/load game.
        Components.Add(new GamerServicesComponent(this));

#if DEBUG
        fps = new FPS(this);
        Components.Add(fps);
        fps.UpdateOrder = 1;
        camera.UpdateOrder = 2;
#endif

    }