Null reference exception for MissileFired.Move

Question

namespace SpaceInvader
{
    /// <summary>
    /// This is the main type for your game
    /// </summary>
    public class SpaceInvaders : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;

        Texture2D StarfieldImg;
        Texture2D InvaderImg;
        Texture2D AltInvaderImg;
        Texture2D RocketLauncherImg;
        Texture2D MissileImg;
        int RocketXPos;
        int AlienDirection;
        int AlienSpeed;
        Invader[] Invaders; 
        double Ticks;
        Missile MissileFired;

        public SpaceInvaders()
        {
            graphics = new GraphicsDeviceManager(this);

            graphics.PreferredBackBufferWidth = 1024;
            graphics.PreferredBackBufferHeight = 768;
            graphics.ApplyChanges();

            Content.RootDirectory = "Content";
        }

        /// <summary>
        /// Allows the game to perform any initialization it needs to before starting to run.
        /// This is where it can query for any required services and load any non-graphic
        /// related content.  Calling base.Initialize will enumerate through any components
        /// and initialize them as well.
        /// </summary>
        protected override void Initialize()
        {
            // TODO: Add your initialization logic here
            RocketXPos = 512;

            AlienDirection = -1;
            AlienSpeed = 16;

            Invaders = new Invader[11];

            int XPos = 512;
            for (int Count = 0; Count < 11; Count++)
            {
                Invaders[Count] = new Invader();
                Invaders[Count].SetXPos(XPos);
                Invaders[Count].SetYPos(100);

                XPos = XPos + 32;
            }

            Ticks = 0;

            MissileFired = null;
            base.Initialize();
        }

        /// <summary>
        /// LoadContent will be called once per game and is the place to load
        /// all of your content.
        /// </summary>
        protected override void LoadContent()
        {
            // Create a new SpriteBatch, which can be used to draw textures.
            spriteBatch = new SpriteBatch(GraphicsDevice);

            // TODO: use this.Content to load your game content here
            StarfieldImg = Content.Load<Texture2D>("Starfield1024x768");
            InvaderImg = Content.Load<Texture2D>("inv1");
            AltInvaderImg = Content.Load<Texture2D>("inv12");
            RocketLauncherImg = Content.Load<Texture2D>("LaserBase");
            MissileImg = Content.Load<Texture2D>("bullet");

        }

        /// <summary>
        /// UnloadContent will be called once per game and is the place to unload
        /// all content.
        /// </summary>
        protected override void UnloadContent()
        {
            // TODO: Unload any non ContentManager content here
        }

        /// <summary>
        /// Allows the game to run logic such as updating the world,
        /// checking for collisions, gathering input, and playing audio.
        /// </summary>
        /// <param name="gameTime">Provides a snapshot of timing values.</param>
        protected override void Update(GameTime gameTime)
        {
            // Allows the game to exit
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
                this.Exit();

            // TODO: Add your update logic here
            MissileFired.Move();

            if (Keyboard.GetState().IsKeyDown(Keys.Space))
            {
                MissileFired = new Missile(RocketXPos, 650);
            }

            if (Keyboard.GetState().IsKeyDown(Keys.Left))
            {
                RocketXPos = RocketXPos - 4;
            }

            if (Keyboard.GetState().IsKeyDown(Keys.Right))
            {
                RocketXPos = RocketXPos + 4;
            }

            if (RocketXPos < 100)
            {
                RocketXPos = 100;
            }

            if (RocketXPos > 924)
            {
                RocketXPos = 924;
            }

            Ticks = Ticks + gameTime.ElapsedGameTime.TotalMilliseconds;

            if (Ticks > 500)
            {
                for (int Count = 0; Count < 11; Count++)
                {
                    Invaders[Count].MoveHorizontal(AlienSpeed * AlienDirection);
                }

                if (Invaders[0].GetXPos() < 96)
                {
                    AlienDirection = +1;

                    int XPos = 96;
                    for (int Count = 0; Count < 11; Count++)
                    {
                        Invaders[Count].MoveVertical(4);
                        Invaders[Count].SetXPos(XPos);
                        XPos = XPos + InvaderImg.Width;
                    }
                }

                if (Invaders[10].GetXPos() > 924)
                {
                    AlienDirection = -1;

                    int XPos = 924 - InvaderImg.Width * 10;
                    for (int Count = 0; Count < 11; Count++)
                    {
                        Invaders[Count].MoveVertical(4);
                        Invaders[Count].SetXPos(XPos);
                        XPos = XPos + InvaderImg.Width;
                    }
                }

                Ticks = 0;
            }

            base.Update(gameTime);
        }


        /// <summary>
        /// This is called when the game should draw itself.
        /// </summary>
        /// <param name="gameTime">Provides a snapshot of timing values.</param>
        protected override void Draw(GameTime gameTime)
        {
            spriteBatch.Begin();

            spriteBatch.Draw(StarfieldImg, Vector2.Zero, Color.White);

            spriteBatch.Draw(RocketLauncherImg, new Vector2(RocketXPos, 650), Color.White);

            if (MissileFired != null)
            {
                Vector2 MissilePos = new
                Vector2(MissileFired.GetPosition().X, MissileFired.GetPosition().Y - MissileImg.Height);
            spriteBatch.Draw(MissileImg, MissilePos, Color.White);
            }



            for (int Count = 0; Count < 11; Count++)
            {
                spriteBatch.Draw(InvaderImg, Invaders[Count].GetPos(), Color.White);
            }

            spriteBatch.End();

            base.Draw(gameTime);
        }
    }
}

When attempting to run this game, MissileFired.Move is underlined with a null reference exception. I think it may have something missing in the draw or update method but i have no clue. The detailed information about this exception states the "Object reference not set to an instance of an object." Help would be great, thank you!.

It certainly looks like you are setting `MissileFired` to null in your `Initialize` method. Then, on your first `Update` call, you invoke `Move` on it, most likely without an object. Why even null out the `MissileFired` object in your `Initialize` method? — Tejs, Apr 17 '12 at 14:52

Adam Houldsworth · Accepted Answer · 2012-04-17T14:56:29.043

2

The object MissileFired is null. When you attempt to call a member on a null object (such as Move()), you get a NullReferenceException.

From what I can see in your code, MissileFired is conditionally created by a key press, so perhaps the code that attempts to use it should do a null check first and not assume it exists:

if (MissileFired != null)
{
    MissileFired.Move();
}

To me this makes some sense, a missile is only going to exist when it is fired, but the code needs to handle both situations.

edited Apr 17 '12 at 14:56

answered Apr 17 '12 at 14:51

Adam Houldsworth

60,104
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Answered my question, thank you! – user1275084 Apr 17 '12 at 15:09

AvrDragon · Answer 2 · 2012-04-17T14:58:50.373

1

    protected override void Initialize()
    {
        ...
        MissileFired = null;
        ...
    }

That's a problem - MissleFired is null after game start, so you can not use it without null-check in your Update() method, before you are sure you have initialized it.

edited Apr 17 '12 at 14:58

answered Apr 17 '12 at 14:53

AvrDragon

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Null reference exception for MissileFired.Move

2 Answers2