Question

When building the code it builds fine but wont run. Debugging it causes a Nullreferenceexception error whenever a sprite uses variables from the sprite class (First instance at Setup()

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;

namespace BilliardsEngine2D
{
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;

        Sprite CueBall;
        Sprite DirectionArrow;
        Sprite RedBall;

        SpriteFont myFont;

       //SoundEffect HitCue;
       //SoundEffect Collision;

        int ScreenWidth, ScreenHeight;

        float distance = 0.0f;
        public static float MasterVolume { get; set; }

        KeyboardState oldState;

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
            graphics.PreferredBackBufferHeight = 500;
            graphics.PreferredBackBufferWidth = 750;
        }

        protected override void Initialize()
        {
            Setup();
            SoundEffect.MasterVolume = 0.01f;
            base.Initialize();
        }

        private void Setup()
        {
            ScreenHeight = graphics.GraphicsDevice.Viewport.Height;
            ScreenWidth = graphics.GraphicsDevice.Viewport.Width;
            CueBall.Rotation = 0.0f;
            DirectionArrow.Rotation = CueBall.Rotation;
            CueBall.Velocity = Vector2.Zero;
            CueBall.Position = new Vector2(ScreenWidth / 2, ScreenHeight / 2);
            DirectionArrow.Position = CueBall.Position;
        }

        protected override void LoadContent()
        {
            spriteBatch = new SpriteBatch(GraphicsDevice);

            CueBall = new Sprite(Content.Load<Texture2D>("Sprites/CueBall"));
            DirectionArrow = new Sprite(Content.Load<Texture2D>("Sprites/Arrow"));
            RedBall = new Sprite(Content.Load<Texture2D>("Sprites/RedBall"));

            myFont = Content.Load<SpriteFont>("LucidaSans");
        }

        protected override void Update(GameTime gameTime)
        {
            KeyboardState newState = Keyboard.GetState();



            if (newState.IsKeyDown(Keys.Escape)) //Pause
            {
                Exit();
            }

            if (newState.IsKeyDown(Keys.Left)) //Rotate Left
            {
                CueBall.Rotation -= 0.05f;
                DirectionArrow.Rotation = CueBall.Rotation;
            }

            if (newState.IsKeyDown(Keys.Right)) //Rotate Right
            {
                CueBall.Rotation += 0.05f;
                DirectionArrow.Rotation = CueBall.Rotation;
            }

            if (newState.IsKeyDown(Keys.Space) && oldState.IsKeyUp(Keys.Space)) //Power Up
        {
        }

        if (newState.IsKeyUp(Keys.Space) && oldState.IsKeyDown(Keys.Space)) //Fire
        {
            HitCueBall();
        }

        oldState = newState;

        SideCollision();
        //BallCollision();

        base.Update(gameTime);
    }

    private void AccelerateCueBall() //
    {
        CueBall.Velocity += new Vector2(
            (float)(Math.Cos(CueBall.Rotation - MathHelper.PiOver2) * 0.05f),
            (float)((Math.Sin(CueBall.Rotation - MathHelper.PiOver2) * 0.05f)));

        if (CueBall.Velocity.X > 7.5f)
        {
            CueBall.Velocity = new Vector2(5.0f, CueBall.Velocity.Y);
        }
        if (CueBall.Velocity.X < -7.5f)
        {
            CueBall.Velocity = new Vector2(-5.0f, CueBall.Velocity.Y);
        }
        if (CueBall.Velocity.Y > 7.5f)
        {
            CueBall.Velocity = new Vector2(CueBall.Velocity.X, 5.0f);
        }
        if (CueBall.Velocity.Y < -7.5f)
        {
            CueBall.Velocity = new Vector2(CueBall.Velocity.X, -5.0f);
        }
    }

    public void SideCollision()
    {
        CueBall.Position += CueBall.Velocity;

        if (CueBall.Position.X + CueBall.Width < 0)
        {
            CueBall.Position = new Vector2(ScreenWidth, CueBall.Position.Y);
        }
        if (CueBall.Position.X - CueBall.Width > ScreenWidth)
        {
            CueBall.Position = new Vector2(0, CueBall.Position.Y);
        }
        if (CueBall.Position.Y + CueBall.Height < 0)
        {
            CueBall.Position = new Vector2(CueBall.Position.X, ScreenHeight);
        }
        if (CueBall.Position.Y - CueBall.Height > ScreenHeight)
        {
            CueBall.Position = new Vector2(CueBall.Position.X, 0);
        }
    }

    private bool CheckBallCollision(Sprite CueBall, Sprite RedBall)
    {
        Vector2 position1 = CueBall.Position;
        Vector2 position2 = RedBall.Position;

        float Cathetus1 = Math.Abs(position1.X - position2.X);
        float Cathetus2 = Math.Abs(position1.Y - position2.Y);

        Cathetus1 *= Cathetus1;
        Cathetus2 *= Cathetus2;

        distance = (float)Math.Sqrt(Cathetus1 + Cathetus2);
        if ((int)distance < CueBall.Width)
            return true;

        return false;
    }

    private void HitCueBall()
    {


        Vector2 Velocity = new Vector2(
            (float)Math.Cos(CueBall.Rotation - (float)MathHelper.PiOver2),
            (float)Math.Sin(CueBall.Rotation - (float)MathHelper.PiOver2));

        Velocity.Normalize();
        Velocity *= 6.0f;
        CueBall.Velocity = Velocity;
        CueBall.Position = CueBall.Position + CueBall.Velocity;
    }

    protected override void UnloadContent()
    {

    }

    protected override void Draw(GameTime gameTime)
    {
        GraphicsDevice.Clear(Color.Green);

        spriteBatch.Begin(SpriteSortMode.Deferred, BlendState.AlphaBlend);

        spriteBatch.Draw(CueBall.Texture, CueBall.Position, null, Color.White, CueBall.Rotation, CueBall.Center, CueBall.Scale, SpriteEffects.None, 1.0f);
        spriteBatch.Draw(DirectionArrow.Texture, DirectionArrow.Position, null, Color.White, DirectionArrow.Rotation, DirectionArrow.Center, 
            DirectionArrow.Scale, SpriteEffects.None, 1.0f);

        spriteBatch.End();
        base.Draw(gameTime);
    }
}

The Sprite Class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework;

namespace BilliardsEngine2D
{
class Sprite
{
    Texture2D texture;

    Vector2 position;
    Vector2 center;
    Vector2 velocity;

    float rotation;
    float scale;

    public Sprite(Texture2D texture)
    {
        this.texture = texture;

        position = Vector2.Zero;
        center = new Vector2(texture.Width / 2, texture.Height / 2);
        velocity = Vector2.Zero;

        Rotation = 0.0f;
        Scale = 1.0f;
    }

    public Texture2D Texture
    {
        get { return texture; }
    }

    public Vector2 Position
    {
        get { return position; }
        set { position = value; }
    }

    public Vector2 Center
    {
        get { return center; }
    }

    public Vector2 Velocity
    {
        get { return velocity; }
        set { velocity = value; }
    }

    public float Rotation
    {
        get { return rotation; }
        set
        {
            rotation = value;
            if (rotation < -MathHelper.TwoPi)
                rotation = MathHelper.TwoPi;
            if (rotation > MathHelper.TwoPi)
                rotation = -MathHelper.TwoPi;
        }
    }

    public float Scale
    {
        get { return scale; }
        set { scale = value; }
    }

    public int Width
    {
        get { return texture.Width; }
    }

    public int Height
    {
        get { return texture.Height; }
    }
}

Any ideas how to fix this.

Answer 1

I believe that your setup method is getting called before the LoadContent method.

According to this documentation LoadContent is called by Initialize.

    protected override void Initialize()
    {
        Setup();
        SoundEffect.MasterVolume = 0.01f;
        base.Initialize();
    }

I’d say it’s fair to assume that base.Initialize() is where LoadContent is called from.

I believe that your code is breaking, because you’re trying to set properties of your Sprite classes in the Setup() method before they’re instanciated in the LoadContent() method.

I’d recommend moving statements where you modify your Sprite properties:

CueBall.Rotation = 0.0f;

after you’ve instantiated them, with new Sprite() in your LoadContent method.

CueBall = new Sprite(Content.Load<Texture2D>("Sprites/CueBall"));

in your LoadContent method

Or you can move your Setup Call to your LoadContent method