A Null Reference Exception

Question

"Object reference not set to an instance of an object."

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

namespace XNAdev
{
    class Sprite
    {
        //The size of the Sprite
        public Rectangle Size;

        //Used to size the Sprite up or down from the original image
        public float Scale = 1.0f;

        //The current position of the Sprite
        public Vector2 Position = new Vector2(115, 0);
        //The texture object used when drawing the sprite
        private Texture2D mSpriteTexture;

        //Load the texture for the sprite using the Content Pipeline
        public void LoadContent(ContentManager theContentManager, string theAssetName)
        {
            mSpriteTexture = theContentManager.Load<Texture2D>(theAssetName);
            Size = new Rectangle(0, 0, (int)(mSpriteTexture.Width * Scale), (int)(mSpriteTexture.Height * Scale));
        }

        //Draw the sprite to the screen
        public void Draw(SpriteBatch theSpriteBatch)
        {
            theSpriteBatch.Draw(mSpriteTexture, Position,
                new Rectangle(0, 0, mSpriteTexture.Width, mSpriteTexture.Height), Color.White,
                0.0f, Vector2.Zero, Scale, SpriteEffects.None, 0);

        }    
    }
}

I am very new at this C# so any help would be great.

I have no idea what my error is.

---

namespace XNAdev
{

    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        Sprite mSprite;
        Sprite mSpriteTwo;
        Sprite mBackgroundOne;
        Sprite mBackgroundTwo;
        Sprite mBackgroundThree;
        Sprite mBackgroundFour;
        Sprite mBackgroundFive;





        public Game1()
        {           

            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }

        protected override void Initialize()
        {
            mSprite = new Sprite();
            mSpriteTwo = new Sprite();

            mBackgroundOne = new Sprite();
            mBackgroundOne.Scale = 2.0f;

            mBackgroundTwo = new Sprite();
            mBackgroundTwo.Scale = 2.0f;

            mBackgroundThree = new Sprite();
            mBackgroundThree.Scale = 2.0f;

            mBackgroundFour = new Sprite();
            mBackgroundFour.Scale = 2.0f;

            mBackgroundFive = new Sprite();
            mBackgroundFive.Scale = 2.0f;

            base.Initialize();
        }

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

            mSprite.Position = new Vector2(125, 245);

            mSpriteTwo.LoadContent(this.Content, "SquareGuy");
            mSpriteTwo.Position.X = 300;
            mSpriteTwo.Position.Y = 300;

            mBackgroundOne.LoadContent(this.Content, "Background01");
            mBackgroundOne.Position = new Vector2(0, 0);            

            mBackgroundTwo.LoadContent(this.Content, "Background02");
            mBackgroundTwo.Position = new Vector2(mBackgroundOne.Position.X + mBackgroundOne.Size.Width, 0);

            mBackgroundThree.LoadContent(this.Content, "Background03");
            mBackgroundThree.Position = new Vector2(mBackgroundTwo.Position.X + mBackgroundTwo.Size.Width, 0);

            mBackgroundFour.LoadContent(this.Content, "Background04");
            mBackgroundFour.Position = new Vector2(mBackgroundThree.Position.X + mBackgroundThree.Size.Width, 0);

            mBackgroundFive.LoadContent(this.Content, "Background05");
            mBackgroundFive.Position = new Vector2(mBackgroundFour.Position.X + mBackgroundFour.Size.Width, 0);            
        }

        protected override void UnloadContent()
        {

        }


        protected override void Update(GameTime gameTime)
        {
            // Allows the game to exit
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
                this.Exit();

            if (mBackgroundOne.Position.X < -mBackgroundOne.Size.Width)
            {
                mBackgroundOne.Position.X = mBackgroundFive.Position.X + mBackgroundFive.Size.Width;
            }

            if (mBackgroundTwo.Position.X < -mBackgroundTwo.Size.Width)
            {
                mBackgroundTwo.Position.X = mBackgroundOne.Position.X + mBackgroundOne.Size.Width;
            }

            if (mBackgroundThree.Position.X < -mBackgroundThree.Size.Width)
            {
                mBackgroundThree.Position.X = mBackgroundTwo.Position.X + mBackgroundTwo.Size.Width;
            }

            if (mBackgroundFour.Position.X < -mBackgroundFour.Size.Width)
            {
                mBackgroundFour.Position.X = mBackgroundThree.Position.X + mBackgroundThree.Size.Width;
            }

            if (mBackgroundFive.Position.X < -mBackgroundFive.Size.Width)
            {
                mBackgroundFive.Position.X = mBackgroundFour.Position.X + mBackgroundFour.Size.Width;
            }

            Vector2 aDirection = new Vector2(-1, 0);
            Vector2 aSpeed = new Vector2(160, 0);

            mBackgroundOne.Position += aDirection * aSpeed * (float)gameTime.ElapsedGameTime.TotalSeconds;
            mBackgroundTwo.Position += aDirection * aSpeed * (float)gameTime.ElapsedGameTime.TotalSeconds;
            mBackgroundThree.Position += aDirection * aSpeed * (float)gameTime.ElapsedGameTime.TotalSeconds;
            mBackgroundFour.Position += aDirection * aSpeed * (float)gameTime.ElapsedGameTime.TotalSeconds;            
            mBackgroundFive.Position += aDirection * aSpeed * (float)gameTime.ElapsedGameTime.TotalSeconds;            
        }

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

            spriteBatch.Begin();

            mBackgroundOne.Draw(this.spriteBatch);
            mBackgroundTwo.Draw(this.spriteBatch);
            mBackgroundThree.Draw(this.spriteBatch);
            mBackgroundFour.Draw(this.spriteBatch);
            mBackgroundFive.Draw(this.spriteBatch);

            mSprite.Draw(this.spriteBatch);
            mSpriteTwo.Draw(this.spriteBatch);
            spriteBatch.End();

            base.Draw(gameTime);
        }
    }
}

Thats the rest of the code

Answer 1

I think you need to initialize mSpriteTexture

Answer 2

Probably you should check

 mSpriteTexture = theContentManager.Load<Texture2D>(theAssetName);
 if (mSpriteTexture != null)
     Size = new Rectangle(0, 0, (int)(mSpriteTexture.Width * Scale), (int)(mSpriteTexture.Height * Scale));

And in the Draw function too.

Answer 3

1. Lookup how to set Visual Studio to break on exceptions.
2. Learn how to use the debugger so you can step through your code to see where the error occurs.

Answer 4

This error means that one of the fields or methods of an object has been tried to be accessed without the object being instantiated.

For you code seems that this is happening with the object mSpriteTexture.

You should add

mSpriteTexture = new Texture2D();

somewhere, but I can not tell where only with this piece of code.

Answer 5

--- moved to question --

Answer 6

I've had a look through it and managed to get it to work using validation, if the sprite you are drawing has a null reference (No texture) it will ignore it and carry on drawing everything else.

Change your Draw() method in the Sprite.cs to this:

//Draw the sprite to the screen
public void Draw(SpriteBatch theSpriteBatch)
{
    if (mSpriteTexture != null)
    {
        theSpriteBatch.Draw(mSpriteTexture, Position,
            new Rectangle(0, 0, mSpriteTexture.Width, mSpriteTexture.Height), Color.White,
            0.0f, Vector2.Zero, Scale, SpriteEffects.None, 0);
    }
}

The problem occurs because you never give "Sprite mSprite;" a texture, only a position.

Quick snippet:

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

    mSprite.Position = new Vector2(125, 245);

    mSpriteTwo.LoadContent(this.Content, "SquareGuy");
    mSpriteTwo.Position.X = 300;
    mSpriteTwo.Position.Y = 300;

As you can see you only give mSprite a position of 125,245, simply assign it a texture like you have with the rest of the sprites and it will work fine.

You don't need to remove the if(mSpriteTexture != null) from the Draw() method after you assign the texture though, if you don't it simply means you won't notice if something isn't being assigned correctly, could be a pain if you're debugging something else later.