Nullreferenceexception after initializing an array

Question

I've been trying to make a test game in XNA (visual studio 2015), but i get a NullReferenceException every time i load this, even though i actually do initialize it. Here is a very short version of it...

    namespace Overbox
    {
        public class Player
        {
             private Player[] characterList = new Player[14];

             virtual public void Initialize()
             {
                  //characterList = new Player[14];
                  characterList[0] = new Hero();

                  //for (int i = 0; i <= characterList.Length; i++)
                  //    characterList[i].Initialize();
                  characterList[0].Initialize();
             }
    }
    virtual public void Draw(SpriteBatch spriteBatch, Texture2D texture)
    {
        //for (int i = 0; i <= characterList.Length; i++)
        //{
        //    if (characterList[i].Active)
        //        characterList[i].Draw(spriteBatch, texture);
        //}
        characterList[0].Draw(spriteBatch, texture); //Here the error occurs
    }



   }
}

If someone wants the whole class for any reason i could edit this, but this is all the related code i could find.

Edit: stack trace

Overbox.exe!Overbox.Player.Draw(Microsoft.Xna.Framework.Graphics.SpriteBatch spriteBatch, Microsoft.Xna.Framework.Graphics.Texture2D texture) Line 53 C# Overbox.exe!Overbox.MediaHandler.Draw(Microsoft.Xna.Framework.Graphics.SpriteBatch spriteBatch) Line 54 C#

Overbox.exe!Overbox.Program.Main(string[] args) Line 15 C#

It's all supposed to be there.

Hero class:

    namespace Overbox.Characters
    {
         class Jacketed : Player
         {
         override public void Initialize()
         {
                //Unrelated setting of variables
         }
         public override void Draw(SpriteBatch spriteBatch, Texture2D texture)
         {
                spriteBatch.Draw(texture, PlayerPosition, Color.White);
         }
    }
}

Very short version of game loop class:

  public class Game1 : Microsoft.XNA.Framework.Game
  {
        public static Player player;
        //I do load the content which playerTexture requires before anyone asks about it
        private Texture2D playerTexture;


        protected override void Initialize()
        {
               player = new Player();
               player.Initialize();
        }
        protected override void Draw(GameTime gameTime)
        {
               spriteBatch.Begin();

               player.Draw(spriteBatch, playerTexture);

               spriteBatch.End();
        }
  }

Answer 1

private Player[] characterList = new Player[14];

Makes an array of Player, but does not actually construct the objects. So what you end up with is basically an array full of null values.

You are missing something like this:

public Player()
{
    foreach (var character in characterList)
    {
        character = new Player();
    }
}

Answer 2

This is a wild guess, but I'm assuming Hero overrides one or both or Initialize and Draw. Let's say Hero looks like this:

class Hero : Player
{
    public override void Initialize()
    {
        // Do some stuff. Don't call base.Initialize()
    }
}

So we've overridden Initialize(), but haven't called base.Initialize(). We've also not overriden Draw(), so the Draw() from Player will be used when called on a Hero instance.

So let's consider when a Player player is constructed. Then player.characterList is also created, but its elements are null. Next, player.Initialize() is called, setting characterList[0] = new Hero(). Note that characterList[0] has its own characterList array as well.

The next thing to happen is characterList[0].Initialize(). This called the Initialize() override in Hero. This does not call Initialize() inside Player, so the characterList inside the Hero instance still has null elements.

Some time in the future, the Draw method is called in the Player. This, in turn, calls characterList[0].Draw(spriteBatch, texture). Since we didn't override the Draw method in Hero, this calls the same method, but on our Hero instance. But remember that the Hero's characterList still has null values? Well, the only thing to happen here is calling characterList[0].Draw(spriteBatch, texture), which as we now know, is calling Draw on a null value.

This is my guess as to what is happening. The details may vary, but I would imagine this sort of thing is the root cause. To be sure, check the stack trace of the exception. You'll notice two frames for the Draw method, letting you know that the exception occurred in a recursive call to Draw.

Answer 3

I've had this solution in mind for a while, but I didn't want to do it because of organizing reasons. All I had to do was to move characterList to the main game loop class and making it public, guaranteeing that it would be initialized before the draw method would be called. This did work.