Question

I’m using C# and XNA 4.0, as well as Farseer Physics Engine (very similar to Box2D), and have a Block class from which I derive OBlock, LBlock, etc.

Block is as follows:

class Block
{
    public Body m_body;
    public virtual void Draw(SpriteBatch spriteBatch) { }

    public virtual void RemoveBody(World world)
    {
        //world.RemoveBody(m_body);
    }
}

I’ve only put those methods, fields, etc in so I can access their overridden versions in a List

So my overridden versions look something like this:
OBlock.cs

class OBlock : Block
{

    private static Texture2D blockImg; //I load this in LoadContent so I don't have loads of Texture2Ds
    public new Body m_body; //Is this right?

    public OBlock(World world, Vector2 position)
    {
        m_body = BodyFactory.CreateBody(world, position); // Create the body, changing it from null
        FixtureFactory.AttachRectangle(Game1.blockSide *2, Game1.blockSide *2, 1.0f, new Vector2(0, 0), m_body); //This bit changes between classes
        m_body.BodyType = BodyType.Dynamic;

    }

    public override void RemoveBody(World world)
    {
        world.RemoveBody(m_body);
    }

    public static void LoadImage(Texture2D tex)
    {
        OBlock.blockImg = tex;
    }

    public override void Draw(SpriteBatch spriteBatch)
    {

        Vector2 position = m_body.Position * Game1.MetreInPixels;
        Vector2 origin = new Vector2(blockImg.Width / 2, blockImg.Height / 2);

        float rotation = m_body.Rotation;

        spriteBatch.Begin();

        spriteBatch.Draw(blockImg, position, null, Color.White, rotation, origin, Game1.BLOCK_SCALE, SpriteEffects.None, 1);

        spriteBatch.End();

        base.Draw(spriteBatch);
    }
}

There’s also LBlock, ZBlock, etc which all look very similar apart from the bit I commented.

I then have them all in

List<Block> blocks //As a field in Game1

blocks = new List<Block>(); // In LoadContent after loading images

What I’m trying to do is access m_body for any Block in the list regardless of type using

blocks[index].m_body.DOSTUFF();

Apparently m_body is always null…

Answer 1

public new Body m_body;

This is not right. The m_body from your Block class (which you should mark abstract btw) is visible in all classes deriving from it. What you’re doing right now is called hiding (which should always be avoided imo), which causes various complications.

You’ve now made it so that each OBlock has two members m_body, one belonging to Block, one belonging to OBlock. It makes it so that any time you reference this.m_body in your OBlock class, you’re assigning the m_body field belonging to OBlock, leaving m_block declared in Block alone. This is one of the weird complication that comes with hiding members:

Block myBlock = new OBlock();
// myBlock.m_block is null, because myBlock is of type block, and remember,
// the m_block belonging to Block is never assigned to, only the one belonging
// to OBlock

Whereas this would work as expected:

OBlock myOBlock = new OBlock();
// myOBlock.m_block isn't null, because it was assigned to in the constructor.

So avoid hiding! Remove

public new Body m_body;

from your code file and you’re good to go, because m_block declared in Block is inherited by all classes deriving from it anyway.