XNA 4.0 - Transparent 3D model

Question

I have a little problem with my 3D model (.fbx) rendered in XNA, look at screenshot.

Screenshot:

It should be a lego brick, which was modelated in Blender, then exported to .fbx and added to XNA project. My problem is transparency, I tried so much with depthbuffer, z-buffer etc., but nothing helped. Here's my code:

namespace Using3DModels
{
/// <summary>
/// This is the main type for your game
/// </summary>
public class Game1 : Microsoft.Xna.Framework.Game
{
    GraphicsDeviceManager graphics;
    SpriteBatch spriteBatch;


    float myModelRotation;
    Vector3 myModelPosition;
    /// <summary>
    /// Stores the model that we are going to draw.
    /// </summary>
    private Model model;

    /// <summary>
    /// Stores the world matrix for the model, which transforms the 
    /// model to be in the correct position, scale, and rotation
    /// in the game world.
    /// </summary>
    private Matrix world = Matrix.CreateTranslation(new Vector3(6, 4, 0));

    /// <summary>
    /// Stores the view matrix for the model, which gets the model
    /// in the right place, relative to the camera.
    /// </summary>
    private Matrix view = Matrix.CreateLookAt(new Vector3(30, 25, -10), new Vector3(5, 5, 0), Vector3.UnitY);

    /// <summary>
    /// Stores the projection matrix, which gets the model projected
    /// onto the screen in the correct way.  Essentially, this defines the
    /// properties of the camera you are using.
    /// </summary>
    private Matrix projection = Matrix.CreatePerspectiveFieldOfView(MathHelper.ToRadians(45), 800f / 480f, 0.1f, 100f);


    public Game1()
    {
        graphics = new GraphicsDeviceManager(this);
        Content.RootDirectory = "Content";
        graphics.PreferMultiSampling = true;
    }

    /// <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()
    {
        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);

        model = Content.Load<Model>("lego6");

        myModelPosition = Vector3.Zero;
        myModelRotation = MathHelper.ToRadians(45f);
    }

    /// <summary>
    /// UnloadContent will be called once per game and is the place to unload
    /// all content.
    /// </summary>
    protected override void UnloadContent()
    {
    }

    /// <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();

        myModelRotation += MathHelper.ToRadians(1f);

        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)
    {
        GraphicsDevice.BlendState = BlendState.Opaque;
        GraphicsDevice.DepthStencilState = DepthStencilState.Default;
        GraphicsDevice.SamplerStates[0] = SamplerState.LinearWrap;



        GraphicsDevice.Clear(Color.DarkGray);

        DrawModel(model, world, view, projection);

        base.Draw(gameTime);
    }

    /// <summary>
    /// Does the work of drawing a model, given specific world, view, and projection
    /// matrices.
    /// </summary>
    /// <param name="model">The model to draw</param>
    /// <param name="world">The transformation matrix to get the model in the right place in the world.</param>
    /// <param name="view">The transformation matrix to get the model in the right place, relative to the camera.</param>
    /// <param name="projection">The transformation matrix to project the model's points onto the screen correctly.</param>
    private void DrawModel(Model model, Matrix world, Matrix view, Matrix projection)
    {
        GraphicsDevice.DepthStencilState = DepthStencilState.Default;

        Matrix[] transforms = new Matrix[model.Bones.Count];
        model.CopyAbsoluteBoneTransformsTo(transforms);


        foreach (ModelMesh mesh in model.Meshes)
        {
            foreach (BasicEffect effect in mesh.Effects)
            {
                effect.World = transforms[mesh.ParentBone.Index] * Matrix.CreateRotationY(myModelRotation) * Matrix.CreateTranslation(myModelPosition);
                effect.View = view;
                effect.Projection = projection;

            }

            mesh.Draw();
        }
    }

}
}

Answer 1

This looks like a culling issue, like Trevor Powell pointed out. A simple way to test if it really is a problem related to culling is to turn it off like this:

RasterizerState rasterizerState = new RasterizerState();
rasterizerState.CullMode = CullMode.None;
GraphicsDevice.RasterizerState = rasterizerState;

Please note that disabling culling is not the best idea (except for some corner cases where you want to achieve some special effect) for a shipping product, because it also draws triangles not facing the camera and thus slowing down the drawing.