的返回值不能修改

我是c#的新手，所以，如果这个问题有点傻，我提前抱歉，当我尝试运行程序时，我会得到一个错误。从visual Studio 2012我得到了这个错误信息"无法修改GameName1.Cartman的返回值。定位"因为它不是一个变量"2次，在第30行和31行。我希望有人能帮我解决这个问题。

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace GameName1
{
    class Cartman
    {
        public Vector2 Position { get; set; }
        public float Scale { get; set; }
        public int Direction { get; set; }
        public float Rotation { get; set; }
        public Texture2D Texture { get; set; }
    public Vector2 Origin {
        get
        {
            return new Vector2(Texture.Width / 2, Texture.Height / 2);
        } 
    }
    public Rectangle BoundingBox
    { 
        get 
        {
            return new Rectangle(
                (int)(Position.X = Texture.Width / 2 * Scale), // line 30
                (int)(Position.Y = Texture.Width / 2 * Scale), // line 31
                (int)(Texture.Width * Scale),
                (int)(Texture.Height * Scale)
                );
        }
    }
    public Cartman()
    {
        Direction = 1; //standaard naar rechts
    }
    }
}

这是我的另一个类:

#region Using Statements
using System;
using System.Collections.Generic;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Storage;
using Microsoft.Xna.Framework.GamerServices;
#endregion
namespace GameName1
{
    /// <summary>
    /// This is the main type for your game
    /// </summary>
    public class Game1 : Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        private Cartman _cartman;
        public Game1()
            : base()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }
        /// <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()
        {
            // TODO: Add your initialization logic here
            _cartman = new Cartman();
            _cartman.Position = new Vector2(100, 100);
            _cartman.Scale = 0.3f;
            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);
            // TODO: use this.Content to load your game content here
            _cartman.Texture = Content.Load<Texture2D>("cartman");
        }
        /// <summary>
        /// UnloadContent will be called once per game and is the place to unload
        /// all content.
        /// </summary>
        protected override void UnloadContent()
        {
            // TODO: Unload any non ContentManager content here
        }
        /// <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)
        {
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed || Keyboard.GetState().IsKeyDown(Keys.Escape))
                Exit();
            // TODO: Add your update logic here
            _cartman.Position = new Vector2(_cartman.Position.X + 1 *_cartman.Direction, _cartman.Position.Y);
            _cartman.Rotation += 0.3f * _cartman.Direction;
            if(!GraphicsDevice.Viewport.Bounds.Contains(_cartman.BoundingBox))
            {
                _cartman.Direction *= -1;
            }
            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.Clear(Color.Orange);
            // TODO: Add your drawing code here
            spriteBatch.Begin();
            spriteBatch.Draw(   _cartman.Texture,
                                _cartman.Position,
                                null,
                                Color.White,
                                _cartman.Rotation,
                                new Vector2 (0,0),
                                _cartman.Scale,
                                SpriteEffects.None,
                                0);
            spriteBatch.End();
            base.Draw(gameTime);
        }
    }
}