無法修改返回值

Question

我是C＃的新手，所以，如果這個問題有點傻，我會提前抱歉，當我嘗試運行程序時出現錯誤。從visual Studio 2012我得到這個錯誤信息 「無法修改'GameName1.Cartman.Position'的返回值，因爲它不是一個變量」2次，在30和31行。我希望有人可以幫助我這個。

這裏是代碼：

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 
    } 
    } 
} 

，這裏是我的其他類：

#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); 
     } 
    } 
} 

Answer 1

你不允許修改Vector2就像是（作爲錯誤指示）。下面是什麼工作：

Position = new Vector2(Texture.Width/2 * Scale, Texture.Width/2 * Scale); 
return new Rectangle(
      (int)(Position.X), 
      (int)(Position.Y), 
      (int)(Texture.Width * Scale), 
      (int)(Texture.Height * Scale) 
      ); 

注意，而不是試圖修改Vector2的成分，我只是創建一個新的，然後在長方形構造函數中使用它。 Lasse V. Karlsen的回答包含了一個很好的解釋爲什麼你不允許這樣做。

Answer 2

當類型是結構時，屬性的返回值不被視爲變量。

所以這個：

(int)(Position.X = Texture.Width/2 * Scale), 

嘗試一個新的值賦給調用Position屬性，但the Vector2 type is a struct, not a class的結果X財產。

這基本上告訴你，你試圖在腳下自己拍攝，因爲返回結構返回副本。你會，如果這個錯誤沒有彈出，修改副本。

換句話說，上面的代碼是與此類似：

var temp = Position; 
(int)(temp.X = Texture.Width/2 * Scale), 

但是，由於變量被視爲一個結構在這種情況下複印，「溫度」的變量並沒有真正存在這樣，因此編譯器會阻止你這樣做。

所以你需要讀出位置向量到一個變量，修改它，然後將整個變量分配回屬性。

有點像這樣：

Position = new Vector2(Texture.Width/2 * Scale, Texture.Width/2 * Scale); 
return new Rectangle(
    (int)Position.X, 
    (int)Position.Y, 
    (int)(Texture.Width * Scale), 
    (int)(Texture.Height * Scale) 
    );