字段初始值设定项不能引用非静态字段、方法或属性.1.

好吧，问题是我得到了两个代码，一个来自我正在读的书，另一个来自写的书。它们同样相同。让我们从使用的书中的代码开始

public class Game1 : Microsoft.Xna.Framework.Game
{        
         SpriteFont font;
         GraphicsDeviceManager graphics;
         SpriteBatch spriteBatch;
         DateTime nowDateTime = DateTime.Now;
         string nowString = nowDateTime.ToString();
    `   

这不会产生任何错误。。。现在对于代码，我把我自己从这本书上写下来了

public class Game1 : Microsoft.Xna.Framework.Game
{
    SpriteFont font;
    GraphicsDeviceManager graphics;
    SpriteBatch spriteBatch;
    DateTime nowDateTime = DateTime.Now;
    string nowString = nowDateTime.ToString();

完全一样的权利？还是我错过了一些重要的东西？我就是一点也不明白，我的错误就在题目里。

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;
namespace WindowsGame4
{
     /// <summary>
    /// This is the main type for your game
    /// </summary>
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        SpriteFont font;
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        DateTime nowDateTime = DateTime.Now;
        string nowString = nowDateTime.ToString();
        public Game1()
        {
            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
            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);
            font = Content.Load<SpriteFont>("Spritefont1");
            // TODO: use this.Content to load your game content here
        }
        /// <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)
        {
            // Allows the game to exit
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
                this.Exit();
            // TODO: Add your update logic here
            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.CornflowerBlue);
            DateTime nowDatetime = DateTime.Now;
            // TODO: Add your drawing code here
            base.Draw(gameTime);
        }
    }
}

public class Game1 : Microsoft.Xna.Framework.Game
{
    SpriteFont font;
    GraphicsDeviceManager graphics;
    SpriteBatch spriteBatch;
    DateTime nowDateTime;
    string nowString;
    public Game1()
    {
        nowDateTime = DateTime.Now;
        nowString = nowDateTime.ToString();
    }
}

string nowString = nowDateTime.ToString();

static DateTime nowDateTime = DateTime.Now;
string nowString = nowDateTime.ToString();