C#中“Singleton” 模式四種實現方法

大家一定用過或者看過“Singleton”模式了吧。
下面給出三種實現方法，分別給出優缺點。

方法一：

public sealed class Singleton
{
   private static readonly Singleton instance = new Singleton();
   
   private Singleton(){}

   public static Singleton Instance
   {
      get
      {
         return instance;
      }
   }
}

優點：簡單明了
缺點：耗費資源

方法二：

public sealed class ClassicSingleton
{
    private static ClassicSingleton instance;
    private static object syncRoot = new Object();

    private ClassicSingleton() { }

    public static ClassicSingleton Instance
    {
        get
        {
            if (instance == null)
            {
                lock (syncRoot)
                {
                    if (instance == null)
                    {
                        //...custom code
                        instance = new ClassicSingleton();
                    }
                }
            }

            return instance;
        }
    }
}

優點：節省資源
缺點：代碼冗長

方法三：

public sealed class Singleton
{
    static Singleton(){Instance = new Singleton();}

    private Singleton(){}

    public static Singleton Instance{get; private set;}
}

優點：既節省資源，又簡單明了
缺點：線程不安全

方法四：

public class Singleton
{
    private static Singleton instance;
    // Added a static mutex for synchronising use of instance.
    private static System.Threading.Mutex mutex;
    private Singleton() { }
    static Singleton()
    {
        instance = new Singleton();
        mutex = new