单例模式实现 (Singleton)

第一个版本：

// Bad code! Do not use!
public sealed class Singleton
{
    private static Singleton instance=null;

    private Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            if (instance==null)
            {
                instance = new Singleton();
            }
            return instance;
        }
    }
}

这个用例是线程不安全的，两个不同线程完全可能同时进入if (instance==null)并进行判断得出结果都为True，此时两个线程都将会去创建各自的实例对象，这就跟单例模式违反了。 

第二个版本：

public sealed class Singleton
{
    private static Singleton instance = null;
    private static readonly object padlock = new object();

    Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            lock (padlock)
            {
                if (instance == null)
                {
                    instance = new Singleton();
                }
                return instance;
            }
        }
    }
}

第二个版本成功解决了第一个版本中存在的线程不安全的问题---加入了锁机制。但问题也随之而来，加入锁机制后，每次要获取instance都会先判断是否被锁，在这里，都讲成为单管道，效率明显减低。

第三个版本：

// Bad code! Do not use!
public sealed class Singleton
{
    private static Singleton instance = null;
    private static readonly object padlock = new object();

    Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            if (instance == null)
            {
                lock (padlock)
                {
                    if (instance == null)
                    {
                        instance = new Singleton();
                    }
                }
            }
            return instance;
        }
    }
}

为了解决第二个版本中的效率问题，在第三个版本中修改了锁机制，但这样真的就解决问题了吗？还真没有。具体见http://csharpindepth.com/Articles/General/Singleton.aspx#dcl

第四个版本：

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

    // Explicit static constructor to tell C# compiler
    // not to mark type as beforefieldinit
    static Singleton()
    {
    }

    private Singleton()
    {
    }

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

第四版本很好的取消掉了锁机制带来的问题，但还是没有解决懒惰加载的问题。

第五个版本：

public sealed class Singleton
{
    private Singleton()
    {
    }

    public static Singleton Instance { get { return Nested.instance; } }
        
    private class Nested
    {
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested()
        {
        }

        internal static readonly Singleton instance = new Singleton();
    }
}

更好的第六个版本：（使用4.0+）

public sealed class Singleton
{
    private static readonly Lazy<Singleton> lazy =
        new Lazy<Singleton>(() => new Singleton());
    
    public static Singleton Instance { get { return lazy.Value; } }

    private Singleton()
    {
    }
}

扩展的第七个版本：

/// <summary>
/// A base class for the singleton design pattern.
/// </summary>
/// <typeparam name="T">Class type of the singleton</typeparam>
public abstract class SingletonBase<T> where T : class
{
  #region Members

  /// <summary>
  /// Static instance. Needs to use lambda expression to construct an instance (since constructor is private).
  /// </summary>
  private static readonly Lazy<T> sInstance = new Lazy<T>(() => CreateInstanceOfT());

  #endregion

  #region Properties

  /// <summary>
  /// Gets the instance of this singleton.
  /// </summary>
  public static T Instance { get { return sInstance.Value; } }

  #endregion

  #region Methods

  /// <summary>
  /// Creates an instance of T via reflection since T's constructor is expected to be private.
  /// </summary>
  /// <returns></returns>
  private static T CreateInstanceOfT()
  {
    try
    {
      return Activator.CreateInstance(typeof(T), true) as T;
    }
    catch (MissingMethodException ex)
    {
      throw new TypeLoadException(
        string.Format(
          System.Globalization.CultureInfo.CurrentCulture,
          "The type '{0}' must have a private constructor to be used in the Singleton pattern.",
          typeof(T).FullName),
          ex);
    }//end try/catch
  }

  #endregion
} 

参考：

http://csharpindepth.com/Articles/General/Singleton.aspx

http://www.codeproject.com/Articles/572263/A-Reusable-Base-Class-for-the-Singleton-Pattern-in