Singleton Pattern Framework Code

Keywords: C#, Design Pattern, Singleton

The following code block is the most simple, however it is thread-safe. Refer to http://msdn2.microsoft.com/en-us/library/ms954629.aspx

// .NET Singleton
sealed class Singleton 
{
    private Singleton() {}
    public static readonly Singleton Instance = new Singleton();
}

some more complex implemention are as follows,

/// <summary>

/// Class implements singleton pattern.

/// </summary>

public class Singleton

{

    // Private constructor to avoid other instantiation

    // This must be present otherwise the compiler provide 

    // a default public constructor

    private Singleton()

    {

    }



    /// <summary>

    /// Return an instance of <see cref="Singleton"/>

    /// </summary>

    public static Singleton Instance

    {

        get

        {

            /// An instance of Singleton wont be created until the very first 

            /// call to the sealed class. This a CLR optimization that ensure that

            /// we have properly lazy-loading singleton. 

            return SingletonCreator.CreatorInstance;

        }

    }



    /// <summary>

    /// Sealed class to avoid any heritage from this helper class

    /// </summary>

    private sealed class SingletonCreator

    {

      // Retrieve a single instance of a Singleton

      private static readonly Singleton _instance = new Singleton();



      /// <summary>

      /// Return an instance of the class <see cref="Singleton"/>

      /// </summary>

      public static Singleton CreatorInstance

      {

        get { return _instance; }

      }

    }

}

上面的代码没有考虑singleton对象的线程安全问题. 在用VS2005向导生成Settings类和Resource类的时候, 你会注意到Setting类需要考虑线程同步问题, 因为程序可能不仅仅会读取外部的config文件, 也有可能会修改config文件的信息. 而Resource类不需要考虑线程同步的问题, 因为一般程序不会修改Resource.res的文件.

那么怎样为singleton类增加线程同步问题呢? 查看Settings.Designer.cs文件, 会发现在创建defaultInstance时候, 用了下面的代码:

private static Settings defaultInstance = ((Settings)(global::System.Configuration.ApplicationSettingsBase.Synchronized(new Settings())));

通过Reflector工具, 可以看到ApplicationSettingsBase.Synchronized()方法是个static方法, 传入的参数就是一个Settings类的实体, 这个方法仅仅是将settings对象的 IsSynchronized属性设置为True, 再看一下ApplicationSettingsBase类的Save()方法, 读取的方法和Save()方法类似. 就明白它是怎样完成线程同步的.

//ApplicationSettingsBase class

public virtual void Save()

{

    if (this.IsSynchronized)

    {

        lock (this)

        {

            this.SaveCore();

            return;

        }

    }

    this.SaveCore();

}