单例模式

一般用法

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

	static Singleton() { }
	private Singleton() { }
	public static Singleton Instance
	{
		get { return _instance; }
	}
}

例外1：需要在不触发初始化的情况下调用其他静态方法；
例外2：需要知道是否已经实例化了。

内部类

/// <summary>
/// 内部类，完全延迟实例化
/// </summary>
public sealed class Singleton2
{
	private Singleton2() { }
	public static Singleton2 Instance { get { return Nested._instance; } }

	private class Nested
	{
		static Nested() { }
		internal static readonly Singleton2 _instance = new Singleton2();
	}
}

.NET 4 (or higher)

/// <summary>
///  推荐使用单例
/// </summary>
public sealed class Singleton3
{
	private static readonly Lazy<Singleton3> _lazy =
		new Lazy<Singleton3>(() => new Singleton3());
	private Singleton3() { }

	public static Singleton3 Instance { get { return _lazy.Value; } }
}