http://martinfowler.com/articles/injection.html

http://www.cnblogs.com/zhouyinhui/archive/2008/07/07/1237613.html

Tame Your Software Dependencies for More Flexible Apps

http://msdn2.microsoft.com/en-us/magazine/cc337885.aspx

Implementing the Singleton Pattern in C#

http://www.yoda.arachsys.com/csharp/singleton.html

Five implementations are given. The author also compared their pros and cons.

Bestone is:

public sealed class Singleton
{
    Singleton()
    {
    }

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

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