class Singleton 
{
    private static readonly object _locker = new object();
    private static volatile Singleton _instance = null;

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

Взято с http://blogs.byte-force.com/xor/archive/2006/01/25...