Test:
public class TestClass
.Instance.Write();

 //另一个实现singleton的方法,不用约束的。
 public static class Singleton<T>
    {
        private static T instance;
        public static T NewInstance()
        {
            if (instance == null)
            {
                instance = (T) Activator.CreateInstance(typeof(T), true);
            }
            return instance;
        }
    }