关于C#扩展方法，得不到效果

Question

扩展方法

Answer 1

static class Program

Answer 2

扩展方法：
静态类里面的静态方法using System;
using System.Collections.Generic;namespace XXXXX.Extensions
{
    public static class StringExtensions
    {
        public static DateTime? ToNullableDateTime(this string obj)
        {
            if (string.IsNullOrEmpty(obj))
                return null;
            else
            {
                DateTime dateTime;
                if (DateTime.TryParse(obj, out dateTime))
                    return dateTime;
                else
                    return null;
            }
        }
}