c#中不能访问嵌套类中的非静态成员

namespace DateTimeExpress
{
    public class Today
    {
        private DateTime _CurrentDateTime;

        public class Month 
        {
            public int LastDayOfCurrentMonth
            {
                get
                {
                    return DateTime.DaysInMonth( _CurrentDateTime.Year, _CurrentDateTime.Month);
                }
            }
        }
        public Today()
        {
        }
    }
}

你可以在c#编程指南的嵌套类型段落中看到一个解释你的问题的例子，他们说:

嵌套类型或内部类型可以访问包含类型或外部类型。来访问包含类型，将其作为构造函数传递给嵌套的类型。

本质上，您需要在嵌套类(Month类)的构造函数中传递对容器(Today类)的引用

public class Today
{
    private DateTime _CurrentDateTime;
    private Month _month;
    public int LastDayOfCurrentMonth { get { return _month.LastDayOfCurrentMonth; }}
    // You can make this class private to avoid any direct interaction
    // from the external clients and mediate any functionality of this
    // class through properties of the Today class. 
    // Or you can declare a public property of type Month in the Today class
    private class Month
    {
        private Today _parent;
        public Month(Today parent)
        {
            _parent = parent;
        }
        public int LastDayOfCurrentMonth
        {
            get
            {
                return DateTime.DaysInMonth(_parent._CurrentDateTime.Year, _parent._CurrentDateTime.Month);
            }
        }
    }
    public Today()
    {
        _month = new Month(this);
        _CurrentDateTime = DateTime.Today;
    }
    public override string ToString()
    {
        return _CurrentDateTime.ToShortDateString();
    }
}

然后像这样调用

Today y = new Today();
Console.WriteLine(y.ToString());
Console.WriteLine(y.LastDayOfCurrentMonth);