using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        private string cc;
        public string abc(string cc)
        {
            string c = "龙";
            int d = 22;
            string a = "你的名字叫" + c;
            string b = "你的年龄是" + d + "岁";
            this.cc=a+b;
            if (d < 25)
            {
                cc = "你还年轻，请努力！";
                cc += cc;
            }
            else if (d > 25 && d <= 35)
            {
                cc = "人到中年，更加努力！";
                cc += cc;
            }
            else
            {
                cc="年老里吧！";
                cc += cc;
            }
            return cc;
        }
        static void Main(string[] args)
        {
            Console.WriteLine("{0}",cc);
            Console.Read();
        }
    }
}

 if (d < 25)
            {
                cc = "你还年轻，请努力！";
                cc += cc;
            }
            else if (d > 25 && d <= 35)
            {
                cc = "人到中年，更加努力！";
                cc += cc;
            }
            else
            {
                cc="年老里吧！";
                cc += cc;
            }
 static void Main(string[] args)
        {
            Console.WriteLine("{0}",cc);
            Console.Read();
        }


改为
string dd="";
  if (d < 25)
            {
                dd = "你还年轻，请努力！";
                cc += dd;
            }
            else if (d > 25 && d <= 35)
            {
                dd = "人到中年，更加努力！";
                cc += dd;
            }
            else
            {
                dd="年老里吧！";
                cc += dd;
            }
 static void Main(string[] args)
        {
            string res=abc();
            Console.WriteLine("{0}",res);
            Console.Read();
        }

得运行abc方法才可以的

使用了你的方法还是不行，不过在Main中实例化Program类再调用其abc方法就能运行了..

可以不是实例化你abc()方法改成静态的就可以了，你试试，main（）方法和abc()方法在同一个类中不用实例化

把abc方法改成静态方法也可以,这样就不用实列化对象了，在Main()应该这样写
        static void Main(string[] args)
        {
           Console.WriteLine(abc());
          Console.ReadLine();
        }
    }
}
另外abc()是带参数的，不用另外声明相同的变量