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

namespace ConsoleApplication3
{
    class Program
    {
        static void Main(string[] args)
        {
            int a, b;
            Boolean flag;
          input:
            Console.WriteLine("请输入数A：");
            a = Convert.ToInt32(Console.ReadLine());
            Console.WriteLine("请输入数B：");
            b = Convert.ToInt32(Console.ReadLine());
            if (a > b)
            {
                Console.WriteLine("A>B");
                flag = true;

            }
            else
            {
                Console.WriteLine("A<B");
                flag = false;
            }
        }
    if (flag == false)
        {
        goto input;
        }
        }
    }
}这后面编译不通过，怎么错误啊，求解释

if (flag == false)
        {
        goto input;
        }
       }//这个括号多了去掉就好了

报错的时候记得看错误代码去解决问题。你定义的bool类型变量flag，与你最后一个if语句不在同一个作用域中。你这些代码，根本就不用flag变量，你仔细看一下，flag基本上没用。如果一定要用flag，为什么不定义变量时就赋一个false的值给它呢？
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
             int a, b;
            Boolean flag;
          input:
            Console.WriteLine("请输入数A：");
            a = Convert.ToInt32(Console.ReadLine());
            Console.WriteLine("请输入数B：");
            b = Convert.ToInt32(Console.ReadLine());
            if (a > b)
            {
                Console.WriteLine("A>B");
                flag = true;
                Console.ReadKey();

            }
            else
            {
                Console.WriteLine("A<B");
                flag = false;
                goto input;

            }
        }
        }
}

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
         
                bool flag = false;
            input:
                Console.WriteLine("请输入A的值");
                int a = int.Parse(Console.ReadLine());
                Console.WriteLine("请输入B的值");
                int b = int.Parse(Console.ReadLine());
                if (a > b)
                {
                    Console.WriteLine("您输入的值a>b，符合要求");
                    Console.ReadKey();
                }
                else if(a==b)
            {
            Console.WriteLine("您输入的值a=b，符合要求");
            Console.ReadKey();
            }
            else if(a < b)
            {
            Console.WriteLine("您输入的值因为a<b，所以请重新输入");
                goto input;

            }
        }
        }
}