这道题我想了好久还是想不出，希望大家帮帮忙，讲解下如何将大小写字母转换。
    编一个程序，定义一个字符变量，使用if else语句，输入一个字符，如果它是大写字母，则把它转换成小写字母，如果它是小写字母，则把它转换成大写字母，否则不转换。然后输出结果。
我是这样做的，不过运行是错的，大家看看那里错。
            char NUM;
            NUM = char.Parse(Console.ReadLine());
            if (NUM >= 'A' & NUM <= 'Z')
            {
                char num = Convert.ToChar(NUM);
                Console.WriteLine(NUM);
            }
            else if (NUM >= 'a' & NUM <= 'z')
            {
                char num = Convert.ToChar(NUM);
                Console.WriteLine(NUM);

            }
            else
                Console.WriteLine();

   这样应该可以！
            Console.WriteLine("请输入要转换的字母！");
            char NUM =Convert.ToChar(Console .ReadLine());
            if (char.IsLower(NUM))
            {
               Console.WriteLine( char.ToUpper(NUM));
            }
            else if (char.IsUpper(NUM))
            {
                Console.WriteLine(char.ToLower(NUM));              
            }
            Console.ReadLine();

            Console.WriteLine("请输入要转换的字母！");
            char NUM =Convert.ToChar(Console .ReadLine());
            NUM = char.Parse(Console.ReadLine());
            if (NUM >= 'A' & NUM <= 'Z')
            {
                if (char.IsLower(NUM))
                {
                Console.WriteLine( char.ToUpper(NUM));
                }
            }
            else if (NUM >= 'a' & NUM <= 'z')
            {
                else if (char.IsUpper(NUM))
                {
                Console.WriteLine(char.ToLower(NUM));              
                }
           }
            else{
                Console.ReadLine();}

直接用char.ToLower(NUM)就可以啊

3楼好

3楼的貌似有错吧
            Console.WriteLine("请输入字母！");
            char Num = char.Parse(Console.ReadLine());
            if (Num >= 65 && Num <= 90)
            {
                Console.WriteLine(Convert.ToChar(Num + 32));
            }
            if (Num >= 97 && Num <= 122)
            {
                Console.WriteLine(Convert.ToChar(Num - 32));
            }
            else
            {
                Console.ReadLine();               
            }