用c#作一个英文字母的大小转换。。。。用到char  

using System;
namespace Text
{
    public class Demo
    {
        public static void Main()
        {
            try
            {
                Console.Write("请输入英文字母：");
                string words = Console.ReadLine();
                Console.Write("大写：{0}", words.ToUpper());
                Console.WriteLine();
                Console.Write("小写：{0}", words.ToLower());
            }
            catch(Exception)
            {
                Console.WriteLine("请输入英文字母！！！");
            }

        }
    }
}

using System;
namespace Text
{
    public class Demo
    {
        public static void Main()
        {
            try
            {
                Console.Write("请输入英文字母：");
                string st= Console.ReadLine();
                Console.WriteLine("大写：{0}", st.ToUpper());
                Console.Write("小写：{0}", st.ToLower());
            }
            catch(Exception)
            {
                Console.WriteLine("请输入英文字母！！！");
            }

        }
    }
}

private void button2_Click(object sender, EventArgs e)
        {   char a1;
            
            try
            {
                a1 = Convert.ToChar(textBox1.Text);
                if (char.IsDigit(a1))
                {
                    MessageBox.Show("你输入的是数字！！");
                    textBox1.Focus(); textBox1.Text = "";
                }
                else if (char.IsLetter(a1))
                {
                    MessageBox.Show("你输入的是字符！！");
                    textBox1.Focus(); textBox1.Text = "";
                }
                else
                {
                    MessageBox.Show("你输入的不对啊！！");
                    textBox1.Focus();
                    textBox1.Text = "";
                }

            }
            catch
            {
                MessageBox.Show("你输入的不是一个吧！！");
            }
        }

自己转换的话，可以这样写：
// 将字符c转换为大写
char upper = (char)(c >= 'a' && c <= 'z' ? c - 'a' + 'A' : c);    // 要进行强制转换
// 将字符c转换为小写
char lower = (char)(c >= 'A' && c <= 'Z' ? c - 'A' + 'a' : c);

            char ch;
            Console.WriteLine("请输入一个字符:");
            ch = char.Parse(Console.ReadLine());
            if (ch >= 'A' && ch <= 'Z')
                ch = (char)(ch + 32);
            Console.WriteLine("{0}",ch);
            Console.ReadLine();

            这个是将大写字母转换为小写字母的......