c# 将字符串转换为大写或小写

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace 将字符串转换为大写小写
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            label1.Text = "输入字符串";
            label2.Text = "转换后的字符串";
            button1.Text = "确认转换";
            radioButton1.Text = "大写";
            radioButton2.Text = "小写";
            radioButton1.Checked = true;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (textBox1.Text == " ")            //判断文本框是否为空
            {
                MessageBox.Show("请输入字符串");
            }
            if(radioButton1.Checked==true)        
            {
                textBox2.Text = textBox1.Text.ToUpper();            //ToUpper()方法将小写字母转换成大写字母
            }
            else 
            {
                textBox2.Text = textBox1.Text.ToLower();           //ToUpper()方法将大写字母转换成小写字母
            }
        }

        private void textBox1_KeyPress(object sender, KeyPressEventArgs e)        //用到了textbox的keypress事件，用来判断输入的是否为想要的字母，如果不是，将禁止输入
        {
            if ((e.KeyChar >= '1' && e.KeyChar <= '9' )||( e.KeyChar >= 'a' && e.KeyChar <= 'z' )|| (e.KeyChar >= 'A' && e.KeyChar <= 'Z'))
            {
                e.Handled = false;        //e.handled=false表示没有处理过，同意输入
            }
            else
            {
                e.Handled = true;        //e.handled=false表示处理过了，禁止输入
            }
        }
    }
}

运行结果