C#字符串大小写转换

		string s;
        private void button1_Click(object sender, EventArgs e)
        {
            s = textBox1.Text;
            string s1 = s.ToUpper();
            textBox1.Text = s1;
            
         }

        private void button2_Click(object sender, EventArgs e)
        {
            s = textBox1.Text;
            string s1 = s.ToLower();
            textBox1.Text = s1;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }