我再textBox1中定义个变量并且运算 后再button中想用这个变量进行判断 但是不行
请问是我变量定义错位置了么？
我后来把变量放在外面了但是还是不对
请问是该用令一个变量进行连接么？

        private void textBox1_TextChanged(object sender, EventArgs e)
        {
          int a = 0;
        int beijing = 18;
        int shanghai = 20;
        int tianjing = 11;
        int shenyang = 0;
         
            switch ("textBox1.Text")
            {
                case "北京":
                    a = beijing;
                    if (a < 20)
                        beijing++;
                    break;
                case "上海": a = shanghai;
                    if (a < 20)
                        shanghai++;
                    break;
                case "天津": a = tianjing;
                    if (a < 20)
                        tianjing++;
                    break;
                case "沈阳": a = shenyang;
                    if (a < 20)
                        shenyang++;
                    break;
                    b = a;
            }

        }
        private void button1_Click(object sender, EventArgs e)
        {
            if (a< 20)
就像这样 但是把a放到上面还是不行

还是不对提示我case "北京":
                    a = beijing
这个a是无法访问的代码