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？

不知所云
什么叫"在textBox1中定义变量"

        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

private int a = 0;        
private void textBox1_TextChanged(object sender, EventArgs e)
        {
        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)

这样呢?

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

我靠  错得还蛮典型的：

private int a = 0;        
private void textBox1_TextChanged(object sender, EventArgs e)
        {
        //定义的时候最好规范点  这么怎么一看都想试字符串 不是想是int
        int beijing = 18;
        int shanghai = 20;
        int tianjing = 11;
        int shenyang = 0;
         
            //第一个地方 "textBox1.Text"  
            switch ("textBox1.Text")
            {
                case "北京":
                    a = beijing;
                    //if 哪怕只有一句话 至少也要把他{}起来  而且需要对齐
                    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;
                    //无法访问的语句
                    b = a;
            }

        }

可以不用{}的吧

也可以不用 但从开发规范来讲 最好是养成一个好的习惯，可以参考下 高质量C/C++这本书  现在网络上也有很多关于C#的