一个C#计算器源代码，不知道哪里出错了

namespace P8_3
{
    public partial class Form1 : Form
    {
        private string s;
        private double x, y;
        private Button btn;
        public Form1()
        {
            InitializeComponent();
        }        private void Form1_Load(object sender, EventArgs e)
        {
            textBox1.Text = "";
            label1.Text = "";        }
        private void buttond_Click(object sender, EventArgs e)
        {
            btn = (Button)sender;
            textBox1.Text = textBox1.Text+btn.Text;
        }
        private void buttonp_Click(object sender, EventArgs e)
        {
            btn = (Button)sender;
            if (btn.Name != "button12")
            {
                x = Convert.ToDouble(textBox1.Text);
                textBox1.Text = "";
                s = btn.Name;
                label1.Text = x.ToString();
            }
            else
            {
                if (label1.Text == "")
                    MessageBox.Show("输入不正确!!!", "信息提示", MessageBoxButtons.OK);
                else
                {
                    y = Convert.ToDouble(textBox1.Text);
                    switch (s)
                    {
                        case "button13":
                            textBox1.Text = (x + y).ToString();
                            break;
                        case "button14":
                            textBox1.Text = (x - y).ToString();
                            break;
                        case "button15":
                            textBox1.Text = (x * y).ToString();
                            break;
                        case "button16":
                            if (y == 0)
                                MessageBox.Show("除零错误!!!", "信息提示", MessageBoxButtons.OK);
                            else
                                textBox1.Text = (x / y).ToString();
                            break;
                    }
                    label1.Text = textBox1.Text;
                }
            }
        }        
    }        
}