1 private void txtNumber1_KeyPress(object sender, KeyPressEventArgs e)
 2         {
 3             //MessageBox.Show(((int)(e.KeyChar)).ToString()); //ASCii查看
 4             //由于本事件可能有多个文本框的触发，所以要通过sender来得到触发该事件的控件
 5             TextBox currentTextBox = sender as TextBox;
 6 
 7             if (e.KeyChar < ‘0‘ || e.KeyChar > ‘9‘)
 8             {
 9                 e.Handled = true;
10             }
11             if (e.KeyChar == 46)//让用户可以输入小数点
12             {
13                 //检测这个number1文本框中是否已经包含小数点
14                 if (currentTextBox.Text.IndexOf(".") == -1)
15                 {
16                     //光标不能在第一位，也就是说小数点不能出现在第一位
17                     if (currentTextBox.SelectionStart > 0)
18                     {
19                         e.Handled = false;
20                     }
21                 }
22             }
23             if (e.KeyChar == 8)  //让用户可以输入退格键
24             {
25                 e.Handled = false;
26             }
27             
28         }
29 
30         private void btnResult_Click(object sender, EventArgs e)
31         {
32             try
33             {
34                 double number1 = Convert.ToDouble(txtNumber1.Text);
35                 double number2 = Convert.ToDouble(txtNumber2.Text);
36                  
37                 switch (comboBox1.Text)
38                 {
39                     case "加":
40                         lbResult.Text = (number1 + number2).ToString();
41                         break;
42                     case "减":
43                         lbResult.Text = (number1 - number2).ToString();
44                         break;
45                     case "乘":
46                         lbResult.Text = (number1 * number2).ToString();
47                         break;
48                     case "除":
49                         lbResult.Text = (number1 / number2).ToString();
50                         break;
51                 }
52             }
53             catch(Exception ex)
54             {
55                 MessageBox.Show("你的输入有问题"+ex.Message);
56             }
57         }
58 
59         private void txtNumber1_TextChanged(object sender, EventArgs e)
60         {
61             lbResult.Text = " "; 
62         }
63 
64         private void frmFourOperations_Load(object sender, EventArgs e)
65         {
66             comboBox1.SelectedIndex = 0; //当combobox的dropdownstyle属性设置为list时 将无法在属性写入初始值 需要通过该语句实现
67         }