private void MainForm_Load(object sender, EventArgs e)
  {
   comboBox1.SelectedIndex = 0;
   comboBox2.SelectedIndex = 1;
   comboBox3.SelectedIndex = 0;
   comboBox4.SelectedIndex = 1;
   comboBox5.SelectedIndex = 0;
   comboBox6.SelectedIndex = 1;
   comboBox7.SelectedIndex = 0;
   comboBox8.SelectedIndex = 1;
  }

  private void button4_Click(object sender, EventArgs e)
  {
   string str1, str2;
   str1=Convert.ToString(comboBox7.SelectedItem);
   str2=Convert.ToString(comboBox8.SelectedItem);
   double d1, d2;
   if (textBox7.Text == "")
   {
    textBox7.Text = "1";
    d1 = 1;
   }
   else
    d1 = Convert.ToDouble(textBox7.Text);
   if (str1 == str2)
   {
    d2 = d1;
    textBox8.Text = Convert.ToString(d2);
   }
   else
   {
    if(str1 == "摄氏度" && str2 == "华氏度")
    {
     d2=1.8*d1+32;
     textBox8.Text = Convert.ToString(d2);
    }
    if(str1 == "摄氏度" && str2 == "开氏度")
    {
     d2=d1+273.15;
     textBox8.Text = Convert.ToString(d2);
    }
    if(str1 == "华氏度" && str2 == "摄氏度")
    {
     d2=(d1-32)/1.8;
     textBox8.Text = Convert.ToString(d2);
    }
    if(str1 == "华氏度" && str2 == "开氏度")
    { 
     d2=(d1-32)/1.8+273.15;
     textBox8.Text = Convert.ToString(d2);
    }
    if (str1 == "开氏度" && str2 == "摄氏度")
    {
     d2 = d1 - 273.15;
     textBox8.Text = Convert.ToString(d2);
    }
    if (str1 == "开氏度" && str2 == "华氏度")
    {
     d2 = (d1 - 273.15) * 1.8 + 32;
     textBox8.Text = Convert.ToString(d2);
    }
   }
  }

  private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
  {
   if (e.KeyChar != '\b' && e.KeyChar != 46)//允许输入退格键和小数点键
   {
    if ((e.KeyChar < '0') || (e.KeyChar > '9'))//允许输入0-9数字 
    {
     e.Handled = true;
    }
   } 
  }