double转string的问题
textBox3.Text = Convert.ToInt32(textBox1.Text) * (0.003102 * 17.16 + 0.1);

无法将类型“double”隐式转换为“string”,

textbox1.Text，这个值怎么转？

[解决办法]
float f = Convert.ToInt32(textBox1.Text) * (0.003102 * 17.16 + 0.1);
textBox3.Text = f.ToString();
[解决办法]

C# code

if (comboBox1.Text == "新胶袋" && comboBox2.Text == "5.5cm"){  double f;  f= double.Parse(textBox1.Text) * (0.003102 * 17.16 + 0.1);  textBox3.Text=f.ToString();}