double a = Convert.ToDouble(textBox1.Text);
double b = Convert.ToDouble(textBox2.Text);
string s = Convert.ToString(comboBox1.Text);
if (s == "+")
textBox3.Text = Convert.ToString(a + b);
else if(s == "-")
textBox3.Text = Convert.ToString(a - b);
else if(s == "*")
textBox3.Text = Convert.ToString(a * b);
else if(s == "/")
textBox3.Text = Convert.ToString(a / b);
代码就是这些 在窗体添加3个textbox 一个combobox一个按钮就可以