c# 小练习

            double a = double.Parse( this.textBox1.Text);
            double b = double.Parse(this.textBox2.Text);
            double c =0;
            switch (this.comboBox1.Text)
            {
                case "+":
                    c = a + b;
                    break;
                case "-":
                    c = a - b;
                    break;
                case "*":
                    c = a * b;
                    break;
                case "/":
                    c = a / b;
                    break;

                default:
                    c =0;
                    break;
            }
            MessageBox.Show(c.ToString());

 double num1 = 0;
            double num2 = 0;
            if (!double.TryParse(this.textBox1.Text, out num1)) { 

                    MessageBox.Show("s");
                    return;
            }
            if (!double.TryParse(this.textBox2.Text, out num2)) {

                MessageBox.Show("s");
                return;
            }
            this.label1.Text = (num1 + num2).ToString();