---

private void button1_Click(object sender, EventArgs e)
{
    double a = Convert.ToDouble(textBox1.Text);
    double b = Convert.ToDouble(textBox2.Text);

    switch (comboBox1.Text)
    {
        case "+":
            textBox3.Text = Convert.ToString(a + b);
            break;
        case "-":
            textBox3.Text = Convert.ToString(a - b);
            break;
        case "*":
            textBox3.Text = Convert.ToString(a * b);
            break;
        case "/":
            if (b == 0)
                MessageBox.Show("На ноль делить нельзя!", "Ошибка");
            textBox3.Text = Convert.ToString(a / b);
            break;
    }
}

private void button2_Click(object sender, EventArgs e)
{
    textBox1.Text = "";
    textBox2.Text = "";
    textBox3.Text = "";
    comboBox1.Text = "";
}