Web ASP.NET C#简单计算器

<div>
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:DropDownList ID="DropDownList1" runat="server">
            <asp:ListItem>选择</asp:ListItem>
            <asp:ListItem>+</asp:ListItem>
            <asp:ListItem>-</asp:ListItem>
            <asp:ListItem>*</asp:ListItem>
            <asp:ListItem>/</asp:ListItem>
            <asp:ListItem>%</asp:ListItem>
        </asp:DropDownList>
        <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>=<asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
        <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />     
    </div>

protected void Button1_Click(object sender, EventArgs e)
    {
        double a = Convert.ToDouble(TextBox1.Text);
        double b = Convert.ToDouble(TextBox2.Text);
        string c = DropDownList1.Text;
        double d = 0;
        d = Class1.Switch(a, b, c, d);
        TextBox3.Text = Convert.ToString(d);      
    }

public class Class1
{
    //四则运算
    public static double Switch(double a, double b, string c, double d)
    {
        switch (c)
        {
            case "+":
                d = a + b;
                break;
            case "-":
                d = a - b;
                break;
            case "*":
                d = a * b;
                break;
            case "/":
                d = a / b;
                break;
            case "%":
                d = a % b;
                break;
            default:
                break;
        }
        return d;
    }
}