asp:Button ID="Button1" 怎么弹出确认对话框?

用户名：<asp:TextBox ID="name" runat="server"></asp:TextBox>
密 码：<asp:TextBox ID="accountPassword" runat="server"></asp:TextBox>
email：<asp:TextBox ID="email" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="注册" onclick="Button1_Click" />
怎么弹出让用户确认执行注册的对话框?
点否不执行任何操作,点是执行注册操作,转到另一个页面

    protected void Button1_Click(object sender, EventArgs e)
    {
        string name = this.name.Text.Trim().ToString();
        string accountPassword = this.accountPassword.Text.Trim().ToString();
        string email = this.email.Text.Trim().ToString();
        bool flag = new ECardAccountDAO().selectByEmail(email);
        if (flag)
        {
            this.lblMessage.Text = "该email已被使用！";
        }
        else
        {
            bool rs = new ECardAccountDAO().Insert(Guid.NewGuid().ToString(), name, email, accountPassword);
            if (!rs)
            {
                Session["name"] = name;
                Session["accountPassword"] = accountPassword;
                Session["email"] = email;
                Response.Redirect("registerSuccess.aspx");
            }
        }
    }