Cách sử dụng Recaptcha trong web asp.net

<%@ Register TagPrefix="recaptcha" Namespace="Recaptcha" Assembly="Recaptcha" %>

<div>
         <recaptcha:RecaptchaControl
    ID="recaptcha"
    runat="server"
    PublicKey="6LckeNQSAAAAAFpwu4ywENBgCFsPYarPC0aGpFub"
    PrivateKey="6LckeNQSAAAAAHJDm6_cj8kSKiBFrgArARhCpUWH"
    Theme="white"
    />
              <asp:Label Visible="false" ID="lblResult" runat="server" />
    <asp:Button ID="btnSubmit" runat="server" Text="Submit" onclick="btnSubmit_Click" />
    </div>

 protected void Page_Load(object sender, EventArgs e)    {
        lblResult.Visible = false;
    }
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        lblResult.Visible = true;
        if (!recaptcha.IsValid)
        {
            lblResult.ForeColor = Color.Red;
            lblResult.Text = "Captcha is invalid.";
        }
        else
        {
            lblResult.ForeColor = Color.Green;
            lblResult.Text = "Captcha is valid";
        }
    }