MSCaptcha Error: System.ArgumentNullException: Value cannot be null

Hi shailendratiwari444,

1. Use latest DLL.

Download the DLL from below link.

http://kirsanov.net/page/ASPNET-CAPTCHA.aspx

2. Set ErrorInputTooFast and ErrorInputTooSlow properties of Captcha control.

Refer below code.

protected void ValidateCaptcha(object sender, ServerValidateEventArgs e)
{
    e.IsValid = false;
    if (!string.IsNullOrEmpty(txtCaptcha.Text.Trim()))
    {
        captcha1.ErrorInputTooFast = "Image text was typed too quickly.";
        captcha1.ErrorInputTooSlow = "Image text was typed too slowly.";
        captcha1.ValidateCaptcha(txtCaptcha.Text.Trim());
        e.IsValid = captcha1.UserValidated;
        if (e.IsValid)
        {
            ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Valid Captcha!');", true);
        }
    }
}