namespace WebApplication1
{
    public partial class DetailsForm : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }``

        [WebMethod]
        public static string InsertMethod(string firstname, string lastname)
        {
            SqlConnection con = new SqlConnection(@"Data Source=KIMBERLYSQLSERVER02;Initial Catalog=Chalegh;User ID=***;Password=***");
            SqlCommand cmd = new SqlCommand("insert into TestTable values('"+ firstname +"','" + lastname +"')", con);
            con.Open();
            cmd.ExecuteNonQuery();
            return "True";
        }

        public static string GenerateVoucher(int length)
        {
            char[] CharArr = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();
            string randomString = string.Empty;
            Random objRandom = new Random();
            return randomString;


        }
    }
}

using (var con =new SqlConnection(yourConnectionString))
{
    SqlCommand cmd = new SqlCommand("insert into TestTable values('@firstName', '@lastName')", con);
    SqlParameter paramFirstName= new SqlParameter();
    paramFirstName.ParameterName = "@firstName";
    paramFirstName.Value         = firstName;

    SqlParameter paramLastName= new SqlParameter();
    paramLastName.ParameterName = "@lastName";
    paramLastName.Value         = lastName;

    cmd.Parameters.Add(paramFirstName);
    cmd.Parameters.Add(paramLastName);
    con.Open();
    cmd.ExecuteNonQuery();
}

String randomString = Guid.NewGuid().ToString("N").Substring(0, 6);

lblVoucherNumber.Text = randomString;

String randomString = Guid.NewGuid().ToString("N");

using(SqlConnection con = new SqlConnection(@"Data Source=KIMBERLYSQLSERVER02;Initial     Catalog=Chalegh;User ID=***;Password=***"))
 {
   using(SqlCommand cmd = new SqlCommand("insert into TestTable values(@firstname,@lastname)", con))
   {
     con.Open();
     int status = cmd.ExecuteNonQuery();
     if(status>0)
        return "True";

        return "False";
    }
  }