Asp.Net 3.5 with TLS1.2 issue with SecurityProtocol

started working on making sure a website work on Tls1.2, Asp.net website:3.5 frame work. found related answer- that we need to install the msupdate and need to change the os registry in order to change at os level, But to change at code level from 3.5 found work around, as suggested in the work around, added the code 1.ADDED TWO CLASSES IN THE CODE

SslProtocolsExtensions.cs

namespace System.Security.Authentication
{
    public static class SslProtocolsExtensions
    {
        public const SslProtocols Tls12 = (SslProtocols)0x00000C00;
        public const SslProtocols Tls11 = (SslProtocols)0x00000300;
    }
}

SecurityProtocolTypeExtensions.cs

namespace System.Net
{
    using System.Security.Authentication;
    public static class SecurityProtocolTypeExtensions
    {
        public const SecurityProtocolType Tls12 = (SecurityProtocolType)SslProtocolsExtensions.Tls12;
        public const SecurityProtocolType Tls11 = (SecurityProtocolType)SslProtocolsExtensions.Tls11;
        public const SecurityProtocolType SystemDefault = (SecurityProtocolType)0;
    }
}

AND ADDED THE Below code in the aspx page, which gives the following error. Servicepointmanager.securityprotocol does not exist in the current context ..have'nt found any direct answer, if have answer please share appreciated

 public const SslProtocols _Tls12 = (SslProtocols)0x00000C00;
        public const SecurityProtocolType Tls12 = (SecurityProtocolType)(0x300 | 0xc00);
        ServicePointManager.SecurityProtocol = (SecurityProtocolType)0x00000C00;