string device = getOSInfo();

public String getOSInfo()
{
    var ua = Request.UserAgent;

    if (ua.Contains("Android"))
        return string.Format("Android {0}", GetMobileVersion(ua, "Android"));

    if (ua.Contains("iPad"))
        return string.Format("iPad OS {0}", GetMobileVersion(ua, "OS"));

    if (ua.Contains("iPhone"))
        return string.Format("iPhone OS {0}", GetMobileVersion(ua, "OS"));

    if (ua.Contains("Linux") && ua.Contains("KFAPWI"))
        return "Kindle Fire";

    if (ua.Contains("RIM Tablet") || (ua.Contains("BB") && ua.Contains("Mobile")))
        return "Black Berry";

    if (ua.Contains("Windows Phone"))
        return string.Format("Windows Phone {0}", GetMobileVersion(ua, "Windows Phone"));

    if (ua.Contains("Mac OS"))
        return "Mac OS";

    if (ua.Contains("Windows NT 5.1") || ua.Contains("Windows NT 5.2"))
        return "Windows XP";

    if (ua.Contains("Windows NT 6.0"))
        return "Windows Vista";

    if (ua.Contains("Windows NT 6.1"))
        return "Windows 7";

    if (ua.Contains("Windows NT 6.2"))
        return "Windows 8";

    if (ua.Contains("Windows NT 6.3"))
        return "Windows 8.1";

    if (ua.Contains("Windows NT 10"))
        return "Windows 10";

    //fallback to basic platform:
    return (ua.Contains("Mobile") ? " Mobile " : "");
}
public String GetMobileVersion(string userAgent, string device)
{
    var temp = userAgent.Substring(userAgent.IndexOf(device) + device.Length).TrimStart();
    var version = string.Empty;

    foreach (var character in temp)
    {
        var validCharacter = false;
        int test = 0;

        if (Int32.TryParse(character.ToString(), out test))
        {
            version += character;
            validCharacter = true;
        }

        if (character == '.' || character == '_')
        {
            version += '.';
            validCharacter = true;
        }

        if (validCharacter == false)
            break;
    }

    return version;
}