В унифицированном API необходимо обновить импорт DLL.

Заменить его следующим образом:

[DllImport ("libc", CallingConvention = CallingConvention.Cdecl)]
internal static extern int sysctlbyname([MarshalAs(UnmanagedType.LPStr)] string property, // name of the property
                                            IntPtr output, // output
                                            IntPtr oldLen, // IntPtr.Zero
                                            IntPtr newp, // IntPtr.Zero
                                            uint newlen // 0
    );

Новая версия этого кода от 24 марта 2013 года:

using System;
using System.Runtime.InteropServices;
using MonoTouch.Foundation;
using MonoTouch.UIKit;

namespace YourNamespace
{
    public class IOSDeviceHardware
    {
        public const string HardwareProperty = "hw.machine";

        public enum IOSHardware {
            iPhone,
            iPhone3G,
            iPhone3GS,
            iPhone4,
            iPhone4RevA,
            iPhone4CDMA,
            iPhone4S,
            iPhone5GSM,
            iPhone5CDMAGSM,
            iPodTouch1G,
            iPodTouch2G,
            iPodTouch3G,
            iPodTouch4G,
            iPodTouch5G,
            iPad,
            iPad3G,
            iPad2,
            iPad2GSM,
            iPad2CDMA,
            iPad2RevA,
            iPadMini,
            iPadMiniGSM,
            iPadMiniCDMAGSM,
            iPad3,
            iPad3CDMA,
            iPad3GSM,
            iPad4,
            iPad4GSM,
            iPad4CDMAGSM,
            iPhoneSimulator,
            iPhoneRetinaSimulator,
            iPadSimulator,
            iPadRetinaSimulator,
            Unknown
        }

        [DllImport(MonoTouch.Constants.SystemLibrary)]
        static internal extern int sysctlbyname([MarshalAs(UnmanagedType.LPStr)] string property, IntPtr output, IntPtr oldLen, IntPtr newp, uint newlen);

        public static IOSHardware Version {
            get {
                var pLen = Marshal.AllocHGlobal(sizeof(int));
                sysctlbyname(IOSDeviceHardware.HardwareProperty, IntPtr.Zero, pLen, IntPtr.Zero, 0);

                var length = Marshal.ReadInt32(pLen);

                if (length == 0) {
                    Marshal.FreeHGlobal(pLen);

                    return IOSHardware.Unknown;
                }

                var pStr = Marshal.AllocHGlobal(length);
                sysctlbyname(IOSDeviceHardware.HardwareProperty, pStr, pLen, IntPtr.Zero, 0);

                var hardwareStr = Marshal.PtrToStringAnsi(pStr);

                Marshal.FreeHGlobal(pLen);
                Marshal.FreeHGlobal(pStr);

                if (hardwareStr == "iPhone1,1") return IOSHardware.iPhone;
                if (hardwareStr == "iPhone1,2") return IOSHardware.iPhone3G;
                if (hardwareStr == "iPhone2,1") return IOSHardware.iPhone3GS;
                if (hardwareStr == "iPhone3,1") return IOSHardware.iPhone4;
                if (hardwareStr == "iPhone3,2") return IOSHardware.iPhone4RevA;
                if (hardwareStr == "iPhone3,3") return IOSHardware.iPhone4CDMA;
                if (hardwareStr == "iPhone4,1") return IOSHardware.iPhone4S;
                if (hardwareStr == "iPhone5,1") return IOSHardware.iPhone5GSM;
                if (hardwareStr == "iPhone5,2") return IOSHardware.iPhone5CDMAGSM;

                if (hardwareStr == "iPad1,1") return IOSHardware.iPad;
                if (hardwareStr == "iPad1,2") return IOSHardware.iPad3G;
                if (hardwareStr == "iPad2,1") return IOSHardware.iPad2;
                if (hardwareStr == "iPad2,2") return IOSHardware.iPad2GSM;
                if (hardwareStr == "iPad2,3") return IOSHardware.iPad2CDMA;
                if (hardwareStr == "iPad2,4") return IOSHardware.iPad2RevA;
                if (hardwareStr == "iPad2,5") return IOSHardware.iPadMini;
                if (hardwareStr == "iPad2,6") return IOSHardware.iPadMiniGSM;
                if (hardwareStr == "iPad2,7") return IOSHardware.iPadMiniCDMAGSM;
                if (hardwareStr == "iPad3,1") return IOSHardware.iPad3;
                if (hardwareStr == "iPad3,2") return IOSHardware.iPad3CDMA;
                if (hardwareStr == "iPad3,3") return IOSHardware.iPad3GSM;
                if (hardwareStr == "iPad3,4") return IOSHardware.iPad4;
                if (hardwareStr == "iPad3,5") return IOSHardware.iPad4GSM;
                if (hardwareStr == "iPad3,6") return IOSHardware.iPad4CDMAGSM;

                if (hardwareStr == "iPod1,1") return IOSHardware.iPodTouch1G;
                if (hardwareStr == "iPod2,1") return IOSHardware.iPodTouch2G;
                if (hardwareStr == "iPod3,1") return IOSHardware.iPodTouch3G;
                if (hardwareStr == "iPod4,1") return IOSHardware.iPodTouch4G;
                if (hardwareStr == "iPod5,1") return IOSHardware.iPodTouch5G;

                if (hardwareStr == "i386" || hardwareStr=="x86_64")
                {
                    if (UIDevice.CurrentDevice.Model.Contains("iPhone"))
                    {
                        if(UIScreen.MainScreen.Scale > 1.5f)
                            return IOSHardware.iPhoneRetinaSimulator;
                        else
                            return IOSHardware.iPhoneSimulator;
                    }
                    else
                    {
                        if(UIScreen.MainScreen.Scale > 1.5f)
                            return IOSHardware.iPadRetinaSimulator;
                    else
                        return IOSHardware.iPadSimulator;
                    }
                }

                return IOSHardware.Unknown;
            }
        }
    }
}