C# Signature:

[DllImport("kernel32.dll")]

public static extern void GetSystemInfo()
     out SystemInfo input
);

public static SystemInfo GetSystemInfo()
{
     var stub = new SystemInfo();
     GetSystemInfo(out stub);
     return stub;
}

User-Defined Types:

    public enum ProcessorArchitecture
    {
        X86 = 0,
        X64 = 9,
        @Arm = -1,
        Itanium = 6,
        Unknown = 0xFFFF,
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct SystemInfo
    {
        public ProcessorArchitecture ProcessorArchitecture; // WORD
        [Obsolete("This field is not currently used in Windows API.")]
        private ushort Reserved; // WORD
        public uint PageSize; // DWORD
        public IntPtr MinimumApplicationAddress; // (long)void*
        public IntPtr MaximumApplicationAddress; // (long)void*
        public IntPtr ActiveProcessorMask; // DWORD*
        public uint NumberOfProcessors; // DWORD (WTF)
        [Obsolete("You should use the ProcessorArchitecture instead of ProcessorType to obtain the processor type.")]
        public uint ProcessorType; // DWORD
        public uint AllocationGranularity; // DWORD
        public ushort ProcessorLevel; // WORD
        public ushort ProcessorRevision; // WORD
    }

Notes:

This API mapping demonstrates mapping a union in C#.

Tips & Tricks:

lpMinimumApplicationAddress, lpMaximumApplicationAddress, dwActiveProcessorMask are pointers(first two was void pointer, while the third is a double word pointer) in kernel32.dll, so the size changes when used from within a 64-bit application. If you would like to use these, you will have to cast them to either an uint or a ulong.

Alternative Managed API:

To determine whether the OS or Process is x64, using .NET 4, see System.Environment (Is64BitProcess, Is64BitOperatingSystem); http://msdn.microsoft.com/en-us/library/z8te35sa.aspx

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