How to marshall an array of structs?

Question

using System;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{
    class Program
    {
        const int SystemPowerInformation = 11;
        const uint STATUS_SUCCESS = 0;

        [StructLayout(LayoutKind.Sequential)]
        struct PROCESSOR_POWER_INFORMATION
        {
            public uint  Number;
            public uint MaxMhz;
            public uint CurrentMhz;
            public uint MhzLimit;
            public uint MaxIdleState;
            public uint CurrentIdleState;
        }

        [DllImport("powrprof.dll")]
        static extern uint CallNtPowerInformation(
            int InformationLevel,
            IntPtr lpInputBuffer,
            int nInputBufferSize,
            [MarshalAs(UnmanagedType.LPArray)]
            out byte[]  lpOutputBuffer,
            int nOutputBufferSize
        );

        static void Main(string[] args)
        {


            byte[] buffer = new byte[4 * Marshal.SizeOf(typeof(PROCESSOR_POWER_INFORMATION))];
            uint retval = CallNtPowerInformation(
                SystemPowerInformation,
                IntPtr.Zero,
                0,
                out  buffer,
                4 * Marshal.SizeOf(typeof(PROCESSOR_POWER_INFORMATION))
            );

            if (retval == STATUS_SUCCESS)
                Console.WriteLine(buffer);
       }
    }
}

I am trying to get some data out of CallNtPowerInformation. I tried to create a struct and call CallNtPowerInformation and marshal the data from it, but that didn't work. So I am trying to see if I can get the data into a byte array, but I get the following:

Object reference not set to an instance of an object.

I believe I am allocating the memory to the buffer.

I am not sure why. Any pointers would be helpful.

Answer 1

Your constant named SystemPowerInformation with value 11 has the wrong name. It should be named ProcessorInformation.

You should declare the p/invoke like this:

[DllImport("powrprof.dll")]
static extern uint CallNtPowerInformation(
    int InformationLevel,
    IntPtr lpInputBuffer,
    int nInputBufferSize,
    [Out] PROCESSOR_POWER_INFORMATION[] processorPowerInformation,
    int nOutputBufferSize
);

In order to call the function you need to allocate a suitably sized array of PROCESSOR_POWER_INFORMATION structs. Like this:

PROCESSOR_POWER_INFORMATION[] powerInfo = 
    new PROCESSOR_POWER_INFORMATION[procCount];

The documentation for CallNtPowerInformation tells you to use GetSystemInfo to work out how many processors you have. You can use Environment.ProcessorCount.

Then you call the function like this:

uint retval = CallNtPowerInformation(
    ProcessorInformation,
    IntPtr.Zero,
    0,
    powerInfo,
    powerInfo.Length*Marshal.SizeOf(typeof(PROCESSOR_POWER_INFORMATION))
);

Here's a complete program:

using System;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{
    class Program
    {
        const int ProcessorInformation = 11;
        const uint STATUS_SUCCESS = 0;

        [StructLayout(LayoutKind.Sequential)]
        struct PROCESSOR_POWER_INFORMATION
        {
            public uint Number;
            public uint MaxMhz;
            public uint CurrentMhz;
            public uint MhzLimit;
            public uint MaxIdleState;
            public uint CurrentIdleState;
        }

        [DllImport("powrprof.dll")]
        static extern uint CallNtPowerInformation(
            int InformationLevel,
            IntPtr lpInputBuffer,
            int nInputBufferSize,
            [Out] PROCESSOR_POWER_INFORMATION[] lpOutputBuffer,
            int nOutputBufferSize
        );

        static void Main(string[] args)
        {
            int procCount = Environment.ProcessorCount;
            PROCESSOR_POWER_INFORMATION[] procInfo =
                new PROCESSOR_POWER_INFORMATION[procCount]; 
            uint retval = CallNtPowerInformation(
                ProcessorInformation,
                IntPtr.Zero,
                0,
                procInfo,
                procInfo.Length * Marshal.SizeOf(typeof(PROCESSOR_POWER_INFORMATION))
            );
            if (retval == STATUS_SUCCESS)
            {
                foreach (var item in procInfo)
                {
                    Console.WriteLine(item.CurrentMhz);
                }
            }
        }
    }
}