Calling un-managed code with pointer

Question

I have a C# project that makes calls out to an unmanaged C++ dll. The wrapper and most of the calls are working OK, so I know that I have the basic structure of how everything ties together OK, but there is one specific call that is giving me fits. The API call requires a pointer to a structure, which contains a list of configuration data.

Here is the call:

m_status = m_XXXXBox.SetConfig(m_channelId, ref SCONFIG_LIST);

Where SCONFIG_LIST is the structure containing the data...

The issue specifically relates to SCONFIG_LIST

Here is the documentation directly from the spec for this API :

Points to the structure SCONFIG_LIST, which is defined as follows:
typedef struct
{
unsigned long NumOfParams; /* number of SCONFIG elements */
SCONFIG *ConfigPtr; /* array of SCONFIG */
} SCONFIG_LIST
where:
NumOfParms is an INPUT, which contains the number of SCONFIG elements in the array
pointed to by ConfigPtr.
ConfigPtr is a pointer to an array of SCONFIG structures.
The structure SCONFIG is defined as follows:
typedef struct
{
unsigned long Parameter; /* name of parameter */
unsigned long Value; /* value of the parameter */
} SCONFIG

Here are the 2 structures that I defined in C#:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public unsafe struct SConfig
{
    public int Parameter;
    public int Value;
}



[StructLayout(LayoutKind.Sequential, Pack = 1)]
public unsafe struct SConfig_List
{
    public int NumOfParams;
    // public List<SConfig> sconfig = new List<SConfig>();  // This throws compile time error
    public List<SConfig> sconfig;
}

I know you can not have field initializers in structs, but I cant seem to figure out how to initialize the sconfig in the struct externally... Here is snippet from the calling method

      SConfig_List myConfig = new SConfig_List();
      SConfig configData = new SConfig();

      configData.Parameter = 0x04;
      configData.Value = 0x10;
      myConfig.NumOfParams = 1;
      myConfig.sconfig.Add(configData);

This throws an error at runtime of "object reference not set to an instance of an object" I understand this error because sconfig has not been initialized - I just cant figure out how to do that....

So my next thought was to get around this, I would just create the SCONFIG_LIST struct like this (without the list inside) - My reasoning for this is that I now do not have to initialize the object, and I could just make multiple calls to dll with a NumOfParams = 1, rather than NumOfParams > 1 and having the dll loop through the struct data.

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public unsafe struct SConfig_List
{
    public int NumOfParams;
    public SConfig sconfig;
}

And here is how I called the method

configData.Parameter = 0x04;
configData.Value = 0x10;
myConfig.NumOfParams = 1;
myConfig.sconfig.Parameter = configData.Parameter;
myConfig.sconfig.Value = configData.Value;

m_status = m_XXXXBox.SetConfig(m_channelId, ref myConfig);

This got rid of the errors to this point, now on the the actual method that calls the dll There are still several questions / issues surrounding the Marshalling, but here it is:

public XXXXErr SetConfig(int channelId, ref SConfig_List config)
{
    unsafe
    {
        IntPtr output = IntPtr.Zero;
        IntPtr input = Marshal.AllocHGlobal(Marshal.SizeOf(config));
        Marshal.StructureToPtr(config, input, true);

        XXXXErr returnVal = (XXXXErr)m_wrapper.Ioctl(channelId, (int)Ioctl.SET_CONFIG, input, output);
        return returnVal;
    } 
}

This gets past all of the initial setup without error, but when I try to actually invoke the dll I get an error : Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

I know this is a mouthful, and I really don't even know exactly what to ask as I am sure there are multiple issues within this post, but any ideas on getting me on the right track?

I have tried so many things at this point I am at a loss, and I just need some direction. I am not looking for a "do this for me" type answer, but rather an explanation and maybe some pointers on getting this done. As with all things, I am sure there are multiple ways of accomplishing the task - maybe a way that works, but is not good form, and a longer more complicated way that may be "better practice"

Any and all suggestions / comments will be greatly appreciated. And if I excluded any relevant data that is required to help me solve this riddle let me know and I will provide what I can.

---

I want to thank the responses so far. I have been trying every combination to try and resolve this myself but I have not had any luck so far. I have found quite a few ways that do NOT work, however :-)

I have tried various combinations of "unsafe" - "MarshalAs", "StructLayout" and several other things I found on the web, now I am begging for mercy.

I have successfully implemented several other calls to this unmanaged dll, but all of them use simple integer pointers etc. My problem is passing the pointer to a Struct containing an array of another struct. If you look at the very top of my original question you can see the documentation from the dll and how it wants thing structured. There is NO return value, I am merely trying to pass some configuration settings to a device through this dll.

I am going to post a framework of my entire project so that maybe I can get someone to hold my hand through this process, and hopefully help others in the future trying to solve this type of issue as well.

Here is skeleton of Wrapper (not all functions displayed)

using System;
using System.Runtime.InteropServices;

namespace My_Project
{
    internal static class NativeMethods
    {
        [DllImport("kernel32.dll")]
        public static extern IntPtr LoadLibrary(string dllToLoad);

        [DllImport("kernel32.dll")]
        public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);

        [DllImport("kernel32.dll")]
        public static extern bool FreeLibrary(IntPtr hModule);
    }

    internal class APIDllWrapper
    {
        private IntPtr m_pDll;

        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
        public delegate int APIIoctl(int channelId, int ioctlID, IntPtr input, IntPtr output);
        public APIIoctl Ioctl;
        //extern “C” long WINAPI APIIoctl
        //(
        //unsigned long ChannelID,
        //unsigned long IoctlID,
        //void *pInput,
        //void *pOutput
        //)

        public bool LoadAPILibrary(string path)
        {
            m_pDll = NativeMethods.LoadLibrary(path);

            if (m_pDll == IntPtr.Zero)
                return false;

            pAddressOfFunctionToCall = NativeMethods.GetProcAddress(m_pDll, "APIIoctl");
            if (pAddressOfFunctionToCall != IntPtr.Zero)
                Ioctl = (APIIoctl)Marshal.GetDelegateForFunctionPointer(
                                                                                        pAddressOfFunctionToCall,
                                                                                        typeof(APIIoctl));
            return true;
        }

        public bool FreeLibrary()
        {
            return NativeMethods.FreeLibrary(m_pDll);
        }
    }
}


And Here is the class that defines the hardware I am trying to communicate with
    namespace My_Project
{
    public class APIDevice
    {
        public string Vendor { get; set; }
        public string Name { get; set; }

        public override string ToString()
        {
            return Name;
        }
    }
}


Interface
    using System.Collections.Generic;

namespace My_Project
{
    public interface I_API
    {
        APIErr SetConfig(int channelId, ref SConfig_List config);
    }
}

The actual Class containing the API Code - this is where the error is, I know that how I have the IntPtrs now is Not correct - But this displays what I am trying to do

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace My_Project
{
    public class API : I_API
    {
        private APIDevice m_device;
        private APIDllWrapper m_wrapper;

        public APIErr SetConfig(int channelId, ref SConfig_List config)
        {
            IntPtr output = IntPtr.Zero;
            IntPtr input = Marshal.AllocHGlobal(Marshal.SizeOf(config));
            Marshal.StructureToPtr(config, input, true);

            APIErr returnVal = (APIErr)m_wrapper.Ioctl(channelId, (int)Ioctl.SET_CONFIG, input, output);
            return returnVal;             
        }       
    }
}

Here is class containing the definitions for the Structs I am using

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace My_Project
{
    public enum APIErr
    {
        STATUS_NOERROR = 0x00,
        ERR_BUFFER_EMPTY = 0x10,
        ERR_BUFFER_FULL = 0x11,
        ERR_BUFFER_OVERFLOW = 0x12
    }

    public struct SConfig
    {
        public int Parameter;
        public int Value;
    }

    public struct SConfig_List
    {
        public int NumOfParams;
        public SConfig[] sconfig;

        public SConfig_List(List<SConfig> param)
        {
            this.NumOfParams = param.Count;
            this.sconfig = new SConfig[param.Count];
            param.CopyTo(this.sconfig);
        }
    }
}

And finally - the actual application calling the dll through the wrapper

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using My_Project;

namespace Test_API
{
    public class Comm
    {
        private I_API m_APIBox;
        APIErr m_status;
        int m_channelId;
        bool m_isConnected;

        public Comm(I_API apiInterface)
        {
            m_APIBox = apiInterface;
            m_isConnected = false;
            m_status = APIErr.STATUS_NOERROR;
        }

        public bool ConfigureDevice()
        {
            SConfig tempConfig = new SConfig();

            tempConfig.Parameter = 0x04;
            tempConfig.Value = 0x10;
            SConfig_List setConfig = new SConfig_List(tempConfig);

            m_status = m_APIBox.SetConfig(m_channelId, ref setConfig);
            if (m_status != APIErr.STATUS_NOERROR)
            {
                m_APIBox.Disconnect(m_channelId);
                return false;
            }
            return true;
        }
    }
}

Answer 1

I started another thread because I was asking the wrong question due to my inexperience

The working solution is here

Marshal array of struct and IntPtr

Thanks for the help

-Lee

Answer 2

You cannot marshal List<>, it must be an array. An array is already marshaled as a pointer so you don't have to do anything special. Go easy on the Pack, no need for the unsafe keyword.

You could add a constructor to the struct to make it easy to initialize it from a List<>. Like this:

[StructLayout(LayoutKind.Sequential)]
public struct SConfig {
    public int Parameter;
    public int Value;
}

[StructLayout(LayoutKind.Sequential)]
public struct SConfig_List {
    public int NumOfParams;
    public SConfig[] sconfig;

    public SConfig_List(List<SConfig> param) {
        this.NumOfParams = param.Count;
        this.sconfig = new SConfig[param.Count];
        param.CopyTo(this.sconfig);
    }
}

Answer 3

To initialize list you just have to add line:

myConfig.sconfig = new List<SConfig>()

before you start adding elements into it.