C# method's type signature is not pinvoke compatible

Question

I want to use API from Omron V4KU, the documentation described like this :

Original c# code :

    const string DllLocation = @"..\..\Libs\Omron\OMCR.dll";

    [DllImport(DllLocation)]
    public static extern LPCOMCR OMCR_OpenDevice(string lpcszDevice, LPCOMCR_OPTION lpcOption);

    public void Start()
    {
        var lpcOption = new LPCOMCR_OPTION();
        var result = OMCR_OpenDevice(null, lpcOption); // error method's type signature is not pinvoke compatible
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct LPCOMCR
    {
        public string lpcszDevice;
        public IntPtr hDevice;
        public uint lpcDevice;
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct LPCOMCR_OPTION
    {
        public uint dwReserved0;
        public uint dwReserved1;
        public uint dwReserved2;
        public uint dwReserved3;
    }

if I missed or wrong in writing code? sorry, my english is bad. thanks for help.

Answer 1

Start by defining the union structure correctly:

// OMCR_OPTION.COM
[StructLayout(LayoutKind.Sequential)]
public struct OmcrCom
{
    public IntPtr Reserved0;
    public uint BaudRate;
    public uint Reserved1;
    public uint Reserved2;
    public uint Reserved3;
    public IntPtr Reserved1;
    public IntPtr Reserved2;
}

// OMCR_OPTION.USB
[StructLayout(LayoutKind.Sequential)]
public struct OmcrUsb
{
    public uint Reserved0;
    public uint Reserved1;
    public uint Reserved2;
    public uint Reserved3;
}

// OMCR_OPTION (union of COM and USB)
[StructLayout(LayoutKind.Explicit)]
public struct OmcrOptions
{
    [FieldOffset(0)]
    public OmcrCom Com;

    [FieldOffset(0)]
    public OmcrUsb Usb;
}

// OMCR
[StructLayout(LayoutKind.Sequential)]
public struct OmcrDevice
{
    public string Device;
    public IntPtr DeviceHandle;
    public IntPtr DevicePointer;
}

[DllImport(dllName: DllLocation, EntryPoint = "OMCR_OpenDevice"]
public static extern IntPtr OmcrOpenDevice(string type, ref OmcrOptions options);

And then call the method, something like:

var options = new OmcrOptions();
options.Com.BaudRate = 115200; // or whatever you need to set

var type = "COM"; // is this USB/COM? not sure

OmcrDevice device;

var devicePtr = OmcrOpenDevice(type, ref options);
if (devicePtr == IntPtr.Zero)
    device = (OmcrDevice)Marshal.PtrToStructure(devicePtr, typeof(OmcrDevice));

Answer 2

Well, for one, the documentation is asking you to pass LPCOMCR_OPTION as a pointer - you're passing it as a value. Using ref should help. There's another problem, though, and that's the return value - again, you're trying to interpret it as a value, while the docs say it's a pointer. However, this is a lot trickier than the first error - as far as I'm aware, your only options are using a C++/CLI interop library, or expecting IntPtr as a return value. In any case, you need to handle proper deallocation of the memory you get this way.

Answer 3

You need to do it like this:

[StructLayout(LayoutKind.Sequential)]
public struct OMCR
{
    [MarshalAs(UnmanagedType.LPStr)]
    public string lpcszDevice;
    public IntPtr hDevice;
    public IntPtr lpcDevice;
}

[StructLayout(LayoutKind.Sequential)]
public struct OMCR_OPTION
{
    public uint dwReserved0;
    public uint dwReserved1;
    public uint dwReserved2;
    public uint dwReserved3;
}

[DllImport(DllLocation, CallingConvention = CallingConvention.???,
    SetLastError = true)]
public static extern IntPtr OMCR_OpenDevice(string lpcszDevice, 
    ref OMCR_OPTION lpcOption);

You need to replace CallingConvention.??? with the appropriate calling convention. We cannot tell from the question what that is. You will have to find out by reading the header file.

The return value is a pointer to OMCR. You need to hold on to this pointer and pass it to OMCR_CloseDevice when you are finished with it.

In order to obtain an OMCR value you would do the following:

OMCR_OPTION Option = new OMCR_OPTION(); // not sure how to initialize this
IntPtr DevicePtr = OMCR_OpenDevice(DeviceType, ref Option);
if (DevicePtr == IntPtr.Zero)
    throw new Win32Exception();
OMCR Device = (OMCR)Marshal.PtrToStructure(DevicePtr, typeof(OMCR));