Reading a complex struct from C++ DLL to C# project

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C++

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struct

Greetings.

I'm working on a scanner management project. Provider sent me the .dll and .h files (C++ native) for I need to build a wrapper to use it in my C# project. Some functions had no problems, but others are really difficult to invoke (or at least I dont have enought experience for doing that).

One of them involves a pointer to struck, that internally has other structs.

This is the code in C++

typedef struct _ImParam
{
  UINT Format;
  UINT Resolution;
  UINT ColorDepth;
} IM_PARAM;

typedef struct _sValues
{
  UINT Xpos;
  UINT Ypos;
  UINT Width;
  UINT Height;
  BOOL Milli; 
} S_VALUES;

typedef struct _sProperties
{
  BOOL Enable;
  S_VALUES Properties;
} S_PROPERTIES;

typedef struct _DevParam
{
  BOOL Enable;
  UINT Font;
  char Symbol;
  IM_PARAM Image1;
  IM_PARAM Image2;
  S_PROPERTIES Properties[10];
  UINT FeedMode;
} DevParam;

// more code, comments, etc. etc.

// The function I want to use
BOOL GetParameters( DWORD ID, DevParam  *dParam );

and this is how I defined the structs in C#

[StructLayout(LayoutKind.Sequential)]
public struct ImParam
{
   public uint Format;
   public uint Resolution;
   public uint ColorDepth;

   public ImParam(uint n)
   {
       Format = n;
       Resolution = 300;
       ColorDepth = 256;
   }
};

[StructLayout(LayoutKind.Sequential)]
public struct sValues
{
   public uint Xpos;
   public uint Ypos;
   public uint Width;
   public uint Height;
   [MarshalAs(UnmanagedType.Bool)]
   public bool Milli;

   public sValues(uint n)
   {
       Xpos = n;
       Ypos = n;
       Width = n;
       Height = n;
       Milli = false;
   }
};

[StructLayout(LayoutKind.Sequential)]
public struct sProperties
{
   [MarshalAs(UnmanagedType.Bool)]
   public bool Enable;
   public sValues Properties;

   public sProperties(int n)
   {
       Enable = false;
       Properties = new sValues(n);
   }
};

[StructLayout(LayoutKind.Sequential)]
public struct DevParam
{
   [MarshalAs(UnmanagedType.Bool)]
   public bool Enable;
   public uint Font;
   public char Symbol;
   public ImParam Image1;
   public ImParam Image2;
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10)]
   public sProperties[] Properties;
   public uint FeedMode;

   public DeviceParameters(int n)
   {
       Enable = true;
       Font = 0;
       Symbol = '?';
       Image1 = new ImParam(3);
       Image2 = new ImParam(3);
       Properties = new sProperties[10];
       for(int i = 0; i < 10; i++)
          Properties[i] = new sProperties(n);
       FeedMode = 1;
   }
};

What I need is to get the struct from the C++.

What I have tried:

I tried adding a ref to the struct

// At the wrapper class
[DllImport(path, EntryPoint = "?GetParameters@@YGHKPAU_DevParam@@@Z")]
public static extern bool GetParameters(int ID, out DevParam dParam);

// At main
DevParam DP; // Got ID previously
bool res = Class1.GetParameters(ID, out DP);
Console.WriteLine("Result: " + res);

Also tried using an IntPtr

// At the dll wrapper class
[DllImport(path, EntryPoint = "?GetParameters@@YGHKPAU_DevParam@@@Z")]
public static extern bool GetParameters(int ID, ref IntPtr dParam);

// At main
int size = Marshal.SizeOf(typeof(DevParam));
IntPtr Ptr = Marshal.AllocHGlobal(size);
bool res = Class1.GetParameters(ID, ref Ptr);
Console.WriteLine("Result: " + res);
var test = Marshal.PtrToStructure(Ptr, typeof(DevParam));
// No idea what I'll do here
Marshal.FreeHGlobal(Ptr);

In both cases the result "res" is true (means function was called correctly) but I'm not getting the struct. With the first solution the struct members have default values (all 0 with numbers, all false with bool), and with the second one, Ptr is 0, that means didn't get a correct memory address.

Any help? Been working in this for weeks already and I'm completely stuck.

Note: Don't have source code of the .dll
Note2: Don't have enough experience with dll imports. Would be good an explanation "for dummies"
Note3: Sorry for such a long post