Develop Reference

Initialization of structs used as ref output parameters in native methods

How do native methods initialize structures that they return through an out IntPtr parameter?
Given the following native code:
[DllImport("foo.dll", EntryPoint = "bar", CallingConvention = CallingConvention.Cdecl)]
internal static extern int GetBars(out int count, out IntPtr output);
and the following struct definition:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode, Pack = 1)]
public readonly record struct BarOut
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 21)]
public readonly string FieldA;
[MarshalAs(UnmanagedType.U1)]
public readonly bool FieldB;
}
the managed implementation is able to make the native call and read all the structs found at the pointer.
The entire code base also works with regular structs such as:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode, Pack = 1)]
public struct BarOut
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 21)]
public string FieldA;
[MarshalAs(UnmanagedType.U1)]
public bool FieldB;
}
I cannot find any documentation that describes how the structs are initialized and therefore I am not clear whether these two output definitions are safely equivalent where the only difference is the record is immutable.
Does substituting records in the codebase present any problems?

C# Marshaling, ubalanced stack and getting PInvoke signature correct

I'm trying to call a C DLL in my C# project using marshaling and have some functions working but I have trouble with others. Like the one below.
My first problem is getting the structs correct and next problem is to return PROFILE_INFO as an array with the list of program files or maybe it won't return a list and proNum is an index.
function in C
extern "C" __declspec(dllexport) int WINAPI GetProgramFileList (unsigned long proNum, PROFILE_INFO *proFile);
typedef struct{
PROINFO proInfo;
__int64 proSize;
PROGRAM_DATE createDate;
PROGRAM_DATE writeDate;
}PROFILE_INFO;
typedef struct{
char wno[33];
char dummy[7];
char comment[49];
char dummy2[7];
char type;
char dummy3[7];
}PROINFO;
typedef struct{
short year;
char month;
char day;
char hour;
char min;
char dummy[2];
}PROGRAM_DATE;
My function
[DllImport(#".\IFDLL.dll", EntryPoint = "GetProgramFileList", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Winapi, SetLastError = true)]
public static extern int GetProgramFileListTest(ulong proNum, ref PROFILE_INFO pro);
[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
public struct PROFILE_INFO
{
[MarshalAs(UnmanagedType.Struct)]
public PROINFO proInfo; // WNo/name/type
public long proSize; // Program size
[MarshalAs(UnmanagedType.Struct)]
public PROGRAM_DATE createDate; // Program creating date
[MarshalAs(UnmanagedType.Struct)]
public PROGRAM_DATE writeDate; // Program updating date
}
[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
public struct PROINFO
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 33)] public string wno; // WNo.
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 7)] private string dummy; // dummy
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 49)] public string comment; // program name
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 7)] private string dummy2; // dummy
public char type; // program type
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 7)] private string dummy3; // dummy
}
[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
public struct PROGRAM_DATE
{
public short year; // Date (Year) 4-digit
public char month; // Date (Month)
public char day; // Date (Day)
public char hour; // Date (Time)
public char min; // Date (Minutes)
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 2)] private char dummy; // Dummy
}
The PROGRAM_DATE createDate in C# struct PROFILE_INFO will throw:
Cannot marshal field 'createDate' of type 'CClient.Models.PROFILE_INFO': The type definition of this field has layout information but has an invalid managed/unmanaged type combination or is unmarshalable.
Changing PROGRAM_DATE fields to string makes it accept it but the function returns an argument(-60) error instead. Though not sure if I'm closer to success.
Other attempts, including attempts to get PROFILE_INFO to return as an array (ref PROFILE_INFO[]), landed in:
A call to PInvoke function has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature.
I have got these descriptions following the C dll:
Explanation
Obtain the program list in the standard area
Argument
proNum [in]
Specify the number of the data to be obtained.
proFile [out]
Store the program information in the standard area, in PROFILE_INFO type.
Before executing this function, make sure to ensure the data area for the number of the data to be obtained.
Return value
If it succeeded, “0” is returned. If there is an error, a value other than “0” is returned.
Other functions I have working is GetProgramDirInfo, SendProgram, ReceiveProgram, SearchProgram, etc. but they don't return any arrays, so I assume marshaling arrays is my problem here. Also I'm trying to avoid using unsafe pointers and I'm unsure if I need to do the copying myself.
Any help is appreciated.

Few points when working with p/invoke:
don't add what's obvious for .NET (structs are structs, ...)
don't add pack if you're unsure, by default .NET should behave like C/C++ on that matter
don't add Ansi information if there's no string in the definition (only when there are strings or TStr, etc.). It doesn't cause problems but is useless
in general, don't add attributes if you don't know what they are used for
int and long in C/C++ are (in general) 32-bit. long is not 64-bit in C/C++
Here is a code that should be better:
[DllImport(#".\IFDLL.dll", EntryPoint = "GetProgramFileList")]
public static extern int GetProgramFileListTest(uint proNum, ref PROFILE_INFO pro);
[StructLayout(LayoutKind.Sequential)]
public struct PROFILE_INFO
{
public PROINFO proInfo; // WNo/name/type
public long proSize; // Program size
public PROGRAM_DATE createDate; // Program creating date
public PROGRAM_DATE writeDate; // Program updating date
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct PROINFO
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 33)] public string wno; // WNo.
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 7)] private string dummy; // dummy
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 49)] public string comment; // program name
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 7)] private string dummy2; // dummy
public char type; // program type
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 7)] private string dummy3; // dummy
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct PROGRAM_DATE
{
public short year; // Date (Year) 4-digit
public char month; // Date (Month)
public char day; // Date (Day)
public char hour; // Date (Time)
public char min; // Date (Minutes)
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 2)] private string dummy; // Dummy
}

C++ union in C# — weird behaviour

I am trying to create some vhd/vhdx files using the VHD API in C#.
There's a C++ union that looks like this:
typedef struct _CREATE_VIRTUAL_DISK_PARAMETERS
{
CREATE_VIRTUAL_DISK_VERSION Version;
union
{
struct
{
GUID UniqueId;
ULONGLONG MaximumSize;
ULONG BlockSizeInBytes;
ULONG SectorSizeInBytes;
PCWSTR ParentPath;
PCWSTR SourcePath;
} Version1;
struct
{
GUID UniqueId;
ULONGLONG MaximumSize;
ULONG BlockSizeInBytes;
ULONG SectorSizeInBytes;
ULONG PhysicalSectorSizeInBytes;
PCWSTR ParentPath;
PCWSTR SourcePath;
OPEN_VIRTUAL_DISK_FLAG OpenFlags;
VIRTUAL_STORAGE_TYPE ParentVirtualStorageType;
VIRTUAL_STORAGE_TYPE SourceVirtualStorageType;
GUID ResiliencyGuid;
} Version2;
struct
{
GUID UniqueId;
ULONGLONG MaximumSize;
ULONG BlockSizeInBytes;
ULONG SectorSizeInBytes;
ULONG PhysicalSectorSizeInBytes;
PCWSTR ParentPath;
PCWSTR SourcePath;
OPEN_VIRTUAL_DISK_FLAG OpenFlags;
VIRTUAL_STORAGE_TYPE ParentVirtualStorageType;
VIRTUAL_STORAGE_TYPE SourceVirtualStorageType;
GUID ResiliencyGuid;
PCWSTR SourceLimitPath;
VIRTUAL_STORAGE_TYPE BackingStorageType;
} Version3;
};
} CREATE_VIRTUAL_DISK_PARAMETERS, *PCREATE_VIRTUAL_DISK_PARAMETERS;
I'm trying to convert that to C#, but not having much luck. I'm not interested in Version3 at all, so am leaving that out.
I've tried a number of things and the best I could get to was having Version2 working (by doing something really bizarre), but I've never managed to get Version1 and Version2 working at the same time.
The solution that has wielded the best results so far has been this, but there has to be something wrong there because Version1 simply doesn't work, and SectorSizeInBytes in Version1 is a ulong rather than uint (if I change it to uint like it should be, I break Version2 and Version1 still doesn't work!)
[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParameters
{
[FieldOffset(0)] public CreateVirtualDiskParametersVersion1 Version1;
[FieldOffset(0)] public CreateVirtualDiskParametersVersion2 Version2;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParametersVersion1
{
public CreateVirtualDiskVersion Version;
public Guid UniqueId;
public ulong MaximumSize;
public uint BlockSizeInBytes;
public ulong SectorSizeInBytes;
public string ParentPath;
public string SourcePath;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParametersVersion2
{
public CreateVirtualDiskVersion Version;
public Guid UniqueId;
public ulong MaximumSize;
public uint BlockSizeInBytes;
public uint SectorSizeInBytes;
public uint PhysicalSectorSizeInBytes;
public string ParentPath;
public string SourcePath;
public OpenVirtualDiskFlags OpenFlags;
public VirtualStorageType ParentVirtualStorageType;
public VirtualStorageType SourceVirtualStorageType;
public Guid ResiliencyGuid;
}
I know theoretically the Version field should be set outside the Version structs and I have tried that as well, but it just breaks things even more funnily enough...
So, can someone advise how to properly translate the above to C#, leaving out the Version3 struct as that's not needed?

Using Pack = 1 to StructLayout attributes eliminates any padding between struct members.
In TCP connections structs are usually passed around without padding so that all programs using the struct can agree on its layout in memory.
However as #David Heffernan pointed out, that may not be the case when passing structs to Windows DLL's. I didn't test the actual call to CreateVirtualDisk because it seemed a bit risky, given that I haven't used this call before and didn't want to clobber my disk if I made a mistake. It looks as if the default packing of 8 bytes (Pack = 0 for default or Pack = 8) may be the correct setting, based on the following quote.
See 64-bit Windows API struct alignment caused Access Denied error on named pipe
The Windows SDK expects packing to be 8 bytes. From Using the Windows Headers
Projects should be compiled to use the default structure packing, which is currently 8 bytes because the largest integral type is 8 bytes. Doing so ensures that all structure types within the header files are compiled into the application with the same alignment the Windows API expects. It also ensures that structures with 8-byte values are properly aligned and will not cause alignment faults on processors that enforce data alignment.
Version is moved to the top of CreateVirtualDiskParameters.
The two unions then follow. Both have the same offset sizeof(CREATE_VIRTUAL_DISK_VERSION).
Also SectorSizeInBytes is uint rather than ulong.
You can let the marshaller do the work of filling string members using the attribute, eg
[MarshalAs(UnmanagedType.LPWStr)] public string ParentPath;
Or, you can represent it as it appears in memory, which is a pointer to a Unicode string:
public IntPtr ParentPath;
and then extract the string yourself with
Marshal.PtrToStringAuto(vdp.Version1.ParentPath)
If you're passing the C# struct to an external DLL, populate it with an unmanaged string
vdp.Version1.ParentPath = (IntPtr)Marshal.StringToHGlobalAuto("I am a managed string");
then free the unmanaged string when you're finished with it
Marshal.FreeHGlobal(vdp.Version1.ParentPath);
Try this.
public enum CREATE_VIRTUAL_DISK_VERSION
{
CREATE_VIRTUAL_DISK_VERSION_UNSPECIFIED = 0,
CREATE_VIRTUAL_DISK_VERSION_1 = 1,
CREATE_VIRTUAL_DISK_VERSION_2 = 2
};
public enum OPEN_VIRTUAL_DISK_FLAG
{
OPEN_VIRTUAL_DISK_FLAG_NONE = 0x00000000,
OPEN_VIRTUAL_DISK_FLAG_NO_PARENTS = 0x00000001,
OPEN_VIRTUAL_DISK_FLAG_BLANK_FILE = 0x00000002,
OPEN_VIRTUAL_DISK_FLAG_BOOT_DRIVE = 0x00000004,
OPEN_VIRTUAL_DISK_FLAG_CACHED_IO = 0x00000008,
OPEN_VIRTUAL_DISK_FLAG_CUSTOM_DIFF_CHAIN = 0x00000010
};
[StructLayout(LayoutKind.Sequential, Pack = 8, CharSet = CharSet.Unicode)]
public struct VIRTUAL_STORAGE_TYPE
{
uint DeviceId;
Guid VendorId;
};
[StructLayout(LayoutKind.Explicit, Pack = 8, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParameters
{
[FieldOffset(0)]
public CREATE_VIRTUAL_DISK_VERSION Version;
[FieldOffset(8))]
public CreateVirtualDiskParametersVersion1 Version1;
[FieldOffset(8))]
public CreateVirtualDiskParametersVersion2 Version2;
}
[StructLayout(LayoutKind.Sequential, Pack = 8, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParametersVersion1
{
public Guid UniqueId;
public ulong MaximumSize;
public uint BlockSizeInBytes;
public uint SectorSizeInBytes;
//public IntPtr ParentPath; // PCWSTR in C++ which is a pointer to a Unicode string
//public IntPtr SourcePath; //string
[MarshalAs(UnmanagedType.LPWStr)] public string ParentPath;
[MarshalAs(UnmanagedType.LPWStr)] public string SourcePath;
}
[StructLayout(LayoutKind.Sequential, Pack = 8, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParametersVersion2
{
public Guid UniqueId;
public ulong MaximumSize;
public uint BlockSizeInBytes;
public uint SectorSizeInBytes;
public uint PhysicalSectorSizeInBytes;
//public IntPtr ParentPath; //string
//public IntPtr SourcePath; //string
[MarshalAs(UnmanagedType.LPWStr)] public string ParentPath;
[MarshalAs(UnmanagedType.LPWStr)] public string SourcePath;
public OPEN_VIRTUAL_DISK_FLAG OpenFlags;
public VIRTUAL_STORAGE_TYPE ParentVirtualStorageType;
public VIRTUAL_STORAGE_TYPE SourceVirtualStorageType;
public Guid ResiliencyGuid;
}

char[] array equivalent in c#

i have a c++ struct which has char[20] like below, and it is packed.
#pragma pack(push, temp_aion_packed, 1)
struct temp
{
char x[20];
char y[20];
};
#pragma pack(pop, temp_aion_packed)
now how can i write this struct in c# so that both are same. i have written like this in c#
[DataContract]
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1),Serializable]
public class temp
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string x;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string y;
}
below is the pinvoke declaration in c#
[DllImport("rmsCAPI.dll", CallingConvention = CallingConvention.StdCall, ExactSpelling = true, EntryPoint = "OrderRequirement")]
public static extern int OrderRequirement(ref temp tmp);
c++ function that calls this struct as parameter
long __stdcall OrderRequirement(struct temp *tmp)
{
string p="";
string q="";
p=temp->x;
q=temp->y;
char buff[2048];
sprintf(buff,"p: %s\n q: %s\n x: %s\n y: %s\n",p,q,temp->x,temp->y);
}
but when i do like this in c# it is giving me the junk data in c++, when i assign values for them in c#. Can any one please assist.
Thank you all for help on the above one, but now i got a new problem which is an extension of this, i am providing everything in detail below.
My structures in c++
#pragma pack(push, temp_aion_packed, 1)
struct temp
{
long req;
struct type m_type;
short id;
char x[20];
char y[20];
};
#pragma pack(pop, temp_aion_packed)
#pragma pack(push, type_aion_packed, 1)
struct type
{
short i;
};
#pragma pack(pop, type_aion_packed)
i have written equivalent c# structs like this
[DataContract]
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1),Serializable]
public struct temp
{
[DataMember]
public long req;
[DataMember]
[MarshalAs(UnmanagedType.Struct)]
public type m_type;
[DataMember]
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string x;
[DataMember]
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string y;
}
[DataContract]
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1),Serializable]
public struct type
{
[DataMember]
public short i;
}
below is my c# pinvoke
[DllImport("rmsCAPI.dll", CallingConvention = CallingConvention.StdCall, ExactSpelling = true, EntryPoint = "OrderRequirement")]
public static extern int OrderRequirement(ref temp tmp);
below is my c++ method that calls struct as parameter
long __stdcall OrderRequirement(struct temp *tmp)
{
char buff[2048];
sprintf(buff,"req: %ld \n id: %d \n x: %s\n",tmp->req,tmp->id,tmp->x);
}
now the problem i am getting is as i have structure variable m_type(of struct "type") declared in struct temp, the variables declared before (long req)that are printing fine in my c++ program but the variable declared after that are not giving me any output. So i think the declaration of structures in c# is messing up and i am unable to figure it out so can any one please help.

You declared the struct as a class in C#. That's fine, but it means that any variable of that type is already a reference. So you don't need to pass by ref. When you pass a class by ref you end up passing a pointer to a pointer to the object. That's one level of indirection too many.
The P/invoke in the C# code should therefore be like this:
public static extern int OrderRequirement(temp tmp);
The other way to fix it is to leave the ref in the function declaration, but to declare the temp type as a struct rather than a class. That's because a struct is a value type. A variable whose type is a struct is a value rather than a reference.
Both solutions work, it's up to you which you choose.
There is another problem in your C++ code.
sprintf(buff,"p: %s\n q: %s\n x: %s\n y: %s\n",p,q,temp->x,temp->y);
You are passing p and q, which are of type std::string to printf and expecting the %s format string to print them. That's an error. You need to call c_str() on the strings. Like this:
sprintf(
buff,
"p: %s\n q: %s\n x: %s\n y: %s\n",
p.c_str(),q.c_str(),temp->x,temp->y
);
The problem with your update is that long is 32 bits in Windows C++ and 64 bits in C#. You need to declare it as int in C#. And you missed the id field altogether.
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1),Serializable]
public struct temp
{
[DataMember]
public int req;
[DataMember]
public type m_type;
[DataMember]
public short id;
[DataMember]
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string x;
[DataMember]
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string y;
}

How to convert a C++ Struct with Union into C#?

Guys I am having difficulties on retrieving struct member values after calling a function in the DLL. I tried to convert the C++ codes into C# but I’m not sure if it is correct or not. Please help me understand my mistakes here (if there is) and how to correct.
My problem here is I can’t correctly retrieved the values of the INNER STRUCTS (Union) after I called the ReceiveMessage function from the DLL. Like for example m_objMsg.MsgData.StartReq.MsgID is always 0.
But when I try to use the C++ .exe program, the MsgID has a correct value. (not 0)
C++ Code:
extern int ReceiveMessage(SESSION, int, Msg*);
typedef struct
{
char SubsId[15];
int Level;
char Options[12];
} ConxReq;
typedef struct
{
char MsgId[25];
} StartReq;
typedef struct
{
long Length;
short Type;
union
{
ConxReq oConxReq;
StartReq oStartReq;
} Data;
} Msg;
/////////////////////////////////////////////////////
Msg oMsg;
int rc=ReceiveMessage(Session, 0, &oMsg);
switch(rc)
{
case 0:
switch(oMsg.Type)
{
case 0: // ConxReq
…
break;
case 1: // StartReq
…
break;
…
}
And here is my attempt to convert this into c#:
[DllImport("MyDLL.dll",
CallingConvention = CallingConvention.Cdecl,
CharSet = CharSet.Ansi)]
protected static extern Int32 ReceiveMessage(IntPtr session,
Int32 nTimeOut,
[MarshalAs(UnmanagedType.Struct)] ref Msg ptrMsg);
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct ConxReq
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 15)]
public string SubsId;
public Int32 Level;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 12)]
public string Options;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct StartReq
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 25)]
public string MsgId;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
protected struct Msg
{
public int Length;
public Int16 Type;
public Data MsgData;
}
StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct Data
{
[FieldOffset(0)]
public ConxReq oConxReq;
[FieldOffset(0)]
public StartReq oStartReq;
}
Msg m_objMsg = new Msg();
m_objMsg.MsgData = new Data();
m_objMsg.MsgData.oConxReq = new ConxReq();
m_objMsg.MsgData.oStartReq = new StartReq();
int rc = ReceiveMessage(m_Session, nTimeOut, ref m_objMsg);
then the SWITCH Condition
And If I add this struct inside the UNION for c++ and c#...
I've got an error stating the "... incorrectly align" or "...overlapped..."
c++
ConxNack oConxNack;
typedef struct
{
int Reason;
} ConxNack;
[StructLayout(LayoutKind.Sequential)]
public struct ConxNack
{
public int nReason;
}
[FieldOffset(0)]
public ConxNack oConxNack;
Thank you so much in advance for your time and help...

Akash is right, have a look here: http://social.msdn.microsoft.com/Forums/en/csharplanguage/thread/60150e7b-665a-49a2-8e2e-2097986142f3
Another option is create two structs and use an appropriate cast once you know which type it is.
hth
Mario

In C++, we know that all members of UNION shared the same memory chunk and can only have one member of an object at a time.
In order to implement this in C#, we need to use the LayoutKind to Explicit and set all the starting point of each member to 0.
In my previous example, An error message is displayed stating that the offset of an object type is incorrectly aligned or overlapped by a non-object type.
Answer is we cannot set all the members to FieldOffSet to 0 since it is not allowed to combine the reference type with the value type.
- Thanks to the explanation of Hans Passant
What I did is to create a copy of the UNION Member Structs and change the type of all the String Member Variables to bytes.
I used bytes since this is a value type so I can put this struct into FieldOffSet(0).
Take note, i adjust the FieldOffSet of the next member variable so i can still get the same size of my string variable.
And also for the struct size since i have byte member at the last.
Thanks to Akash Kava and Mario The Spoon for giving me an idea and providing me a useful link.
After calling the function in the DLL and passed this Struct Obj (ref m_objMsg) as a paramter, I need to extract the values.
One way is to have a pointer that points to the address of the struct in the unmanaged memory and convert this pointer a new
Struct with the corresponding member variables (my original structs).
NEW STRUCTS (BYTES)
////////////////////////////////////////////////////////////////
[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi, Size = 31)]
public struct ConxReq
{
[FieldOffSet(0)]
public byteSubsId;
[FieldOffSet(15)]
public Int32 Level;
[FieldOffSet(19)]
public byte Options;
}
[StructLayout(LayoutKind.Explicit, Size = 4)]
public struct ConxNack
{
[FieldOffSet(0)]
public int nReason;
}
[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi, Size = 25)]
public struct StartReq
{
[FieldOffSet(0)]
public byte MsgId;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
protected struct Msg
{
public int Length;
public Int16 Type;
public Data MsgData;
}
StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct Data
{
[FieldOffset(0)]
public ConxReq oConxReq;
[FieldOffset(0)]
public ConxNack oConxNack;
[FieldOffset(0)]
public StartReq oStartReq;
}
////////////////////////////////////////////////////////////////
MY ORIGINAL STRUCTS
////////////////////////////////////////////////////////////////
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct MyConxReq
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 15)]
public string SubsId;
public Int32 Level;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 12)]
public string Options;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct MyStartReq
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 25)]
public string MsgId;
}
[StructLayout(LayoutKind.Sequential)]
public struct MyConxNack
{
public int nReason;
}
///////////////////////////////////////////////////////////////
Since I have a Msg.Type, i know what kind of struct (type) I could cast the object.
Like for example
ReceiveMessage(m_Session, nTimeOut, ref oMsg);
switch (oMsg.Type)
{
case 0: // ConxReq
IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf(oMsg.MsgData.ConxReq); // use the new struct (bytes)
Marshal.StructureToPtr(oMsg.MsgData.ConxReq, ptr, false);
MyConxReq oMyConxReq = new MyConxReq;
oMyConxReq = (MyConxReq) Marshal.PtrToStructure(ptr, typeof(MyConxReq)); // convert it to the original struct
Marshal.FreeHGlobal(ptr);
Then you can use now the oMyConxReq object to acccess the member variables directly.
Please let me know if you have other or better way to do this...
Kindly advise if what I did is correct or if I missed something.
Thank you so much!!! :)

You have to use StructLayout(LayoutKind.Explicit) and FieldOffsets to make union.