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Marshaling complex C structs with c unions for C#

i am desperate to get a complex c datatype correctly marshaled for C#. I already read all the other posts regarding that topic and i am running out of ideas although it seems to me to be quite close to the solution.
The main issue is that the c-struct is having a union of two different struct types. On with only basic types and one including arrays, which causes trouble.
I have created an example to showcase the situation. The struct worring me is called dataStreamConfiguration. The c code looks like this, the struct in question is at the bottom of the example c-code:
#include "stdint.h"
#include "stddef.h"
typedef enum viewCapEnum {
X = 0,
}viewCapEnum;
typedef struct fraction{
uint8_t nominator;
uint8_t denominator;
}fraction;
typedef struct comSize{
fraction A;
fraction B;
}comSize;
typedef enum someEnum{
A = 0,
B,
C,
D
}someEnum;
typedef struct someSize{
fraction X;
fraction Y;
}someSize;
typedef struct featTemplateCap{
someEnum A;
someSize Size;
}featTemplateCap;
typedef struct featTypeCap{
someEnum AB;
someSize CD;
}featTypeCap;
typedef struct viewCap{
uint8_t A;
uint8_t B;
size_t BCount;
viewCapEnum ViewCapEnum[50];
comSize MinComSize;
size_t CapaCount;
featTemplateCap TemplCap[14];
size_t TypeCapaCount;
featTypeCap FeatTypeCapa[14];
uint8_t GCount;
}viewCap;
typedef struct featX{
uint16_t A;
uint16_t B;
int16_t C;
int16_t D;
}featX;
typedef struct pathCap{
uint8_t Count;
uint8_t Size;
featX Feat;
}pathCap;
typedef struct dataStreamConfiguration{
size_t FeatureSelector;
union {
viewCap AsViewCap;
pathCap AsPathCap;
}dataStream;
}dataStreamConfiguration;
The marshalling of datatypes between C and the C# world is working for almoust all but this dataStreamConfiguration struct. So I got the following code, where instead of mapping (somehow) a union to c# both datatypes have been put one after another. So clearly this was not working correctly. It looked like that:
public unsafe struct UInt32Struct {
public UInt32 value;
}
public unsafe struct fraction{
public Byte nominator;
public Byte denominator;
}
public unsafe struct comSize{
public fraction A;
public fraction B;
}
public unsafe struct someSize{
public fraction X;
public fraction Y;
}
public unsafe struct featTemplateCap{
public UInt32 A;
public someSize Size;
}
public unsafe struct featTypeCap{
public UInt32 AB;
public someSize CD;
}
public unsafe struct viewCap{
public Byte A;
public Byte B;
public UInt16 BCount;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 50]
public UInt32Struct[] ViewCapEnum;
public comSize MinComSize;
public UInt16 CapaCount;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 14]
public featTemplateCap[] TemplCap;
public UInt16 TypeCapaCount;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 14]
public featTypeCap FeatTypeCapa[14];
public Byte GCount;
}
public unsafe struct featX{
public UInt16 A;
public UInt16 B;
public Int16 C;
public Int16 D;
}
public unsafe struct pathCap{
public Byte Count;
public Byte Size;
public featX Feat;
}
public unsafe struct dataStreamConfiguration{
public UInt16 FeatureSelector;
public viewCap AsViewCap;
public pathCap AsPathCap;
}
So to get the union to c# I came across the LayoutKind.Explicit and did following:
[StructLayout(LayoutKind.Explicit)]
public unsafe struct dataStreamConfiguration{
[FieldOffset(0)]
public UInt16 FeatureSelector;
[FieldOffset(2)]
public viewCap AsViewCap;
[FieldOffset(2)]
public pathCap AsPathCap;
}
This was not working due to the alignment of the object types, which are incorrectly aligned or overlapped by non-object fields.. I googled a lot. Adjusted the alignment to 4 by [StructLayout(LayoutKind.Explicit, Pack=4)]. However, 4,8,16,32, whatever alignment i have choosen, I got the same error during runtime - incorrectly aligned or overlapped issue.
Next thing I did - I felt quite lucky about - was to unroll all the arrays in C# datatype for all the arrays in viewCap struct. As I have read that this might cause alignment issues. Well, It didn't work. And I found that the memory has been modified, so I could not find the values I have seen in C appearing now in C#. Most of the values in C# are 0. Ok.
To get rid of this memory modification stuff I put in C# to all other structs [StructLayout(LayoutKind.Sequential)] to keep the order of elements as they are in C. Sadly it didn't help much, I could not find the values of the c-struct in c# either. However, it was finally working, when I got rid of the union and deleted either AsViewCap or AsPathCap (my weak moment of blind rage). Ok, but that was not the solution.
Last help was having a try with IntPtr, so i have created a new struct called dataStreamConfigurationPtr:
public unsafe struct dataStreamConfigurationPtr{
public UInt16 FeatureSelector;
public void* Ptr;
}
[StructLayout(LayoutKind.Sequential)]
public unsafe struct dataStreamConfiguration{
public UInt16 FeatureSelector;
public viewCap AsViewCap;
public pathCap AsPathCap;
}
Instead of having an overlapping memory with StructLayout.Explicit I used an void* to point to the unmanaged memory location. For this I used the old struct definition to get the memory and instead having a union I took the first version where both types are laid out one over another. The idea was to use it like that:
MyFunction(dataStreamConfigurationPtr X, int Status){
//Create obj and appropraite space for data
dataStreamConfiguration DataStream = new dataStreamConfiguration();
DataStream.FeatureSelector = X.FeatureSelector;
unsafe{
IntPtr Ptr = new IntPtr(&X.Ptr);
DataStream.AsViewCap = Marshal.PtrToStructure<viewCap>(Ptr);
DataSteram.AsPathCap = Marshal.PtrToStructure<pathCap>(Ptr);
}
WCFCallback(DataStream, Status);
}
Now the IntPtr is pointing to the right memory, however, this works only for the first item of the structs. So for viewCap the first item A has its correct data whereas item B, BCount,.. all the other item seem to have at least misalligned values or accidental values.. I am quite desperate what to do now, i feel i am so close to a solution but have no idea how to get the other data of the struct from c to c#.
Any suggestions and comments are highly welcome!
Best regards,
Tobias

I would assume you have two use cases and want to interprete the union part based FeatureSelector either as AsViewCap or as AsPathCap on C# side.
That means I assume that you don't intend to do type punning.
One could create two structs then on the managed C# side:
public struct dataStreamConfigurationAsViewCap
{
public UInt64 FeatureSelector;
public viewCap AsViewCap;
}
public struct dataStreamConfigurationAsPathCap
{
public UInt64 FeatureSelector;
public pathCap AsPathCap;
}
You could then only inspect FeatureSelector first and based on the result either interprete it dataStreamConfigurationAsViewCap or as dataStreamConfigurationAsPathCap.
Size
You have several variables with size_t on C side (FeatureSelector, BCount, CapaCount, TypeCapaCount) which you map all to UInt16, which is wrong. UInt16 is the minimum size in the C standard, but the usual implementations especially on platforms running .NET are larger, see also this nice answer. For example on my macOS machine it is 8 bytes.
Maybe it would be a good idea to start with a smaller test case and expand it step by step so that you can identify such problems. And when you encounter a problem, you can more easily create a minimal, complete and testable example.
One approach in this direction could be the following:
Small Test Case
some.h
#ifndef some_h
#define some_h
#include <stdio.h>
typedef struct viewCap {
uint8_t A;
uint8_t B;
} viewCap;
typedef struct pathCap {
uint16_t X;
uint16_t Y;
size_t Num;
} pathCap;
typedef struct dataStreamConfiguration {
size_t FeatureSelector;
union {
viewCap AsViewCap;
pathCap AsPathCap;
} dataStream;
} dataStreamConfiguration;
dataStreamConfiguration *dscViewCap(void);
dataStreamConfiguration *dscPathCap(void);
extern void free_struct(dataStreamConfiguration *ptr);
#endif /* some_h */
some.c
#include "some.h"
#include <stdlib.h>
#include <stdint.h>
#include <stddef.h>
dataStreamConfiguration *dscViewCap(void) {
dataStreamConfiguration *dsc = calloc(1, sizeof(dataStreamConfiguration));
dsc->FeatureSelector = 0;
dsc->dataStream.AsViewCap.A = 42;
dsc->dataStream.AsViewCap.B = 84;
return dsc;
}
dataStreamConfiguration *dscPathCap(void) {
dataStreamConfiguration *dsc = calloc(1, sizeof(dataStreamConfiguration));
dsc->FeatureSelector = 1;
dsc->dataStream.AsPathCap.X = 0xAAAA;
dsc->dataStream.AsPathCap.Y = 0x5555;
dsc->dataStream.AsPathCap.Num = 0x3333333333333333;
return dsc;
}
void free_dsc(dataStreamConfiguration *ptr) {
free(ptr);
}
int main(void) {
return 0;
}
UnionFromC.cs
using System.Runtime.InteropServices;
namespace UnionFromC
{
public static class Program
{
public struct viewCap
{
public Byte A;
public Byte B;
}
public struct pathCap
{
public UInt16 X;
public UInt16 Y;
public UInt64 Num;
}
public struct dataStreamConfigurationAsViewCap
{
public UInt64 FeatureSelector;
public viewCap AsViewCap;
}
public struct dataStreamConfigurationAsPathCap
{
public UInt64 FeatureSelector;
public pathCap AsPathCap;
}
[DllImport("StructLib", EntryPoint = "dscViewCap")]
private static extern IntPtr NativeDSCViewCap();
[DllImport("StructLib", EntryPoint = "dscPathCap")]
private static extern IntPtr NativeDSCPathCap();
[DllImport("StructLib", EntryPoint = "free_dsc")]
private static extern void NativeFreeDSC(IntPtr ptr);
static void Main()
{
IntPtr _intPtrViewCap = NativeDSCViewCap();
var viewDSC = Marshal.PtrToStructure<dataStreamConfigurationAsViewCap>(_intPtrViewCap);
Console.WriteLine("\nInterpretation as view cap:");
Console.WriteLine($" FeatureSelector: {viewDSC.FeatureSelector}");
Console.WriteLine($" A: {viewDSC.AsViewCap.A}");
Console.WriteLine($" B: {viewDSC.AsViewCap.B}");
NativeFreeDSC(_intPtrViewCap);
IntPtr _intPtrPathCap = NativeDSCPathCap();
var pathDSC = Marshal.PtrToStructure<dataStreamConfigurationAsPathCap>(_intPtrPathCap);
Console.WriteLine("\nInterpretation as path cap:");
Console.WriteLine($" FeatureSelector: {pathDSC.FeatureSelector}");
Console.WriteLine($" A: {pathDSC.AsPathCap.X:X4}");
Console.WriteLine($" B: {pathDSC.AsPathCap.Y:X4}");
Console.WriteLine($" Num: {pathDSC.AsPathCap.Num:X8}");
NativeFreeDSC(_intPtrPathCap);
}
}
}
Output of the Test
Interpretation as view cap:
FeatureSelector: 0
A: 42
B: 84
Interpretation as path cap:
FeatureSelector: 1
A: AAAA
B: 5555
Num: 3333333333333333

This is one solution how to get data from C/C++ to C#. Here I will describe what I did wrong and what one has to take care of.
To recall in our minds my requirement has been (still is) that any data represented as union in C/C++, needs to be represented as such in C#. This means for following structure:
typedef struct dataStreamConfiguration{
size_t FeatureSelector;
union {
viewCap AsViewCap;
pathCap AsPathCap;
}dataStream;
}dataStreamConfiguration;
Any data inside AsViewCap must have its kind of representation in AsPathCap, because its simple the same memory. If one of these two are modified, the other is also. To handle C/C++ unions in C# you need to provide a memory layout. As Stephan Schlecht already meantioned it is vital to know the alignment! My project is compiled for 32bit the alignment is at the 4 Byte boarder. Thus my initial layout in my question was simply wrong. You need to check the layout in your C/C++ project and adjust it in the C# struct definion properly: Here is my corrected code, both union members start at 4th Byte:
[StructLayout(LayoutKind.Explicit)]
public unsafe struct dataStreamConfiguration{
[FieldOffset(0)]
public UInt16 FeatureSelector;
[FieldOffset(4)]
public viewCap AsViewCap;
[FieldOffset(4)]
public pathCap AsPathCap;
}
Doing this you are half way there! Yes! But there is one more thing. With this change the Code will compile but, you will get an exception at runtime. Yes, at runtime. Quite quick but it is so. The error message something like: "object-field at offset 4 is incorrectly aligned or overlapped by a non-object field"
C# is bugging is because in C# there are basic types like Integer etc. and reference types.
These reference types can give us errors if we do not handel them correctly. C# has a very nice working marshalling, but in case of unions, its up to you to make it as nice as it can get.
Explenation:
What went wrong in my code is that the struct viewCap is having arrays, marshalled by the C# marshaller. The marshaller is doing his duty and creating an array. However, an array is a reference type and created on the heap. What you will get on the stack (data transfer C++ <-> C#) is the reference address to the array on the heap. Hmpf! Thus the second structure in the union with its basic types would overlapping the address and thus invalidating the reference. Gladly the runtime environment stops us from doing that :-) Moreover, C# is defragmenting the memory. If the struct layout is not efficient regarding memory usage C# will reoder the content. You can avoid this by annotating the struct which the layout kind: Sequential.
Remember:
If you got an array inside a type which is part of a union (C/C++), you cannot use the C# marshaller! Use Layoutkind Explicit for unions and Sequential for structs!
Solution:
There might be several other solutions I am not aware of, but what is 100% working is to unroll the arrays. Yes, it's a lot of work. But it works!
So the final struct looks like this:
[StructLayout(LayoutKind.Sequential)]
public unsafe struct viewCap{
public Byte A;
public Byte B;
public UInt16 BCount;
public UInt32Struct ViewCapEnum_0;
public UInt32Struct ViewCapEnum_1;
public UInt32Struct ViewCapEnum_2;
public UInt32Struct ViewCapEnum_3;
[...]
public comSize MinComSize;
public UInt16 CapaCount;
public featTemplateCap TemplCap_0;
public featTemplateCap TemplCap_1;
public featTemplateCap TemplCap_2;
public featTemplateCap TemplCap_3;
[...]
public UInt16 TypeCapaCount;
public featTypeCap FeatTypeCapa_0;
public featTypeCap FeatTypeCapa_1;
public featTypeCap FeatTypeCapa_2;
public featTypeCap FeatTypeCapa_3;
[...]
public Byte GCount;
}
Happy runtime environment, happy life!
Yes, for this specific solution you would need to adjust the code which is using the arrays. However, its bulletproof and understandable how this works under the hood which makes it easy for maintenance. As my code is generated, an unrolled array is no big deal.

Is there an equivalent of c++ union with int and string member variables in c#?

In c++ we can use same address for string and int variables through use of unions. I am looking to define a union in c# which has two member variables of type int and string.
I have tried 3 ways to define union in which two those shown below work, but they dont look exact equivalent to union of c++. Third one which seems like equivalent of union shows error "Application is in break mode"
This works but doesn't seem like its equivalent of c++ union, as addresses consecutive of integers stay same while that of strings change. (One pair of int and string must have same address in case of a union).
[StructLayout(LayoutKind.Explicit, Size = 128)]
public struct File_element
{
[FieldOffset(0)]
public int Line_no;
[FieldOffset(4)]
public string Line;
}
This works too but I have to define two objects instead of a single, still not sure its equivalent of a union of c++ though
[StructLayout(LayoutKind.Explicit, Size = 128)]
public struct File_element
{
[FieldOffset(0)]
public int Line_no;
}
[StructLayout(LayoutKind.Sequential)]
public struct File_element2
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
public string Line;
}
What I am looking for which I think will be exact equivalent of c++ unions will be this one, but this shows an error ("Application is in break mode") when I try to run
[StructLayout(LayoutKind.Explicit, Size = 128)]
public struct File_element
{
[FieldOffset(0)]
public int Line_no;
[FieldOffset(0)]
public string Line;
}
Error which I see is "Application is in break mode"

I guess you want this:
[StructLayout(LayoutKind.Explicit)]
public unsafe struct File_element
{
[FieldOffset(0)]
public int Line_no;
[FieldOffset(0)]
public fixed byte Line[128];
}
Equal to
union File_element
{
int Line_no;
char Line[128];
}
You can use Line field like a normal array:
unsafe
{
File_element f = new File_element();
f.Line_no = 0x060708;
Console.WriteLine(f.Line[0]); //8
Console.WriteLine(f.Line[1]); //7
Console.WriteLine(f.Line[2]); //6
}
You can also add a property to convert Line to string
public unsafe struct File_element
{
....
public string LineString
{
get
{
fixed (byte* p = Line)
return new string((sbyte*)p);
}
}
}
f.Line_no = 0x373635;
Console.WriteLine(f.LineString); //567

Marshal unmanaged structure with enum member to c#

I have an unmanaged C++ code and I want to convert it to managed C# code, the unmanaged code is shown below, I've searched but I did not find my answer...I want to know the right way for marshaling the below code, I am not sure how to marshal an enum and then reference it in an structure..the MMTPConxNack structure is a member of a union inside another structure.
the hierarchy is shown below:
typedef enum
{
MMTPCnxNckRsn_NoAnswer=-2,
MMTPCnxNckRsn_SendError=-1,
MMTPCnxNckRsn_Ok=0,
MMTPCnxNckRsn_InvalidMember,
MMTPCnxNckRsn_HubNotReady,
MMTPCnxNckRsn_UnknownMember,
MMTPCnxNckRsn_LastCnxTooRecent,
MMTPCnxNckRsn_InvalidVersion,
MMTPCnxNckRsn_InvalidOptions,
MMTPCnxNckRsn_TooManyCnx
} MMTPCnxNckRsn;
typedef struct
{
MMTPCnxNckRsn Reason;
} MMTPConxNack;
typedef struct
{
long Length;
short Type;
union
{
MMTPConxReq ConxReq;
MMTPConxAck ConxAck;
MMTPConxNack ConxNack; // the structure with an enum inside
MMTPErrInd ErrInd;
} Data;
} MMTPMsg;
actually I want to marshal the MMTPConxNack structure..and I use FieldOffset to define the size. Thanks in advance.

The enum looks like this:
public enum MMTPCnxNckRsn {
MMTPCnxNckRsn_NoAnswer = -2,
MMTPCnxNckRsn_SendError = -1,
MMTPCnxNckRsn_Ok = 0,
MMTPCnxNckRsn_InvalidMember,
MMTPCnxNckRsn_HubNotReady,
MMTPCnxNckRsn_UnknownMember,
MMTPCnxNckRsn_LastCnxTooRecent,
MMTPCnxNckRsn_InvalidVersion,
MMTPCnxNckRsn_InvalidOptions,
MMTPCnxNckRsn_TooManyCnx
}
The containing struct is:
public struct MMTPConxNack {
public MMTPCnxNckRsn Reason;
}
The union is:
[StructLayout(LayoutKind.Explicit)]
public struct MMTPMsgDataUnion
{
[FieldOffset(0)]
public MMTPConxReq ConxReq;
[FieldOffset(0)]
public MMTPConxAck ConxAck;
[FieldOffset(0)]
public MMTPConxNack ConxNack;
[FieldOffset(0)]
public MMTPErrInd ErrInd;
}
This is the tricky part. You use LayoutKind.Explicit and FieldOffset to specify that all the members of the C++ union is overlayed on each other. Obviously you need to have definitions for the other 3 types contained in this union, definitions that we cannot see in the C++ code in the question. I presume you already know how to define those.
Once you have the union declared, the final structure is simple:
public struct MMTPMMsg
{
public int Length;
public short Type;
public MMTPMsgDataUnion Data;
}

You need a C# enum and struct to match the native ones:
public enum MMTPCnxNckRsn
{
MMTPCnxNckRsn_NoAnswer,
MMTPCnxNckRsn_SendError,
MMTPCnxNckRsn_Ok,
MMTPCnxNckRsn_InvalidMember,
MMTPCnxNckRsn_HubNotReady,
MMTPCnxNckRsn_UnknownMember,
MMTPCnxNckRsn_LastCnxTooRecent,
MMTPCnxNckRsn_InvalidVersion,
MMTPCnxNckRsn_InvalidOptions,
MMTPCnxNckRsn_TooManyCnx
}
public struct MMTPConxNack
{
public readonly MMTPCnxNckRsn Reason;
}
And you marshal it like this:
var managedItem = (MMTPConxNack)Marshal.PtrToStructure(pointer,typeof(MMTPConxNack));

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.

calling unmanaged function char returns char *

I have a function in unmanaged C/C++ code (dll) that returns a structure containing a char array. I created C# struct to receive this return value uppon calling the function. And uppon calling this function i get 'System.Runtime.InteropServices.MarshalDirectiveException'
This is C declaration:
typedef struct T_SAMPLE_STRUCT {
int num;
char text[20];
} SAMPLE_STRUCT;
SAMPLE_STRUCT sampleFunction( SAMPLE_STRUCT ss );
This is C# declaration:
struct SAMPLE_STRUCT
{
public int num;
public string text;
}
class Dllwrapper
{
[DllImport("samplecdll.dll")]
public static extern SAMPLE_STRUCT sampleFunction(SAMPLE_STRUCT ss);
}
I am using 1-byte ASCII.
Does anyone has a hint or a solution on how to do this?

The trick to converting a C array member is to use the MarshalAs(UnmanagedType.ByValTStr). This can be used to tell the CLR to marshal the array as an inlined member vs. a normal non-inlined array. Try the following signature.
[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential, CharSet=System.Runtime.InteropServices.CharSet.Ansi)]
public struct T_SAMPLE_STRUCT {
/// int
public int num;
/// char[20]
[System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValTStr, SizeConst=20)]
public string text;
}
public partial class NativeMethods {
/// Return Type: SAMPLE_STRUCT->T_SAMPLE_STRUCT
///ss: SAMPLE_STRUCT->T_SAMPLE_STRUCT
[System.Runtime.InteropServices.DllImportAttribute("<Unknown>", EntryPoint="sampleFunction")]
public static extern T_SAMPLE_STRUCT sampleFunction(T_SAMPLE_STRUCT ss) ;
}
This signature is brought to you by the PInovke Interop Assistant (link) available on CodePlex. It can automatically translate most PInvoke signatures from native code to C# or VB.Net.

This is not an easy structure for P/Invoke to marshal: it's easier to marshal structures that contain char* instead of char[] (although you're then left with the problem of allocating the char* from unmanaged code and later freeing it from managed code).
Assuming you're sticking with the current design, one option is to declare the string array as:
public fixed char text[20];
Unfortunately you must then add the unsafe keyword to any code that accesses this array.

Struct definition in C:
#pragma pack(push, 1)
typedef struct T_SAMPLE_STRUCT {
int num;
char text[20];
};
#pragma pack(pop)
Definition in C#:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
public struct T_SAMPLE_STRUCT
{
[MarshalAs(UnmanagedType.I4)]
public int num;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string text;
}

I have managed to do this by separating function to:
void receiveStruct( SAMPLE_STRUCT ss )
void returnStruct(SAMPLE_STRUCT &ss)
I have changed struct definition as JaredPar told me:
[StructLayoutAttribute(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct T_SAMPLE_STRUCT
{
/// int
public int num;
/// char[20]
[MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 20)]
public string text;
}
And now it works.
Thanks!

As an interesting aside, since you already have the answer, the basic issue here is that the struct just needs to be the right size. Since managed types don't have inline arrays, you just have to make up the space that it would otherwise need. When you write in C++/CLI, you'll often see the StructLayoutAttribute with an explicit Size parameter. This makes the runtime allocate the right amount of memory for the type, which allows it to be blittable to the native side. It follows that these should work, as well:
[StructLayout(LayoutKind.Sequential, Size=24)]
public struct T_SAMPLE_STRUCT
{
public int num;
// to get the string here, you'd need to get a pointer
public char firstChar;
}
// or
[StructLayout(LayoutKind.Sequential)]
public struct T_SAMPLE_STRUCT
{
public int num;
public byte c0;
public byte c1;
public byte c2;
public byte c3;
public byte c4;
public byte c5;
public byte c6;
public byte c7;
public byte c8;
public byte c9;
public byte c10;
public byte c11;
public byte c12;
public byte c13;
public byte c14;
public byte c15;
public byte c16;
public byte c17;
public byte c18;
public byte c19;
}
Of course, these are much harder to use from managed code (you'd need to copy memory or use pointers), but they illustrate the concept of a blittable type, which is primarily how types are passed between native and managed code.

First you have to put
StructLayout[Sequential] attribute on your struct and I think it will work
[ StructLayout( LayoutKind.Sequential, CharSet=CharSet.Ansi )]
struct SAMPLE_STRUCT
{
public int num;
[ MarshalAs( UnmanagedType.ByValArray, SizeConst=20 )]
public char[] text;
}