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C# <-> C++ : Marshaling an [In,Out] struct containing Char[]

I have a C++ .dll (unable to make changes to the code) from which I am trying to call a function that takes a reference parameter of a Struct type (also defined in the .dll).
The function requires the Struct to have the 'paramName' and 'groupName' fields populated, and based on those fields, it will return the Struct with the remaining fields populated.
I am not getting any marshaling errors, however, the library call returns an error code and a debug log shows that it is receiving an empty string for the two string fields (see below for how fields are set). My assumption is that the size of this struct is not aligning between the managed and unmanaged representation, and thus the struct is not blittable.
Here is the C++ method signature:
int GetConfigs(int contextHandle, Configs* configs);
And the C++ Configs struct:
struct Configs {
int myInt;
float myFloat;
bool flag;
char name[64];
char group[64];
}
The C# function wrapper:
[DllImport(DLLNAME, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
public static extern int GetConfigs(int contextHandle, [MarshalAs(UnmanagedType.Struct), In, Out] ref Configs configs);
The C# struct definition:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct Configs
{
public int myInt;
public float myFloat;
[MarshalAs(UnmanagedType.U1)]
public bool flag;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string name;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string group;
}
As per the Microsoft documentation I have declared the C++ char[]'s to be represented in C# by strings and marshaled as ByValTStr with a set size.
Also from Microsoft documentation:
bool is not blittable, so I mark it with an explicit MarshalAs.
float is also not blittable, but declaring these fields as a float or a double made no difference in the original issue.
Lastly, the C# calling code:
var configs = new Library.Configs
{
name = "testName",
group = "testGroup"
};
var returnCode = Library.GetConfigs(GetContextId(), ref configs);
The return code comes back as a failure code and the library debug output file shows that the struct argument had:
name = []
(name is clearly set to what I expect it to be at the time of the call when I debug through the C# code)
Instead of declaring the string fields as
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string name;
I tried:
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 64)]
public byte[] name;
but that also made no difference.

The float/double was what was throwing off the struct byte size- the library was expecting a 4 byte floating point number but the data marshaler by default keeps those as 8 bytes. Fixing that along with the boolean as mentioned in the comments above solved the problem:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct Configs
{
public int myInt;
[MarshalAs(UnmanagedType.R4)]
public float myFloat;
[MarshalAs(UnmanagedType.U1)]
public bool flag;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string name;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string group;
}

How to pass char* in struct from c# to c++

It feels like my problem is similar to this.
So far, I have a struct defined in C++ like this:
typedef struct struct_type1
{
uint64 nameLen;
char * name;
} STATUSSTRUCT;
and a function defined as:
extern int _stdcall getStatus(STATUSSTRUCT * status);
and presumably the function like this:
int _stdcall getStatus(STATUSSTRUCT * status)
{
status->nameLen = someLength;
status->name = someName;
return 1;
}
Note that I can't actually change the C++ code (for various reasons) nor the header file.
My C# code looks like this:
public struct STATUSSTRUCT
{
public UInt64 nameLen;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 4128)]
public byte[] name;
}
STATUSSTRUCT status;
[DllImport("test.dll", CallingConvention = CallingConvention.StdCall)]
public static extern int getStatus(ref STATUSSTRUCT status);
public void refreshStatus() {
status = new STATUSSTRUCT();
status.nameLen = 4128;
status.name = new byte[4128];
getStatus(ref status);
}
However, calling refreshStatus is giving me System.AccessViolationException.
Can someone help me figure out how I can call this function in C++ from C#?

Your structure expects a pointer to an array; you are marshalling the array. One side of the transaction is expecting to get the address "123 Sesame Street" and you are providing an exact replica of the apartment building at that address. That's not going to work.
To get marshalling code correct you need to have a thorough and deep understanding of memory management in C#. My advice is that you obtain the services of an expert.

You may try to use StringBuilder instead of byte[] and LPStr instead of ByValArray:
public struct STATUSSTRUCT
{
public UInt64 nameLen;
[MarshalAs(UnmanagedType.LPStr, SizeConst = 4128)]
public StringBuilder name;
}
status = new STATUSSTRUCT();
status.nameLen = 4128;
status.name = new StringBuilder(4128);
getStatus(ref status);
https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.unmanagedtype(v=vs.110).aspx

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;
}

Marshal C++ struct array into C#

I have the following struct in C++:
#define MAXCHARS 15
typedef struct
{
char data[MAXCHARS];
int prob[MAXCHARS];
} LPRData;
And a function that I'm p/invoking into to get an array of 3 of these structures:
void GetData(LPRData *data);
In C++ I would just do something like this:
LPRData *Results;
Results = (LPRData *)malloc(MAXRESULTS*sizeof(LPRData));
GetData( Results );
And it would work just fine, but in C# I can't seem to get it to work.
I've created a C# struct like this:
public struct LPRData
{
/// char[15]
[MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 15)]
public string data;
/// int[15]
[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 15)]
public int[] prob;
}
And if I initialize an array of 3 of those (and all their sub-arrays) and pass it into this:
GetData(LPRData[] data);
It returns with success, but the data in the LPRData array has not changed.
I've even tried to create a raw byte array the size of 3 LPRData's and pass that into a function prototype like this:
GetData(byte[] data);
But in that case I will get the "data" string from the very first LPRData structure, but nothing after it, including the "prob" array from the same LPRData.
Any ideas of how to properly handle this?

I would try adding some attributes to your struct decloration
[StructLayout(LayoutKind.Sequential, Size=TotalBytesInStruct),Serializable]
public struct LPRData
{
/// char[15]
[MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 15)]
public string data;
/// int[15]
[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 15)]
public int[] prob;
}
*Note TotalBytesInStruct is not intended to represent a variable
JaredPar is also correct that using the IntPtr class could be helpful, but it has been quite awhile since I have used PInvoke so I'm rusty.

One trick when dealing with pointers is to just use an IntPtr. You can then use Marshal.PtrToStructure on the pointer and increment based on the size of the structure to get your results.
static extern void GetData([Out] out IntPtr ptr);
LPRData[] GetData()
{
IntPtr value;
LPRData[] array = new LPRData[3];
GetData(out value);
for (int i = 0; i < array.Length; i++)
{
array[i] = Marshal.PtrToStructure(value, typeof(LPRData));
value += Marshal.SizeOf(typeof(LPRData));
}
return array;
}

A similar topic was discussed on this question, and the one of the conclusions was that the CharSet named parameter must be set to CharSet.Ansi. Otherwise, we would be making a wchar_t array instead of a char array. Thus, the correct code would be as follows:
[Serializable]
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct LPRData
{
[MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 15)]
public string data;
[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 15)]
public int[] prob;
}

The PInvoke Interop Assistant may help. http://clrinterop.codeplex.com/releases/view/14120

Did you mark GetData parameter with OutAttribute?
Combining the InAttribute and
OutAttribute is particularly useful
when applied to arrays and formatted,
non-blittable types. Callers see the
changes a callee makes to these types
only when you apply both attributes.