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.
Related
I'm sure there is a easy answer, but I'm stuck. Been trying to understand deserializing unmanaged data structures into managed data and for simple structs everything is fine, but running into issues when the structs are more complex (Multiple member structs, arrays of structs, etc)
I tried Sequential, Auto and Explicit. Auto and Explicit throw errors, and sequential works but only on simpler types.
Below is my sample code. If I deserialize the binary data as PacketHeader, the info is correct. But If I deserialize as BaseDataPacket, it is not correct. Not even the PacketHeader even though it was when I did it by itself.
I would like to avoid doing explicit because there are potentially a lot of different structs I'd have to put offsets for, but I will if that is the only way.
Main Struct
[Serializable, StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct BaseDataPacket
{
public PacketHeader Header;
public PacketData[] Pdata;
}
Packet Header Struct
[Serializable, StructLayout(LayoutKind.Sequential,Pack =1)]
public struct PacketHeader
{
public ushort Format;
public byte MajorVersion;
public byte MinorVersion;
public byte PacketVersion;
public PacketType PacketId;
public ulong SessionUID;
public float SessionTime;
public uint FrameIdentifier;
public sbyte Index;
public sbyte SecondaryIndex;
}
packet data struct
[Serializable, StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct PacketData
{
public byte d1;
public byte d2;
public byte d3;
public float f1;
public float f2;
public float f3;
public byte b1;
public float f4;
public float f5;
}
And heres my code to marshal
public static T FromBinaryReader<T>(BinaryReader reader)
{
// Read in a byte array
byte[] bytes = reader.ReadBytes(Marshal.SizeOf(typeof(T)));
// Pin the managed memory while, copy it out the data, then unpin it
GCHandle handle = GCHandle.Alloc(bytes, GCHandleType.Pinned);
T returnStruct = (T)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(T));
handle.Free();
return returnStruct;
}
Any thoughts? Maybe this is a duplicate question i didnt find?
I am trying to use C# to use a C++ library that includes a struct that is documented as being laid out like this:
struct {
long outervar;
long othervar;
union {
struct {
long a;
} firststruct;
struct {
long p;
long q;
long r;
long s;
} secondstruct;
struct {
long x;
long y;
long z;
} thirdstruct;
} myunion;
} outerstruct;
Ideally I would like to implement something like this:
struct outerstruct
{
UInt32 outervar;
UInt32 othervar;
[FieldOffset(0)]
struct firststruct
{
UInt32 a;
}
[FieldOffset(0)]
struct secondstruct
{
UInt32 p;
UInt32 q;
UInt32 r;
UInt32 s;
}
[FieldOffset(0)]
struct thirdstruct
{
UInt32 x;
UInt32 y;
UInt32 z;
}
}
Of course I can't use FieldOffset or MarshalAs on structs, but I am at a loss as to how to implement this. If anybody has some ideas how to implement this in C# I would appreciate it.
Thanks!
[FieldOffset] is fine on struct fields. But your declaration just has nested types (note that in the C++ code, the structs are actually anonymous, but used to declare a new field...your C# code just declares a named struct, but no actual field). Note that you will also need to include [StructLayout(LayoutKind.Explicit)], as well as provide offsets for all fields in a struct.
Try something like this:
struct firststruct
{
UInt32 a;
}
struct secondstruct
{
UInt32 p;
UInt32 q;
UInt32 r;
UInt32 s;
}
struct thirdstruct
{
UInt32 x;
UInt32 y;
UInt32 z;
}
[StructLayout(LayoutKind.Explicit)]
struct union
{
[FieldOffset(0)]
firststruct firststruct;
[FieldOffset(0)]
secondstruct secondstruct;
[FieldOffset(0)]
thirdstruct thirdstruct;
}
struct outerstruct
{
UInt32 outervar;
UInt32 othervar;
union union;
}
You certainly can use FieldOffset and that is the answer. But you neeed to define the union as a distinct type.
Define each struct in the usual way. I'm going to assume you know how to do this.
Then define the union:
[StructLayout(LayoutKind.Explicit)]
public struct MyUnion
{
[FieldOffset(0)]
public FirstStruct firststruct;
[FieldOffset(0)]
public SecondStruct secondstruct;
[FieldOffset(0)]
public ThirdStruct thirdstruct;
}
Finally put it all together:
[StructLayout(LayoutKind.Sequential)]
public struct OuterStruct
{
public int outervar;
public int othervar;
public MyUnion myunion;
}
The key is always to make the union a distinct type and use a field offset of zero for every member.
FWIW, C++ long on Windows is a signed 32 bit integer. That's int in C#.
I am trying to use functions in C++ dll from C#, but I got an error: "attempt to read or write protected memory. This is often indication that other memory is corrupt"
Anyone know how to fix ?
Here is C++ functions:
typedef void *DGNHandle;
__declspec(dllexport) DGNHandle CPL_DLL DGNOpen( const char *, int );
__declspec(dllexport) DGNElemCore CPL_DLL *DGNReadElement( DGNHandle )
Here is structure in C++:
typedef struct {
int offset;
int size;
int element_id; /*!< Element number (zero based) */
int stype; /*!< Structure type: (DGNST_*) */
int level; /*!< Element Level: 0-63 */
int type; /*!< Element type (DGNT_) */
int complex; /*!< Is element complex? */
int deleted; /*!< Is element deleted? */
int graphic_group; /*!< Graphic group number */
int properties; /*!< Properties: ORing of DGNPF_ flags */
int color; /*!< Color index (0-255) */
int weight; /*!< Line Weight (0-31) */
int style; /*!< Line Style: One of DGNS_* values */
int attr_bytes; /*!< Bytes of attribute data, usually zero. */
unsigned char *attr_data; /*!< Raw attribute data */
int raw_bytes; /*!< Bytes of raw data, usually zero. */
unsigned char *raw_data; /*!< All raw element data including header. */
} DGNElemCore;
And below converted codes are in C#:
[StructLayout(LayoutKind.Sequential )]
public class DGNElemCore
{
public int attr_bytes;
public byte[] attr_data;
public int color;
public int complex;
public int deleted;
public int element_id;
public int graphic_group;
public int level;
public int offset;
public int properties;
public int raw_bytes;
public byte[] raw_data;
public int size;
public int style;
public int stype;
public int type;
public int weight;
}
[DllImport("DgnLib.dll", EntryPoint = "DGNOpen")]
public static extern IntPtr DGNOpen(string fileName, int bUpdate);
[DllImport("DgnLib.dll", EntryPoint = "DGNReadElement")]
public static extern DGNElemCore DGNReadElement(IntPtr DGNHandle)
Codes for testing:
DGNElemCore element = new DGNElemCore();
element = DgnFile.DGNReadElement(dgnFile.oDgnFile) **//Throw error**
Your declaration of DGNElemCore in your C# code is wrong - it needs to exactly match your C structure (especially in size) as otherwise the marshalling code will attempt to marshal memory incorrectly. An example definition which will work (as in not cause problems during marshalling) would be the following
[StructLayout(LayoutKind.Sequential )]
public class DGNElemCore
{
int offset;
int size;
int element_id;
int stype;
int level;
int type;
int complex;
int deleted;
int graphic_group;
int properties;
int color;
int weight;
int style;
int attr_bytes;
IntPtr attr_data;
int raw_bytes;
IntPtr raw_data;
}
Note in particular
The order of members in the C# classes match those in the C struct (although this wont cause an error when calling your function it will give you incorrect values when accessing the members of the marshalled struct)
The char* fields are marshalled as IntPtrs - attempting to marshal pointers to arrays as arrays won't work by default as arrays are larger than pointers, resulting in the marshaller attempting to marshal more memory than is available.
Also I've noticed that your P/Invoke method declarations are wrong. The DGNOpen function returns the structure itself (not a pointer) and so should look more like the following.
public static extern DGNElemCore DGNOpen(string fileName, int bUpdate);
The DGNReadElement function accepts a struct (not a pointer) and returns a pointer to that strucut (not a struct) and so should look more like this
public static extern IntPtr DGNReadElement(DGNHandle handle);
Attributes can be used to change the way that the marshaller works, which can in turn be used to alter the signature of these methods, however if you do this you need to be careful to ensure that the marshalling will still match up to your C++ function declarations.
The problem is that the #include headers might contain declarations that can be misinterpreted by the C++/CLI compilers. C function declarations for example. Best thing to do is to the tell the compiler explicitly about it.
#pragma managed(push, off)
#include "c_include.h"
#pragma managed(pop)
Then you can use C++ libraries from within C++/CLI application like you do with C++ apps. The only thing I always try to do is to wrap 3rd library behind Proxy or Facade design pattern, so that the client would always work with managed classes. This is especially important if your C++/CLI app is a library used by other .NET apps.
Moreover, normally public API classes (or functions) of your DLL should be exposed using following construction:
#ifdef YOUR_DLL_EXPORTS
#define YOUR_API __declspec(dllexport)
#else
#define YOUR_API __declspec(dllimport)
#endif
class YOUR_API ClassToExpose {};
Hope this help
I try to Marshal a unmanaged c++ dll in c#, but the marshaller fails when creating my union.
Why does this code fail?
[StructLayout(LayoutKind.Sequential)]
public struct StructWithArray
{
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
public int[] MySimpleArray;
//More Stuff
}
[StructLayout(LayoutKind.Explicit)]
public struct Union
{
[FieldOffset(0)]
public int Int; //Or anything else
[FieldOffset(0), MarshalAs(UnmanagedType.Struct)]
public StructWithArray MyStructWithArray;
//More Structs
}
And then constructing the Union:
Union MyUnion = new Union();
It fails if I run the code with the following Message: (Translated)
{"The Type "Union" of the Assembly [...] could not be loaded because it contained an Objectfield at Offset 0, which is not aligned correctly or got overlapped by a field which isnt a ObjectField":"Union"}
Any Suggestions?
Ps: The original code is heavily simplified to show only the Problem. There are much more Structs, and the Union is also contained by another Struct.
Declare MySimpleArray as a fixed unsafe array:
[StructLayout(LayoutKind.Sequential)]
public unsafe struct StructWithArray
{
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
public fixed int MySimpleArray[2];
//More Stuff
}
[StructLayout(LayoutKind.Explicit)]
public struct Union
{
[FieldOffset(0)]
public int Int; //Or anything else
[FieldOffset(0), MarshalAs(UnmanagedType.Struct)]
public StructWithArray MyStructWithArray;
//More Structs
}
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;
}