I'd like to create Foo objects in C# from an unmanaged array of Foo structs created in C++.
This is how I think it should work:
On the C++ side:
extern "C" __declspec(dllexport) void* createFooDetector()
{
return new FooDetector();
}
extern "C" __declspec(dllexport) void releaseFooDetector(void* fooDetector)
{
FooDetector *fd = (FooDetector*)fooDetector;
delete fd;
}
extern "C" __declspec(dllexport) int detectFoo(void* fooDetector, Foo **detectedFoos)
{
FooDetector *fd = (FooDetector*)fooDetector;
vector<Foo> foos;
fd->detect(foos);
int numDetectedFoos = foos.size();
Foo *fooArr = new Foo[numDetectedFoos];
for (int i=0; i<numDetectedFoos; ++i)
{
fooArr[i] = foos[i];
}
detectedFoos = &fooArr;
return numDetectedFoos;
}
extern "C" __declspec(dllexport) void releaseFooObjects(Foo* fooObjects)
{
delete [] fooObjects;
}
On C# side:
(I ommitted some fancy code making it possible to call the C++ functions from within C# for better readability);
List<Foo> detectFooObjects()
{
IntPtr fooDetector = createFooDetector();
IntPtr detectedFoos = IntPtr.Zero;
detectFoo(fooDetector, ref detectedFoos);
// How do I get Foo objects from my IntPtr pointing to an unmanaged array of Foo structs?
releaseFooObjects(detectedFoos);
releaseFooDetector(fooDetector);
}
But I don't know how to retrieve the objects from the IntPtr detectedFoos. It should be possible somehow...
Any hints?
UPDATE
Let's assume, Foo is a simple detection rectangle.
C++:
struct Foo
{
int x;
int y;
int w;
int h;
};
C#:
[StructLayout(LayoutKind.Sequential)]
public struct Foo
{
public int x;
public int y;
public int width;
public int height;
}
Is it possible to read from unmanaged memory and create new managed objects from it before releasing the unmanaged memory?
I don't know how may Foo objects will be detected, so I don't know, how much memory to allocate in C# before calling detectFoo(). That's why I alloc/free memory in C++ and just pass a pointer to it. But somehow I can't retrieve the detectedFoos pointer address under C#. How do I do that?
You must re-declare Foo in your C# project. Assuming you know the count of Foos and the value of sizeof(Foo) you should be able to use System.Runtime.Interopservices.Marshal.PtrToStructure() to retrieve your Foo structures one at a time.
You have to define your struct again in C#, and it depends on your stuct. Your struct have to be blitable (the memory layout of the C# struct ave to be the same as it is for the C struct)
Have a look at "Marshalling structs"
Or post your real "Foo" struct and i can show you the C# version
UPDATE:
Because your struct seems to be blitable, you can simple cast the pointer to unmanaged memory to a pointer to the struct defined in c#:
If it is ok for your application to use unsafe code you can write:
unsafe List<Foo> detectFooObjects()
{
List<Foo> res = new List<Foo>()
IntPtr fooDetector = createFooDetector();
IntPtr detectedFoos = IntPtr.Zero;
int nNumFoos = detectFoo(fooDetector, ref detectedFoos );
for(int i=0;i<nNumFoos;i++)
{
Foo** ppDetectedFoos = detectedFoos.ToPointer();
Foo* pFoo = *ppDetectedFoos
res.Add(*pFoo); //copies the struct because is a struct
ppDetectedFoos++:
}
releaseFooObjects(detectedFoos);
releaseFooDetector(fooDetector);
return res;
}
I ended up solving my problem by using a C++/CLI wrapper class.
Related
I have an unmanaged DLL that exports only a C style factory method that returns a new instance of a class (simplified here to look simple).
hello.h
#if defined(HWLIBRARY_EXPORT) // inside DLL
# define HWAPI __declspec(dllexport)
#else // outside DLL
# define HWAPI __declspec(dllimport)
#endif
struct HelloWorld{
public:
virtual void sayHello() = 0;
virtual void release() = 0;
};
extern "C" HWAPI HelloWorld* GetHW();
hello.cpp
#include "hello.h"
struct HelloWorldImpl : HelloWorld
{
void sayHello(){
int triv;
std::cout<<"Hello World!";
std::cin>>triv;
};
void release(){
this->HelloWorldImpl::~HelloWorldImpl();
};
HelloWorld* GetHW(){
HelloWorld* ptr = new HelloWorldImpl();
return ptr;
};
Now, I can use dllimport to access GetHW() but is there a way to access the member functions of the returned 'struct'... ie, sayHello and release?
I was also stuck with the same problem. This question was asked a while before. I commented to it for any better solution but didn't get any reply yet. So, reposting it.
When i googled, able to find out two solutions.
Solution1: Expose all the member functions in the C-style for the existing dll. Which i cant do, as it is a 3rd party dll.
Solution2: Write a managed C++ dll exposing the functionality of native C++ dll, which later can be used in your C# dll. Here many classes/functions are present. So, creating would take most of the time.
i got the above solutions from the link below.
How To Marshall
Please let me know if there is any better solution other than the above two solutions?
i have the source code for C++ solution. But what i though was not to touch C++ dll. If there is any possibility to do it in C#, it would be great.
If there is no alternative, i need to follow any one of the specified two solutions.
The C++ code is using the way abstract classes are implemented by the Visual C++ compiler. http://blogs.msdn.com/b/oldnewthing/archive/2004/02/05/68017.aspx. This memory layout is "fixed" because it is used for implementing COM interfaces. The first member of the struct in memory will be a pointer to a vtable containing the function pointers of your methods. So for a
struct HelloWorldImpl : public HelloWorld
{
public:
int value1;
int value2;
}
the "real" layout in memory would be:
struct HelloWorldImpl
{
HelloWorldVtbl *vtbl;
int value1;
int value2;
}
where vtbl would be:
struct HelloWorldVtbl
{
void *sayHello;
void *release;
}
Just for the sake of doing a complete response, I'm writing the example for this signatures:
struct HelloWorld {
public:
virtual int sayHello(int v1, int v2, int v3) = 0;
virtual void release() = 0;
};
C# code:
[DllImport("NativeLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr GetHW();
[StructLayout(LayoutKind.Sequential)]
struct HelloWorldVtbl
{
public IntPtr sayHello;
public IntPtr release;
}
Your functions are void Func(void) or int Func(int, int, int), but in truth they have a hidden parameter, this, so you can write them as:
int sayHello(HelloWorld*, int, int, int);
void release(HelloWorld*);
so in C# the delegate is
[UnmanagedFunctionPointer(CallingConvention.ThisCall)]
public delegate int Int32MethodInt32Int32Int32(IntPtr ptr, int v1, int v2, int v3);
[UnmanagedFunctionPointer(CallingConvention.ThisCall)]
public delegate void VoidMethodVoid(IntPtr ptr);
Then you can use
IntPtr ptr = GetHW();
IntPtr vtbl = Marshal.ReadIntPtr(ptr, 0);
HelloWorldVtblhw = (HelloWorldVtbl)Marshal.PtrToStructure(vtbl, typeof(HelloWorldVtbl));
Int32MethodInt32Int32Int32 sayHello = (Int32MethodInt32Int32Int32)Marshal.GetDelegateForFunctionPointer(hw.sayHello, typeof(Int32MethodInt32Int32Int32));
int res = sayHello(ptr, 1, 2, 3);
Console.WriteLine(res);
VoidMethodVoid release = (VoidMethodVoid)Marshal.GetDelegateForFunctionPointer(hw.release, typeof(VoidMethodVoid));
release(ptr);
Here is my code:
[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct Foo
{
UInt32 StartAddr;
UInt32 Type;
}
[DllImport(DllName, EntryPoint="_MyFunc", CallingConvention = CallingConvention.Cdecl)]
static extern unsafe IntPtr MyFunc([MarshalAs(UnmanagedType.LPArray)] Foo[] Foos);
List<Foo> Foos = new List<Foo>();
Foo1 = new Foo();
Foo1.StartAddr = 1;
Foo1.Type = 2;
Foos.Add(Foo1);
MyFunc(Foos.ToArray());
In the C-based DLL I print out the value of Foos[0].StartAddr and Foos[0].Type. This works great.
Now I want to add a parameterless constructor to the struct which means I have to switch to a class. By only changing the C# declaration from "struct" to "class" results in corrupted values being passed to the C-based DLL.
I believe this should work but I presume I am missing a step. How can I pass an array of C# classes as a array of structs to C code?
Thanks! Andy
If you need default item in your struct you can add static property to it
[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct Foo
{
UInt32 StartAddr;
UInt32 Type;
public static Foo Default
{
get
{
Foo result = new Foo();
result.StartAddr = 200;
result.Type = 10;
return result;
}
}
}
And when you need to create new Foo struct just call Foo.Default
I've worked on a C# version of a C++ API but I can't manage to get it right.
Since the whole API was too big the first time I posted, I've cut it down to some core functions for which I really need some help to make a managed version of.
So basically, this is a C++ dll containing exported functions that are used to communicate with a technical analysis software.
C++ function I'd like to convert
#define PLUGINAPI extern "C" __declspec(dllexport)
PLUGINAPI int GetFunctionTable( FunctionTag **ppFunctionTable )
{
*ppFunctionTable = gFunctionTable;
// must return the number of functions in the table
return gFunctionTableSize;
}
GetFunctionTable is called by the software which provides a pointer to an array of FunctionTag called gFunctionTable:
typedef struct FunctionTag
{
char *Name;
FunDesc Descript;
} FunctionTag;
FunctionTag gFunctionTable[] = {"ExampleA",{ VExampleA, 0, 0, 0, 0, NULL },
"ExampleB",{ VExampleB, 1, 0, 1, 0, NULL }
};
FunctionTag structure contains an embedded structure called Fundesc:
// FunDesc structure holds the pointer to actual
// user-defined function that can be called by AmiBroker.
typedef struct FunDesc
{
AmiVar (*Function)( int NumArgs, AmiVar *ArgsTable );
UBYTE ArrayQty; // number of Array arguments required
UBYTE StringQty; // number of String arguments required
SBYTE FloatQty; // number of float args
UBYTE DefaultQty; // number of default float args
float *DefaultValues; // the pointer to defaults table
} FunDesc;
Finally, Fundesc contains AmiVar type:
#pragma pack( push, 2 )
typedef struct AmiVar
{
int type;
union
{
float val;
float *array;
char *string;
void *disp;
};
} AmiVar;
#pragma pack(pop)
C# conversion so far
Now, this is what I've written so far in an attempt to get my C# dll to "mimic" the C++ original API. The GetFunctionTable() exported function:
namespace AmiBrokerFrontDll
{
internal static class AmiBrokerFrontDll
{
[DllExport("GetFunctionTable", CallingConvention = CallingConvention.Cdecl)]
public static Int32 GetFunctionTable(ref FunctionTag[] ppFunctionTable)
{
FillFunction();
ppFunctionTable=gFunctionTable;
return gFunctionTableSize;
}
Then comes the definition of FunctionTag structure and gFunctionTableSize:
[StructLayoutAttribute(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct FunctionTag
{
[MarshalAs(UnmanagedType.LPStr)]
public string Name;
public FunDesc Description;
}
public static FunctionTag[] gFunctionTable=new FunctionTag[1];
public static FunctionTag gfunc;
static Int32 gFunctionTableSize = Marshal.SizeOf(gFunctionTable) / Marshal.SizeOf(gfunc);
public static void FillFunction()
{
gFunctionTable[0].Name = "VExempleA";
gFunctionTable[0].Description.Function += VExempleDeMacd;
//ArrayQty, StringQty, FloatQty, DefaultQty, DefaultTablePtr
gFunctionTable[0].Description.ArrayQty = 0;
gFunctionTable[0].Description.StringQty = 0;
gFunctionTable[0].Description.FloatQty = 2;
gFunctionTable[0].Description.DefaultQty = 0;
gFunctionTable[0].Description.DefaultValues = new IntPtr();
}
FunDesc declaration includes a delegate:
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate AmiVar FunctionDelegate(int NumArgs, ref AmiVar ArgsTable);
public struct FunDesc
{
[MarshalAs(UnmanagedType.FunctionPtr)]
public FunctionDelegate Function;
public byte ArrayQty; // The number of Array arguments required
public byte StringQty; // The number of String arguments required
public byte FloatQty; // The number of float args
public byte DefaultQty; // The number of default float args
public IntPtr DefaultValues; // The pointer to defaults table
}
Finally, we have an AmiVar structure:
[StructLayoutAttribute(LayoutKind.Explicit, Size = 8)]
public struct AmiVar
{
[FieldOffset(0)]
public Int32 type;
[FieldOffset(4)]
public Single val;
[FieldOffset(4)]
public IntPtr array;
[FieldOffset(4)]
[MarshalAs(UnmanagedType.LPStr)]
public string name;
[FieldOffset(4)]
public IntPtr disp;
}
Sorry this is too long. Unfortunately, I couldn't make a small consise question.
So this code compiled (maybe not anymore since this is an extract from the bigger picture) but when loading the resulting dll from the technical analysis software, I received an ACCESS VIOLATION ERROR. I believe this means the C# conversion doesn't map the C++ variables size correctly. With the arrays of structures and delegates this project has become too difficult for me to solve alone.
Any help will be much appreciated!
Thanks,
Guillaume
I can't help in your specific case, but I can tell you a couple things that will make your life easier:
Function pointers created from managed delegates should never, ever, ever, be stored in unmanaged code. I don't say this lightly. There are claims that if you create a function pointer from a delegate with GetFunctionPointerForDelegate, that the appropriate thunks will get created and won't ever get garbage collected. This is NOT true. I have watched function pointers that were good on one call go sour on the next. The safest bet is to guarantee that function pointers will never get stored beyond the use of an unmanaged call.
P/Invoke is OK for some tasks, but by far the easiest way to integrate a non-C#-friendly C++ library is to make a better/more appropriate wrapper for it in C++/CLI. See here for a description of one way to approach the problem.
I simply want to call a C function from a DLL in C#. This C function returns a struct.
Here the .h-file declaration of the c dll:
typedef struct t_Point{
int x;
int y;
} Point;
Point myFuncs();
Now I want to use this function in C#. Wrapper.cs:
using System.Text;
using System.Runtime.InteropServices;
namespace CSharp_mit_OpenCV
{
[StructLayout(LayoutKind.Sequential)]
public struct Point
{
public int x;
public int y;
};
class Wrapper
{
[DllImport("OpenCV Test.dll", CharSet= CharSet.Auto)]
[return: MarshalAs(UnmanagedType.Struct)]
public static extern Point myFuncs();
}
}
The usage is as follows:
Point p = Wrapper.myFuncs();
(Naming is probably not the best)
myFuncs only declares a struct, puts some values to x and y and returns it. The problem: The values I get in C# are different than those generated in the C function. It should be 4 and 2 and it is 0 and 111226272. What's the problem here?
Thanks for any help!
Your marshall code seems correct, what is the pack layout of the unmanaged method? LayoutKind.Sequential will treat your ints as 4 byte. Check if this is correct.
I want to use PInvoke to bring to managed side something this:
(C code)
typedef struct{
//some fields...
} A;
type struct{
A* a;
} B;
int getB(B* destination){ //destionation will be an output parameter to C#
//puts an B in 'destination'
return 0;
}
Now, I need a way to tell managed side how to marshalling B from C to C# structure or class. I've tryed many things such as IntPtr fields, MarchalAs atributes, but with no success. I will not expose here the code that I've tryed to keep the question simple. However i could do it as long answers arrive.
If it were me, I would just use unsafe code and use pointers on the C# side:
public unsafe class UnmanagedStuff {
public struct A {
// some fields
}
public struct B {
public A* a;
}
// Add appropriate PInvoke attribute here
public static extern int getB(B* destination);
public static void UseBForSomething() {
B b;
getB(&b);
// Do something with b
}
}
You can do that using the Marshal class.
// Define a C# struct to match the unmanaged one
struct B
{
IntPtr a;
}
[DllImport("dllName")]
extern int getB(IntPtr destination);
B GetB()
{
IntPtr ptrToB = IntPtr.Zero;
getB(ptrToB);
return (B)Marshal.PtrToStructure(ptrToB, typeof(B));
}