I have a legacy DLL written in C that I'd like to call from a C# .NET application. The problem is that the DLL interface for the C DLL is fairly complicated. It's something like this:
__declspec(dllexport) void __stdcall ProcessChunk(
void *p_prochdl,
const BIG_INPUT_STRC *p_inparams,
BIG_OUTPUT_STRC *p_outparams
);
The BIG_INPUT_STRC/BIG_OUTPUT_STRC contain all kinds of things... pointers to buffer arrays, enumerated parameters, integer parameters, etc. In short, they're complicated.
First question: Is there an easy way to get all of the structure information that is contained in the DLL header file into the C# class, or do you need to literally copy and paste everything to C# and re-define it? This seems redundant.
Related to that, what is the correct way to pass structs into the unmanaged DLL from C#?
Finally, is there an example of how to correctly pass buffer arrays from C# into the unmanaged DLL? Alternatively, how can I pass a two-dimensional array into the DLL?
Thanks,
-Greg
Its quite straight forward to do this soft of thing in C# using P/Invoke.
I belive you are going to have to define the data structures in C# manually.
I would reccomend you take a look at this MSDN article on the subject
You'll need to extensively use .net marshalling. First you need to re-define C structs in your C# code, then make sure everything gets marshalle properly using the MarshalAs attribute.
If you need to pass a pointer-to-structure in C# back to the C function you can use the Marshal.StructToPtr function.
Buffer arrays, assuming they're defined as byte[] you can marshal using the following technique:
byte[] buffer = ...;
fixed(byte *pBuffer = buffer)
{
// do something with the pBuffer
}
The fixed statement makes sure that the bufffer doesn't get moved in the memory by the garbage collector, making the pointer within the statement 'fixed' or 'pinned'.
As for the multi dimensional arrays, it depends on the underlying C implementation, you might for example work with a pointer to the array and adjust the position based on the number of dimension and the number of elements in each dimension like:
someValue = buffer[(elementsPerDimensions * x) + y];
From your description it already seems pretty complicated, have you considered making your library COM friendly?
What a pill.
I'd avoid trying to make the library COM friendly. That's more trouble than its worth.
I'd build a second native library with functions designed to be P/INVOKED to build up the records for the first and call it.
Alternately, you could make a C++ native/managed library to handle the marshaling.
Related
I am currently messing around with C# in order to be able to approach ASP.NET with an acceptable knowledge base. In order to do so I occasionally look at my old C++ code and see if I can figure out how to make it work in C#.
So I figured that I will look into arrays, a pretty common way to store data. What I was wondering is what would the C# equivalent of creating a two dimensional array via pointers would look like?
I know that in C++ one way to do it could look like this:
double **myArray;
myArray = new double * [rows];
for(int I = 0; I < rows; I++) {
myArray[I] = new double[cols];
}
I also know that in C# you could easily just say double [rows,cols] and that would create a 2D array. I know that C++ and C# most likely treat pointers differently, but just for the sake of interest, how would I mimic a similar behavior in C#?
Cheers.
C# arrays and C++ pointers are incredibly similar, with the biggest difference is that C# has a layer of managed logic on top of the array data.
An array of pointers in C# is possible. You could do it, for example, with:
unsafe
{
double*[] ptrArray = new double*[5];
ptrArray[2][0] = 4.0;
double*[,] ptr2DArray = new double*[5,5];
ptr2DArray [2, 2][0] = 4.0;
}
However, like I said in my previous comment, using pointers in C# is discouraged, and generally you want to use a managed approach whenever possible. Personally, I cannot fathom a need for creating an array of pointers (much less a 2D array) and I imagine that if someone's program needs a structure like this, then they probably need to rethink their approach.
Usually pointers are very less used in C#, When you use pointers in C# it is considered as unsafe context.
Please check following MSDN Pointer Document that illustrates how to use pointers.
I would like to say don't use pointers if not required. You can use Ref Keyword and Out keyword to pass your parameter as a reference.
When you use pointer you loose all managed code benefits.
You can check C# ref is it like a pointer in C/C++.
Apart from that i think that all arrays are already Reference Type even if it contains all members of value type.
Check Value Types and Reference Types in C#.
I hope it helps.
I have a C# array<System::Byte> and I want that to translate to a C++ byte*. How can I do that? I am using C++/CLR because it lets me use managed/unmanaged code in the same project. I'm basically writing a DLL and making a few methods that can be called via C#, but that contain unmanaged C++ code.
So basically, my C++/CLR method header is this:
void testMethod(array<Byte>^ bytesFromCSharp);
and inside of that testMethod I would like to translate the bytesFromCSharp into a byte* which can be used by other unmanaged C++ code. I malloced the byte* array and wrote a for loop to copy byte by byte but it feels like there should be a better solution.
edit: Example of Hans' technique from his answer below:
//C#
byte[] myBytes = {1,2,3,4};
//C++/CLI
void testMethod(array<byte>^ myBytes) {
pin_ptr<byte> thePtr = &myBytes[0];
byte* bPtr = thePtr;
nativeCode(bPtr);
...
}
Use the pin_ptr<> template class to pin the array and generate a pointer that native code can use. Pinning it ensures that the array cannot be moved by the garbage collector while the native code is using it.
Just make sure that the native code cannot use the array anymore after the pin_ptr<> variable goes out of scope. Which also means that native code storing the pointer for later use is not okay. If it does then you have to make a copy.
I would like to pass C# reference to C/C++ code(pointer), and than get it back from pointer to C#reference.
I have many pairs (uint,Object). I created sorting mechanism in C code, because its much faster than C#. There is only key(uint) and value(object reference) needed. C code is using key for sorting, and is not changing the value(pointer). Is there any simple way to do it? Do i need to use marshalling? C function will be called many times(maybe even a million times), so i am affraid it will be with marshalling too slow, and i don´t even know how to do it with marshalling. I believe when objects address is changed by GC, address of C# reference is not changed. So there will not be needed to place object in pined memory. Am i right about this?
Now i am able to call C function using DllImport, i am able to store C# reference to C pointer, but i am not able to get address stored in C pointer to C# reference.
Any ideas on how to do this?
It is not possible to pass any variables directly from manged c# code to native c++ code. The solution is pinvoke where you can marshal the data.
This works fine but you should know that this is not everytime the fastes solution. On every call the memory must be copied and maybe converted depending on the data types.
My solution to a similar problem, I had some old code in Fortan.
I convert this code to c, then created a managed c++ project.
In c#, I called this manged c++ project.
the c# code looks like this:
unsafe
{
double* input = (double*)utils.Memory.Alloc(sizeof(double) * n);
double* output = (double*)utils.Memory.Alloc(sizeof(double) * n);
//calling the c++ code
c_plus_plus.code(input,output);
//now output contains the output
// (you can use the same array as input and output
}
I hope this helps.
I have a C++ COM server that fills a C# caller's structure with data.
The structure is defined in the C++ IDL, something like the following:
interface Icontrol : IDispatch{
[
uuid(...),
version(1.0) ]
typedef struct testStructure
{
int x;
int y;
int z;
...
} testStructure;
...
[id(9)] HRESULT getStruct([ref,in,out] testStructure * theData);
...
Then, in the C# code:
EO_Lib.testStructure test = new EO_Lib.testStructure();
EO_Lib.getStruct(ref test);
I can make this work no problem with a regular .DLL by simply using MarshallAs in a C# structure for the fields non-native to C#. But I can't get that to work on a COM .DLL. I suspect it's because of my lack of knowledge about IDL.
What I need to be able to do is call getStruct() with a new C# type that I've created with appropriate MarshallAs() information. How do I do this?
I am using Visual Studio 2010 MFC/ATL C++ and C# .NET 4 Framework, if it helps.
While not an actual solution to the question, the following obviates the reason for my question.
From: http://msdn.microsoft.com/en-us/library/75dwhxf7(v=vs.100).aspx
Structures that are returned from platform invoke calls must be blittable types. Platform invoke does not support non-blittable structures as return types.
So basically, you can't do what I wanted to do.
If you ignore the COM aspect, BrokenGlass's solution here answers the convert-one-structure-to-another question.
It sounds like you are trying to tackle this from the Managed side of things. Not knowing many details about your architecture (like is this your COM server, as in you wrote it/own the code?) I'm going to have to make some assumptions in order to provide any solutions that approach from the C++ side, specifically the MIDL (IDL) aspect.
You mention the IDL and from what I can tell, at the very least you have access to the IDL that generates the type library. So, forget about what the CLR (managed) side for a bit and get it all working in the un-managed environments (COM).
Once that is done, determine what you want the struct to look like for your managed clients. Using that information, define a new type in your C++/MIDL project(s). If you don't have an ACF (Application Config File) file for your IDL, create one (its just an IDL file with the exact same name but with an ".acf" extension instead of ".idl". In the ACF file declare a user_marshal type that maps your COM struct to the new "managed friendly" struct that you just defined. Something like this:
// ACF file
typedef [user_marshal(testStructure_ForDotNet)] testStructure;
// where testStructure is your "original" struct and "testStructure_ForDotNet"
// is the one you just defined.
Finally, you'll need to define your conversion functions (Marshaling functions). Refer to this resource for more information:
http://msdn.microsoft.com/en-us/library/windows/desktop/aa367296(v=vs.85).aspx
Anyway, hope that helps (anyone).
I have a structure that represents a wire format packet. In this structure is an array of other structures. I have generic code that handles this very nicely for most cases but this array of structures case is throwing the marshaller for a loop.
Unsafe code is a no go since I can't get a pointer to a struct with an array (argh!).
I can see from this codeproject article that there is a very nice, generic approach involving C++/CLI that goes something like...
public ref class Reader abstract sealed
{
public:
generic <typename T> where T : value class
static T Read(array<System::Byte>^ data)
{
T value;
pin_ptr<System::Byte> src = &data[0];
pin_ptr<T> dst = &value;
memcpy((void*)dst, (void*)src,
/*System::Runtime::InteropServices::Marshal::SizeOf(T::typeid)*/
sizeof(T));
return value;
}
};
Now if just had the structure -> byte array / writer version I'd be set! Thanks in advance!
Using memcpy to copy an array of bytes to a structure is extremely dangerous if you are not controlling the byte packing of the structure. It is safer to marshall and unmarshall a structure one field at a time. Of course you will lose the generic feature of the sample code you have given.
To answer your real question though (and consider this pseudo code):
public ref class Writer abstract sealed
{
public:
generic <typename T> where T : value class
static System::Byte[] Write(T value)
{
System::Byte buffer[] = new System::Byte[sizeof(T)]; // this syntax is probably wrong.
pin_ptr<System::Byte> dst = &buffer[0];
pin_ptr<T> src = &value;
memcpy((void*)dst, (void*)src,
/*System::Runtime::InteropServices::Marshal::SizeOf(T::typeid)*/
sizeof(T));
return buffer;
}
};
This is probably not the right way to go. CLR is allowed to add padding, reorder the items and alter the way it's stored in memory.
If you want to do this, be sure to add [System.Runtime.InteropServices.StructLayout] attribute to force a specific memory layout for the structure. In general, I suggest you not to mess with memory layout of .NET types.
Unsafe code can be made to do this, actually. See my post on reading structs from disk: Reading arrays from files in C# without extra copy.
Not altering the structure is certainly sound advice. I use liberal amounts of StructLayout attributes to specify the packing, layout and character encoding. Everything flows just fine.
My issue is just that I need a performant and preferably generic solution. Performance because this is a server application and generic for elegance. If you look at the codeproject link you'll see that the StructureToPtr and PtrToStructure methods perform on the order of 20 times slower than a simple unsafe pointer cast. This is one of those areas where unsafe code is full of win. C# will only let you have pointers to primitives (and it's not generic - can't get a pointer to a generic), so that's why CLI.
Thanks for the psuedocode grieve, I'll see if it gets the job done and report back.
Am I missing something? Why not create a new array of the same size and initialise each element seperately in a loop?
Using an array of byte data is quite dangerous unless you are targetting one platform only... for example your method doesn't consider differing endianness between the source and destination arrays.
Something I don't really understand about your question as well is why having an array as a member in your class is causing a problem. If the class comes from a .NET language you should have no issues, otherwise, you should be able to take the pointer in unsafe code and initialise a new array by going through the elements pointed at one by one (with unsafe code) and adding them to it.