I have a C++ DLL with partial documentation only and no source code access. I need to use functions of this C++ library in my C# application. This is what I have so far:
[DllImport(cpplib.dll)]
public static extern long someFunctionsWithNoParameters();
When I declare and call a function from the C++ DLL in my C# application like this (function with no arguments) and than call it, it works, the function returns a long value returned by the C++ function.
However I don't know how to handle functions with pointer or reference parameters, or parameters defined as in and out by the C++ function. For example this C++ function:
long functionWithParameters(long &State, char *pName, int nLen)
This is how the function is declared (I have access to the header file of the DLL, but not to the source). The parameters State and pName are declared as out parameters and the parameter nLen is declared as in parameter. How do I declare this C++ function in my C# application under the [DllImport] line and than use it (what form of parameters should I pass in and how to read the out parameters)? Is there some conversion convention between the C/C++ pointer and reference types to some C# types?
Thank you!
You need to declare a matching call convention for the imported function. You could try __cdecl or __stdcall, like this:
[DllImport(cpplib.dll, CallingConvention=CallingConvention.Cdecl)]
public static extern long someFunctionsWithNoParameters();
About pointers and references (which are the same in most practical implementations), you need to use unsafe context and C# pointers, consult MSDN for more detail.
Related
I am passing a C# object (PSObject) to managed C++ in the following way. But it crashes in the commented statement. Am I missing something?
Am I passing PSObject correctly to managed C++?
or am I accessing it wrongly?
I am using clr:oldsyntax.
Actually C# will pass a PSObject to managed C++,
in managed C++ I want to examine the properties present in PSObject. (An object returned by running PowerShell commands)
In C#, using delegate concept I am calling managed C++ method to pass PSObject.
The delegate is declared as below:
delegate bool CFuncDelegate(PSObject Arg);
funcObj (a IntPtr) is a pointer to addData function in C++ (I didn't write that code here, since it is not relevant) and I am calling addData in csharp as:
CFuncDelegate func = (CFuncDelegate)Marshal.GetDelegateForFunctionPointer(funcObj, typeof(CFuncDelegate));
bool isCompleted = func(psoObject); //psoObject is a PSObject
and in managed C++,
static bool __clrcall addData(System::Management::Automation::PSObject* curPsObj)
{
log(Marshal::StringToHGlobalUni(curPsObj->AdaptedMemberSetName));
//prints psadapted
System::Object* value = curPsObj->Properties->get_Item("DisplayName");
//crashes
}
It would be better if someone just post two lines of code to pass object from C# and accessing it in managed C++. Thanks in advance.
I think it's really time that you abandoned oldsyntax and moved to C++/CLI. Anyway, even doing that is not going to solve your problem. You define a delegate like this:
delegate bool CFuncDelegate(PSObject Arg);
And then you assign a delegate variable like this:
CFuncDelegate func = (CFuncDelegate)Marshal.GetDelegateForFunctionPointer(
funcObj, typeof(CFuncDelegate));
The documentation for Marshal.GetDelegateForFunctionPointer says:
Converts an unmanaged function pointer to a delegate.
So, your code can only work if funcObj is an unmanaged function. And your addData method is certainly not that. You need to stop using GetDelegateForFunctionPointer here. It is simply not compatible with calling the managed function addData.
I've no idea where funcObj comes from. You said:
funcObj (a IntPtr) is a pointer to addData function in cpp (i didnt write that code here, since it is not relevant)
In fact it is not only relevant to the problem, it is the root cause of the problem. What I would expect to see here would be for you to add the C++ assembly to your C# project as a reference, at which point addData could be referenced directly.
Of course, none of this even mentions the fact that your calling conventions are mis-matched. The managed code uses clrcall and your unmanaged function pointer is taken to be stdcall.
Update
There's some more information in the comments. Pass addData as a delegate. You'll need to declare the delegate type in the C# code which I believe you reference from your C++ assembly.
I am writing a C# library to be used by native C++ application. I am using C++/CLI as the Interoperability mechanisim.
I require to pass a callback function from C++ to C# (using C++/CLI as the intermediate layer). C# library needs to call the C++ function with a zero terminated string of wide characters; i.e. the prototype of the callback function is
Func(LPCWSTR pszString);
There are other parameters but they are immaterial for this discussion.
I searched net and found Marshal.GetDelegateForFunctionPointer Method wich I can use. The problem with this is that it converts System.String from C# to char* and not wchar_t* which I am looking for.
Also, what is the best method of achieving this- code example including the C++/CLI portion, if possible.
C++/CLI dll is dependent on C# dll. Method needs to be called synchronously.
GetDelegateForFunctionPointer will work, but you need to add a [MarshalAs(UnmanagedType.LPWStr)] attribute to the parameter in your delegate declaration in order for String to get converted into wchar_t*:
delegate void MyDelegate([MarshalAs(UnmanagedType.LPWStr)] string foo)
IntPtr func = ...;
MyDelegate del = (MyDelegate)Marshal.GetDelegateForFunctionPointer(func,
typeof(MyDelegate));
To pass a modifiable string, give a StringBuilder. You need to explicitly reserve space for the unmanaged function to work with:
delegate void MyDelegate([MarshalAs(UnmanagedType.LPWStr)] StringBuilder foo)
StringBuilder sb = new StringBuilder(64); // reserve 64 characters.
del(sb);
See the little-known UnmanagedFunctionPointer attribute, which is like DllImport for delegates, if you'd like to use CharSet or whatnot.
So I have this C .dll source code which I want to use in my C# application. Instead of doing bunch of DllImports I've decided to write a wrapper for it in C++/CLI.
My C function takes a pointer to a struct with 4 callback functions in it:
typedef struct
{
PFN_IN readFp;
PFN_OUT writeFp;
}OPEN_ARGS;
C++/CLI shares the same .h file, therefore uses the same typedef.
C# code has it's own definition of this structure and delegates for CBs, because I can't attach .h to C# project.
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate Int32 PFN_OUT(IntPtr arg, IntPtr pSrc, Int32 len);
[StructLayout(LayoutKind.Sequential)]
public struct OPEN_ARGS
{
public PFN_IN readFp;
public PFN_OUT writeFp;
};
So, when I add my C++/CLI dll explicitly to C# project references, the compliler wouldn't accept calls to C++/CLI functions saying
"Error 2 Argument 2: cannot convert from 'WrapperTest.Program.OPEN_ARGS' to 'SAR_OPEN_ARGS'"
But if I include the C++/CLI dll implicitly like that
[DllImport("Wrapper.dll", CharSet = CharSet.Auto, EntryPoint = "?CLIOpen##YAHHUOPEN_ARGS###Z")]
public static extern int CLIOpen(int a, OPEN_ARGS args);
It will work just fine.
So is there a way to tell C# compiler to ignore this type cast error, or may be other way to wrap C code functions?
EDIT: cleaned up variable names for better readabiltiy
What if you did this another way. Since you have a C++/CLI DLL handling interop duties between the C DLL and the C# assembly, you could expose an equivalent API, only using more .NET-like concepts.
For example, instead of exposing the struct with function pointers, you could expose a class that has three events. The C# assembly would add handlers for those events. Inside the C++ DLL, it would use the function pointers that the C DLL expects, but their implementation would fire the .NET events that the C# assembly is handling.
This would provide a much better experience using the DLL on the C# side, and likely get rid of the interop compiler errors that you're encountering.
Please consider using SWIG to generate the wrapper code for all your pinvoke.
http://www.swig.org/Doc1.3/CSharp.html
So for managed C++, you can use the #pragma managed/unmanaged compiler directives instead of pInvoke, which it looks like you are using. Then you can compile managed and native code together into the same assembly, even the same CPP file.
Then you could do something like:
#pragma managed
// include your native headers here
#include "foo.h" // whatever you call it.
#using <System.dll> // what ever else you need here...
// Have to wrap a function in a class, since .NET doesn't allow free standing functions.
public ref class foo
{
public static int sarCLIOpen(int a, SARWrapperTest::Program::SAR_OPEN_ARGS args)
{
// do something to convert your managed args to native args.
::SAR_OPEN_ARGS native_args = ...
// then call your native function
return sarCLIOpen(a, native_args );
}
};
I´m trying to call a method that is in a C++ dll declarated as __declspec(dllexport) to use in C#, but I don´t know how to return a string value from C++ and how to declare the signature using DllImport in C#.
C++ code "VNVAPI.dll"
__declspec(dllexport) char * GetGpuName(int phyGPUid)
{
CNvidia * pInstance = CNvidia::GetInstance();
char szName[512]={0};
pInstance->GetGpuName(phyGPUid,szName,512);
return szName;
}
C# method signature:
[DllImport("VNVAPI.dll")]
public static extern char GetGpuName(int phyGPUid);
Error generated:
A call to PInvoke function
'Core!Core.Hardware.IO.NVAPI::GetGpuName'
has unbalanced the stack. This is
likely because the managed PInvoke
signature does not match the unmanaged
target signature. Check that the
calling convention and parameters of
the PInvoke signature match the target
unmanaged signature.
Thanks.
As has been pointed out by others you need to specify the C calling convention in your P/Invoke and also use string on the managed side to marshal the null terminated char*.
However you should rejig the C++ routine to take a char* as an input parameter, together with a buffer length parameter. You then write into this buffer in the native code. This avoids the current problem that the data, as you presently have the code, is returned from the stack which, of course, is unwound as the function returns.
The suggestion to use static will make this memory global and so avoid stack unwind problems, at the expense of thread safety. Yes it will likely work for this use case but its a bad habit to get into.
The error message might be confusing.
Without going into detail, try this:
static char szName[512]={0};
If you still get the error, the you need to specify the calling convention in the DllImport attribute.
Edit:
Also make the return type string for C# method declaration.
The error message suggests checking the calling convention. I suspect that the function is using C calling convention, which would explain the unbalanced stack. I would suggest that you specify the calling convention (as #leppi suggested) in your DllImport attribute. Like this:
[DllImport("VNVAPI.dll", CallingConvention=CallingConvention.Cdecl)]
According to the Strings samples, the return value should be string:
static extern string GetGpuName(int phyGPUid);
I'm interfacing with a native 3rd party C++ DLL via C# and the provided interop layer looks like below:
C#:
[DllImport("csvcomm.dll")]
public static extern int CSVC_ValidateCertificate(byte[] certDER, int length);
C++:
CSVC_Status_t CSVCOMM_API CSVC_ValidateCertificate(BYTE* certDER, DWORD length,
DWORD context = CONTEXT_DEFAULT);
Note, there are only two parameters in the C# extern definition since the the C++ function provides a default value for the third parameter. Is this correct? I was receiving some non-deterministic results when using the provided definition, but when I added the third parameter like below, it seems to be working correctly each time rather than sporadically.
[DllImport("csvcomm.dll")]
public static extern int CSVC_ValidateCertificate(byte[] certDER, int length,
int context);
Any ideas? Would the addition of the 3rd parameter really fix this issue?
The optional parameter in C++ is resolved at compile time. When you call into this via P/Invoke, you need to always specify all three parameters.
If you want to have an optional parameter, you'll need to make a C# wrapper around this method with an overload that provides the optional support (or a C# 4 optional parameter). The actual call into the C++ library should always specify all three arguments, however.