Given the following c++ class in foo.dll
class a{
private:
int _answer;
public:
a(int answer) { _answer = answer; }
__declspec(dllexport) int GetAnswer() { return _answer; }
}
I would like the pInvoke GetAnswer from C#. To do that, I use the following method:
[DllImport("foo.dll", CallingConvention = CallingConvention.ThisCall, EntryPoint= "something")]
public static extern int GetAnswer(IntPtr thisA);
And I pass in an IntPtr that points to an a (that I got from somewhere else, it's not important). CallingConvention = CallingConvention.ThisCall makes sure it's handled correctly
What's cool about this question is that I know I'm right so far because it's already working great! Using Depends.exe, I can see that "GetAnswer" is exported as ?GetAnswer#a##UAEHXZ (Or something close - the point being that it's been name mangled). When I plug the mangled name into the "something" for the EntryPoint everything works great! It took me about a day before it dawned on me to use Depends.exe, so I'm going to leave this here as a help to anybody who has a similar issue.
My REAL Question is: Is there any way to disable C++ name mangling on GetAnswer so that I don't need to put the mangled name in as my entry point. Having the mangled name in there seems like it could break, because my understanding of name mangling is that it can change if the compiler changes. Also it's a pain in the butt to use Depends.exe for every instance method that I want to pInvoke.
Edit: Forgot to add what I've tried:
I don't seem to be able to put extern "C" on the function declaration, although I can stick it on the definition. This doesn't seem to help though (which is obvious when you think about it)
The only other solution I can think of is a c-style function that wraps the instance method and takes an instance of an a as a parameter. Then, disable name mangling on that wrapper and pInvoke that. I'd rather stick with the solution that I already have, though. I already told my co-workers that pInvoke is great. I'm going to look like an idiot if I have to put special functions in our c++ library just to make pInvoke work.
You cannot disable mangling for a C++ class method, but you may well be able to export the function under a name of your choice using /EXPORT or a .def file.
However, your entire approach is brittle because you rely on an implementation detail, namely that this is passed as an implicit parameter. And what's more, exporting individual methods of a class is a recipe for pain.
The most sensible strategies for exposing a C++ class to .net languages are:
Create flat C wrapper functions and p/invoke those.
Create a C++/CLI mixed mode layer that publishes a managed class that wraps the native class.
Option 2 is preferable in my opinion.
You may be able to use the comment/linker #pragma to pass the /EXPORT switch to the linker which should allow you to rename the exported symbol:
#pragma comment(linker, "/EXPORT:GetAnswer=?GetAnswer#a##UAEHXZ")
Unfortunately, this does not resolve your need to look up the mangled name using depends or some other tool.
You do not have to disable the mangled name which actually contains lots of information of how the function itself is declared, it basically represents the whole signature of the function after the function name gets de-mangled. I understand you already found a word-around and the other answer has been marked as a correct answer. What I am writing below is how we can make it work as you desired.
[DllImport("foo.dll", CallingConvention = CallingConvention.ThisCall, EntryPoint = "#OrdinalNumber")]
public static extern int GetAnswer(IntPtr thisA);
If you replace "#OrdinalNumber" with the real ordinal number of GetAnsweer, such as "#1", it will work as you desired.
You may just consider the EntryPoint property is the same as the function name we pass to GetProcAddress where you can either pass the function name or the ordinal number of the function.
Your approach to calling non-static function members of a C++ class is indeed correct and thiscall is used correctly and that is exactly thiscall calling convention comes in play in C# P/Invoke. The issue with this approach is that you will have to look into the DLL's PE information, Export Function Information and find out the ordinal number for each function you would like to call, if you have a big number of C++ functions to call, you may want to automate such a process.
From the Question Author: The solution I actually went with
I ended up going with a c-style function that wraps the instance method and takes an instance of an a as a parameter. That way, if the class ever does get inherited from, the right virtual method will get called.
I deliberately chose not to go with C++/CLI because it's just one more project to manage. If I needed to use all of the methods on a class, I would consider it, but I really only need this one method that serializes the class data.
Related
Please find the below Examples
void ExitLibrary(); // C++ function
I am able to covert to C# like below
[DllImport("VisualFOD.dll", CharSet = CharSet.Ansi)]
public static extern void ExitLibrary();
This method will be used to start the live capturing
bool StartLive(DShowLib::Grabber* f_Grabber, HDC f_hHdc, RECT f_WindowRect);
This method will be used to start the live capturing
[DllImport("TIS_UDSHL09_vc71.dll", CharSet = CharSet.Ansi)]
public static extern bool StartLive(IntPtr f_Grabber, IntPtr f_hHdc, Rectangle f_WindowRect);
I am not able to access the actual class DShowLib::Grabber as mentioned in the 2nd statement C++ function.
Do you have any idea how to declare and get the library class name?
This dll is not a COM/.Net dll to include as a reference to C# environment.
If I understand your question correctly, you won't be able to access the DShowLib::Grabber class directly in C# if it resides inside the DLL (and is accessed by a C++ header file).
Two solutions are available. Firstly, you can wrap the native DLL class in C++/CLI class, such that you can include the native C++ header files; Secondly, you can write your own DLL, where you can flatten the DLL class and P/Invoke your flatten methods.
By flattening I mean:
class C
{
void M(int i, float j);
}
The method M has to be called like:
__declspec(dllexport) BOOL __stdcall C_M(C * instance, int i, float j)
{
if (instance == nullptr)
return FALSE;
instance->M(i, j);
return TRUE;
}
However, please take into consideration comments for your question. If you can use ready-made wrapper for library you are interested in, you shall do so :)
There were many attempts to make C#/C++ interop easier, but by far managed C++ from Microsoft is probably the best (though it is not part of C++ standard).
Another promising option is CXXI from Mono,
http://tirania.org/blog/archive/2011/Dec-19.html
which provides a standard compliant approach.
It still needs some hacking (Google Summer of Code 2012 maybe), but it should finally support Visual C++ and Windows.
Yes. You can instantiate and access a C++ class object directly from your own C# code via PInvoke, but it is very complicated and involves a lot of work, please go here and read what that C++ PInvoke Interop SDK can do.
http://www.xinterop.com/index.php/2013/04/13/introduction-to-c-pinvoke-interop-sdk/
(It is a commercial software though. I am the author)
I'm using from C# a C++ library that wasn't quite developed for using from another languages, but it's difficult to change something now. I'm using [DllImport] calls, and nearly everything works fine.
But know I need to use a "backward" call from C++ to C#. So, in C# I need to subscribe to the C++ function. In C++ it is implemented like this:
INTF_API void __stdcall Intf_InstallHandler(Intf_MailHdlPtrType Hdl)
typedef void (STDCALL *Intf_MailHdlPtrType)(InstanceNoType instno,
const MailType* mail, rsuint16 mailsize);
(I've removed a few arguments from the method to make it simplier. They are two bytes - not important)
So in C# I have:
public delegate void MailEventHandler(byte Instance, MailType Mail, UInt16 MailLength);
MailHandlerDef defHandler = (f) =>
{
var structure =
(MailType)Marshal.PtrToStructure(f, typeof(MailType));
handler(structure);
};
Externs.Intf_InstallMailHandler(defHandler);
[DllImport(DLL_NAME, CallingConvention = CallingConvention.StdCall)]
public static extern void Intf_InstallMailHandler(byte Instance, [MarshalAs(UnmanagedType.FunctionPtr)] MailHandlerDef MailHandler, PrimitiveType Primitive);
We have different primitives (in C++ dll every method is a primitive, and every method is implemented as a call-and-wait method, or as an async method, where mailHandler is called on the end of the computation), and all primitives work, except one.
If this primitive is called, the NullRef exception is thrown, without any stacktrace. And I already lost my mind in trying to search what causing this. Intresting is that this doesn't work at our large application (even I tried to switch everyting off, and just call it at start up), but on my small test application it works.
Any help is appreciated.
you could use a .Net CLI C++ wrapper which would then call your standard C++ code.
I am trying to convert C# project to Delphi, the C# code itself is importing functions from native dll let us name it ‘dmp.dll’ which I don’t have the signature of its native functions and I have to look at how these functions are imported in c# and try to import them in Delphi code, and I did import a lot of them and they work fine, but now I am struggling with this function (StartLogging). This is how they imported in C#:
[DllImport("dmp.dll", CharSet = CharSet.Auto, EntryPoint = "StartLogging")]
public static extern int StartLogging(String AdapterName,
PLOG_RECORD_CALLBACK LogRecordCallback,
SafeWaitHandle StopLoggingEvent);
no problem about PLOG_RECORD_CALLBACK, but the problem is on this parameter SafeWaitHandle which is class exists in Microsoft.Win32.SafeHandles namespace. How can I port it to Delphi? What is the equivalent data type to it in Delphi?
And here is how they use it in the C# Code:
AutoResetEvent StopEvent = new AutoResetEvent(false);
The class AutoResetEvent exists in System.Threading
Then they call the method like this:
StartLogging(comboBox1.Text, CallbackProcedure, StopEvent.SafeWaitHandle);
Then at the last and to stop the logging they use:
StopEvent.Set();
I am really confused and I don’t know how to do this, appreciate your help.
Thanks
You can probably use SyncObjs TEvent. If you create it with the ManualReset argument set to False, it should work about the same way. Just pass the Event.Handle (which is a THandle and is compatible with anything in the API expecting one).
I don't have a sample of using a non-manual reset event, but an example of creating a TEvent can be found in the accepted answer here; to make it a non-manually reset (IOW, AutoReset), just change the second parameter to False.
Like David Hefferman said in a response to the approved answer: TSimpleEvent (System.SyncObjs) works the same way as the AutoResetEvent of C#.
It can be used like:
_WaitEvent := TSimpleEvent.Create(nil, resetmanual {boolean}, false, '', false);
_WaitEvent.SetEvent;
_WaitEvent.WaitFor;
If you use the manual reset, just simply use:
_WaitEvent.ResetEvent;
In another question I asked, a comment arose indicating that the .NET framework's Array.Copy method uses unmanaged code. I went digging with Reflector and found the signature one of the Array.Copy method overloads is defined as so:
[MethodImpl(MethodImplOptions.InternalCall), ReliabilityContract(Consistency.MayCorruptInstance, Cer.MayFail)]
internal static extern void Copy(Array sourceArray, int sourceIndex, Array destinationArray, int destinationIndex, int length, bool reliable);
After looking at this, I'm slightly confused. The source of my confusion is the extern modifier which means (MSDN link):
The extern modifier is used to declare
a method that is implemented
externally.
However, the method declaration is also decorated with a MethodImplOptions.InternalCall attribute, which indicates (MSDN link):
Specifies an internal call. An
internal call is a call to a method
that is implemented within the common
language runtime itself.
Can anyone explain this seemingly apparent contradiction?
I would have just commented on leppie's post, but it was getting a bit long.
I'm currently working on an experimental CLI implementation. There are many cases where a publicly exposed method (or property) can't be implemented without knowledge of how the virtual machine is implemented internally. One example is OffsetToStringData, which requires knowledge of how the memory manager allocates strings.
For cases like this, where there is no C# code to express the method, you can treat each call to the method in a special way internal to the JIT process. As an example here, replacing the call byte code with a ldc.i4 (load constant integer) before passing it to the native code generator. The InternalCall flag means "The body of this method is treated in a special way by the runtime itself." There may or may not be an actual implementation - in several cases in my code the call is treated as an intrinsic by the JIT.
There are other cases where the JIT may have special information available that allows heavy optimization of a method. One example is the Math methods, where even though these can be implemented in C#, specifying InternalCall to make them effectively intrinsics has significant performance benefits.
In C#, a method has to have a body unless it is abstract or extern. The extern means a general "You can call this method from C# code, but the body of it is actually defined elsewhere.". When the JIT reaches a call to an extern method, it looks up where to find the body and behaves in different ways per the result.
The DllImport attribute instructs the JIT to make a P/Invoke stub to call a native code implementation.
The InternalCall flag instructs the JIT to treat the call in a self-defined way.
(There are some others, but I don't have examples off the top of my head for their use.)
InternalCall means provided by the framework.
extern says you are not providing code.
extern can be used in 2 general situations, like above, or with p/invoke.
With p/invoke, you simply tell the method where to get the implementation.
I am working on my phd in chemistry and for that reason I need to write a software application to help me with the imaging of samples under a microscope. This microscope is fitted with an x-y-z nanopositioning stage. The stage is controlled using an unmanaged DLL written in VC++ by the hardware vendor. I could provide you with more specifics of needed but let me start with this;
One of the methods in the dll allows me to read settings for the axis of motion:
C++ syntax:
BOOL E7XX_qSVO (int ID, const char* szAxes, BOOL* pbValueArray)
Where BOOL is int 0 or 1 according to the convention.
My C# wrapper contains:
[DllImport("E7XX_GCS_DLL.dll", EntryPoint = "E7XX_qSVO")]
public static extern int qSVO(int iId, string sAxes, int []iValArray);
This seems correct to me. However when I try something like this in my main application (to query axis 1,2 and 3):
Int32 [] _iValues = new Int32[3];
E7XXController.qSVO(m_iControllerID, "123", _iValues);
I consistently get an array like this:
{6, 0, 10} while I should get {0, 0 , 0} according to the display on the device itself. The complementary function:
BOOL E7XX_SVO (int ID, const char* szAxes, const BOOL* pbValueArray) to set the same status bits on the stage also don't work...
Other commands in the dll work perfectly. I can pass strings and doubles in and out without troublem but not the BOOL type...
Do you guys have any idea what could be wrong?
BOOL in C++ is actually an "int" so make sure you use System.Int32 and not System.Boolean.
Alternatively it might be using COM data types i.e. VARIANT_BOOL, in which case you need do need System.Boolean.
Have you tried running dependency viewer to confirm the function prototype?
Have you tried specifying the MarshalAs attribute, e.g.:-
[DllImport("E7XX_GCS_DLL.dll", EntryPoint = "E7XX_qSVO")]
public static extern int qSVO(int iId,
[MarshalAs(UnmanagedType.AnsiBStr)]string sAxes,
[MarshalAs(UnmanagedType.LPArray)]int[] iValArray);
Also, have you tried literally passing the sAxes string as a char array, e.g.:-
[DllImport("E7XX_GCS_DLL.dll", EntryPoint = "E7XX_qSVO")]
public static extern int qSVO(int iId,
[MarshalAs(UnmanagedType.LPArray)]char[] sAxes,
[MarshalAs(UnmanagedType.LPArray)]int[] iValArray);
I've found that with interop you often need to experiment a fair bit to get it working, so try different combinations, check out the different members of the UnmanagedType enumeration and also play about with other people's suggestions too. However, please try the above two techniques and let me know whether they help sort it!
This isn't going to answer your question directly but a sweet tool for getting common Interop Win32 calls easily is using Pinvoke.net The even have a Visual Studio plugin.
http://www.pinvoke.net/index.aspx
The docs for your function say:
The VC++ compiler needs an extern "C" modifier. The declaration must also specify that these functions are to be called like standard Win-API functions. That means the VC++ compiler needs to see a WINAPI or __stdcall modifier in the declaration.
The default calling convention for DllImport is WinApi. On Windows, that's StdCall. But on CE, that's Cdecl. You want to make sure that you use the right calling convention. You might want to tray adding:
CallConvention = CallingConvention.StdCall
Also, specify our string character set:
CharSet = CharSet.Ansi
But it should work even without these. It's quite odd as your code looks right.