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Convert C++ function for use in C#

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.

How to PInvoke an Instance Method by disabling Name Mangling

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.

using C++ DLL in C# windows application:Getting error "Entry point not found"

I am new to using C++ libraries in C# and also to the C++ programming in general. I have a DLL built from a C++ code which I believe is a 'managed' code as the name of the DLL is "TestManaged.dll". I am not 100% sure if the dll/C++ code is managed/unmanaged.
I want to use classes and methods of this DLL in my C# windows forms application code. There are multiple classes in this DLL. When I chekced these classes and methods inside those classes in Object Browser, all of them have Public identifier.
So far, I have added this DLL to my references of C# application code. There are three classes I would talk about in my question: Product, ReqStatus, ProductData. I could create an object(s) for various classes of this DLL as follows.
Product testCall = new ProductClass();
There is another class called ProductData in this DLL and I could get the C++ code for this class which is as follows. In this case, ProductData is shown as class in Object Browser in C# where as it is actually a struct in C++ code. I am not sure if this is important to answer my question (at the end).
Following is a C++ code that defines ProductData struct - ProductData.h file.
#ifdef WIN32_MANAGED
public ref struct ProductData
#else
struct ProductData
#endif
{
UINT32 ProductId; //!< Product ID
UINT32 PRoductRev; //!< Build Revision
};
Following is a C++ code that defines ReqStatus enum - ReqStatus.h file. I have created the same enum in my C# code with no identifier specified.
enum ReqStatus
{
SUCCESS, //!< Method was successful
//Connection errors
NOT_CONNECTED, //!< Connection not open
CONN_TIMEOUT, //!< Connection timed out commuincating with device
};
Now, there are two methods I want to call and have problems with both:
Method 1: is a getProductData method inside Product class which accepts object of ProductData type as a parameter and returns the ReqStatus which is an enum type in C++. So following is the declaration of the gerProductData method (as seen in the Object Browser):
public ReqStatus getProductData(ProductData data)
The same method's C++ delcaration is: (The actual method is too long and hence just giving the declaration): This method is inside Prodcut.cpp file
ReqStatus Product::getProductData(ProductData PLATFORM_PTR data)
PLATFORM_PTR is defined as below in Platform.h
#ifdef WIN32_MANAGED
#define PLATFORM_PTR ^
#else
#define PLATFORM_PTR *
#endif
Method 2: is a getConnected method inside Product class which accepts a character array (I am not sure of this) and an object of ProductData type as a parameter and returns the ReqStatus which is an enum type in C++. So following is the declaration of the getConnected method (as seen in the Object Browser):
public ReqStatus getConnected(sbyte* someChar, ProductData data)
The same method's C++ delcaration is: (The actual method is too long and hence just giving the declaration): This method is inside Prodcut.cpp file
ReqStatus Product::getConnected(const char *someChar, ProductData PLATFORM_PTR data)
C++ code calls the methods as follows:
private : Product^ _testProduct;
testProduct = gcnew Product();
ProductData ^ data = gcnew ProductData();
int portNum = Decimal::ToInt16(7);
char portName[32];
_snprintf(&portName[0], sizeof(portName),"COM%d", portNum);
ReqStatus status = _testProduct->getConnected(&portName[0], data); //Calling getConnected
There is an internal call to getProductData method inside the getConnected method.
ReqStatus status = getProductData(data); //data is the same which was passed to the getConnected method
MY C# code is as follows and I got errors at both method calls: I have put errors on the same line in the below code snippet. Both methods are independent. Its just that the getProductData is called from getConnected method in C++ code. I wanted to check if I can call both individually.
ProductData pData = new ProductData(); // OK
Product _testProduct = new Product(); // OK
ReqStatus status1 = _testProduct.getConnected("COM5", pData ); //Error 1: The best overloaded method getConnected has some invalid arguments
ReqStatus status2 = (ReqStatus)_testProduct.getProductData(pData ); // Error 2: Method is inaccessible due to its protection level
For Error 1, I tried solutions from various articles on StackOverflow and other forums but, could not solve it. Just for a reference, I tried to change the "SomePortCOM" as follows but it din't work.
UPDATE: This code works fine now and I don't see Error 1(Invalid arguments). Now, I only need to get rid of the Error 2 (Protection level error). Kindly provide any suggestions. Thank you.
String str = "COM5";
byte[] bytes = Encoding.ASCII.GetBytes(str);
unsafe
{
fixed (byte* p = bytes)
{
sbyte* sp = (sbyte*)p;
//SP is now what you want
ReqStatus status1 = _testProduct.getConnected(sp, pData );
}
}
For Error2, I searched so many blogs and found that one of the possible solution could be the use of DLLImport, I tried that as well and I have following issue:
C# declaration of DLLImport:
[DllImport("TestManaged.dll",EntryPoint="getConnected")]
public static extern ReqStatus getConnected(String SerialPort, ref ProductData pData);
I am calling this function as below from my C# code:
ProductData pData = new ProductData();
String str = "COM7";
ReqStatus status1 = getConnected(str, ref pData);
However, I am getting Entry point not found error. I tried to run the dumpbin function to get the list of functions exported by this DLL. But, I do not see any functions. Rather just a random output as below.
Microsoft (R) COFF/PE Dumper Version 10.00.40219.01
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file C:\Rumit\TestManaged.dll
File Type: DLL
Summary
2000 .data
22000 .rdata
1000 .reloc
1000 .rsrc
13000 .text
UPDATE:
Also, I do not see any methods in this DLL via Dependency Walker.
Now, I have got the source code for C++. But I am fairly new to C++ coding. In case any change is required to C++ code, kindly give the directions.
Regards,
Rumit

enum ReqStatus
That's your biggest hang-up. That declares a native enum type, it is not usable in managed code and makes any code that uses it inaccessible. You must declare the managed version of it with the enum class keyword, like this:
public enum class ReqStatus {
// etc...
}

The unsafe block around your code will make it so your assembly cannot be verified for security protocols, so be wary of that. When I've called methods from C++ (native or not) from C# I've had to use PInvoke (Platform Invoke) to call them. For a protection level, I know you said everything is public in the C++, but if you're new to C++ you may have made a quick syntax mistake. In C#, all methods need to be preceded by a storage specifier (public, protected, etc...) but in C++ you place a storage specifier followed by a colon and everything between THAT storage and the next declared storage will be of that storage type. Maybe this is your issue?

Thank you Hans, for the pointing out the problem. Just that, I made the enum definition as 'public'. But, I'm not sure if you put the 'class' by mistake or was it intentional as it was giving me so many errors as it wasn't taking it as an enum and was asking for an object at every place I used enum. Let me know if I misunderstood anything here.
So, I got it worked just by making the enum public. However, I am still not able to find how to pass the correct values to that the C++ function from C#. (Error1 in my original post).
When I debuged the C++ code, it passes the "0x0034E808 "COM5" value (I assume it's a memory location and a value?) for the first parameter of the getConnected method. I tried to pass this value by implementing the unsafe method (explained in my original post for Error 1), it passes "0x0277aab8" (again seems some memory address), but could not get connected to it (Getting serial port timeout errors). Am I passing the value incorrectly compared to C++ method?
Regards,
Rumit

Calling DLL using C#

typedef struct
{
// The contents of this struct are platform-dependent and subject to
// change. You should not manipulate the contents of this struct directly.
/*New stuff*/
// HWND m_hWnd;
// HDEVNOTIFY m_hDevNotify;
} fdGlove;
FGLOVEAPI fdGlove *fdOpen(char *pPort, bool bOnlyAllowSingleConnection = false);
FGLOVEAPI int fdClose(fdGlove *pFG);
FGLOVEAPI int fdGetGloveHand(fdGlove *pFG);
FGLOVEAPI int fdGetGloveType(fdGlove *pFG);
I have a DLL file called fglove.dll and I need using C# to call it.
I have written code like this:
[StructLayout(LayoutKind.Sequential)]
public class fdGlove
{
}
[DllImport("fglove.dll")]
public static extern fdGlove fdOpen(string pPort, bool bOnlyAllowSingleConnection);
but when I debug the program it is has an error (Unable to find an entry point named 'fdOpen' in DLL 'fglove.dll'.)
Can someone point out what I have done wrong?

fdOpen is using a default parameter - which can only mean you are trying to export a C++ function out of a DLL. The result is that "fdOpen" is getting "name mangled" in the export table as something that looks like "fdOpen#YAXPB_W0I#Z".
You're better off exporting this function as C. Declare and define fdOpen as follows:
extern "C" fdGlove* __stdcall fdOpen(char* pPort, bool bOnlyAllowSingleConnection);
Other possible issues:
The DLL isn't in the same directory as the EXE trying to load it.
You forgot to export the function from the DLL. You need to use a .DEF file or the __declspec(dllexport) attribute on the function definition. Use "dumpbin /exports fglove.dll" to figure out if this is the case.
Confusion between stdcall and cdecl compile options. I get it confused, so try replacing "_stdcall" with "_cdecl" above. Or try it without either attribute.

The compiler of fglove is most likely doing so type of name mangling.
Use DUMPBIN fglove.dll to get the real names.
Then use [DllImport("fglove.dll", EntryPoint='...')] where ... is the real name.

look at the name of the exported symbol in Dependency Walker. More than likely it will be _fdImport or something similar and you will need to update your DLLImportAttribute to match the exported name.

C# - DLLImport and function default values

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.