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Managed C++ Wrapper for C Code - Identical function names issues

I have been tasked with creating a Wrapper for a C-library to be used in C#. I have no control over the C-library's source code and I only have access to its header file and to the static library file (.lib).
I have followed several tutorials and got this working when creating an unmanaged C++ class which is wrapped using CLI/C++ and used in C#. No problem. The problem I am facing know though, is as C does not use namespaces, I have trouble figuring out how to tell the compiler when I want to call the function from the .lib file itself, rather than my identically named wrapper function. If it helps understanding, my header file only consists of function definitions, typedefs (structs, enums), but no classes (not even sure if C headers usually do?).
What I have done:
Created a VS C++ CLR Class Library (.NET Framework) project
Linked my .lib file to all configurations in linker->inputs as an additional dependency.
Created two files: Wrapper.h and Wrapper.cpp.
#included the header file corresponding to my .lib in the Wrapper.h file
This is where I get a bit confused, mostly due to the tutorials all covering how to link a C++ library rather than a C library. The difference there being the lack of namespace in C, and (in my case) the lack of class.
Uncertainties:
I do not know if I need a ref class Analytics{} in my Wrapper.h or not. I assume I do if I want to use all the functions statically, but as there is no corresponding class in the C library, can I just name this whatever I want?
In order to properly link the C library in my header file (Wrapper.h), I need to use the same function definition as in the original header file. Do I repeat the use of "extern" and such keywords? Can I freely add static and accessibility modifiers?
At this point I want to "implement" the function in the Wrapper.cpp file. Here my problem about identical function names comes in. The function I want to implement has the same name in the original header file as it does in the Wrapper header file (obviously, as it would not work otherwise, right?). The wrapper header file contains a namespace and a class (for now class name is Analytics) so I can declare the function as Analytics::void SanSetApplicationContext(){...}, but as the original C library header file does not contain classes or namespaces, there is no way for me to call that function and distinguishing between them. The C++ compiler always prefers the local definition and I am stuck with a function calling itself for all eternity. I hope you understand my point.
I am probably doing something wrong and/or misunderstanding something, but how would you guys suggest I approach this issue? I will append my files contents below. In the files I have so far only tried implementing a single function. Code should be fixed and hopefully working
Wrapper.h
#pragma once
using namespace System;
namespace Wrapper {
public ref class Analytics
{
public:
static void SanSetApplicationContext(String^ ctx);
};
}
Wrapper.cpp
#include "pch.h"
#include "Wrapper.h"
#include "../SWA_lib/Analytics.h"
namespace Wrapper {
void Analytics::SanSetApplicationContext(String^ ctx) {
//Convert String^ to const char*
msclr::interop::marshal_context mCtx;
const char* convertedStr = mCtx.marshal_as<const char*>(ctx);
::SanSetApplicationContext(convertedStr);
}
}
Analytics.h
...
extern void SanSetApplicationContext(const char *ctx);
...
Update
Updated the code to reflect my changes made based on your comments. Thanks!
Update 2
#Bodo asked me to explain another issue of mine, which is how I would handle wrapping functions that return opaque handles. The example used is the following: typedef struct SanEvent_s *SanEvent; which is found in my C header file. So basically, this library provides an API. The API execution always starts with a call to an Initialize(); function, and ends with a call to the Terminate(); function. I have no previous experience of using this API, but from the documentation, I assume that all objects, references and what not are freed/destroyed when Terminate() is called, as none of their examples show destroying objects.
Now, creating a SanEvent is done like this (according to documentation):
SanEvent event;
event = SanNewEvent("The Main Event");
This opaque handle is considered protected on the C# side, so there does not seem to be a way for me of returning it all the way there. My idea is to keep the events in some kind of Collection, in the C++/CLI wrapper where the type can be used, and only return the index of the Event to C# (The index the even would have in a List or something). I am not sure this is a good idea, but this is as far as I have come with my plan. Something that represents a SanEvent needs to be returned to C# as I need to be able to reference the event in the future, in order to add additional information to the event via other functions. This idea would of course need tailored "helper functions" on the C++ side which mediate between C# and C. I am sorry if the information is vague, but I don't really have a lot to go on myself as of now.

Adding "::" to the beginning of a function call tells the compiler to use the function found in global namespace rather than local namespace/class. So:
namespace Wrapper {
void Analytics::SanSetApplicationContext(const char *ctx) {
::SanSetApplicationContext(ctx);
}
}
Should call the c version correctly

Managed C++ Static Constructor not called in .net4

I've recently moved a project I'm working on from .NET 3.5 to .NET 4. I'm using C#, Managed C++ and Unmanaged C++.
In one of my Managed C++ (interop) I'm having a static constructor:
public ref class StaticPool : public BaseStaticPools
{
public:
static StaticPool()
{
InitializePools();
}
static Poolable^ Dequeue()
{
return (Poolable^)Dequeue(Poolable::typeid);
}
private:
static void InitializePools()
{
BaseStaticPools::CreatePool(Poolable::typeid);
}
};
In .NET 3.5 once Dequeue() had been called for the first time it would trigger the static initialization, which runs the static constructor. Once I moved to .NET 4.0, the static constructor was never called.
I know there have been changes in static initializations in .NET 4.0, but according to all I read it should work fine.

In .NET, type initializers may only be called the first time a field is accessed. This is controlled by the [BeforeFieldInit] attribute.
I filed a bug report, which is only available to beta testers, despite being marked "Public".
Here's the explanation from Microsoft, which you may find helpful:
For C++, this is the intended behavior. We mark our classes with BeforeFieldInit, and so the initialization the CLR is performing is correct. We do not offer a way in C++/CLI to change this behavior. If you require the class constructor to run, you are able to call System.Runtime.CompilerServices.RuntimeHelpers::RunClassConstructor explicitly.
Since we're invoking the standard here, the line from Partition I, 8.9.5 says this:
If marked BeforeFieldInit then the type’s initializer method is executed at, or sometime before, first access to any static field defined for that type.
That section actually goes into detail about how a language implementation can choose to prevent the behavior you're describing. C++/CLI chooses not to, rather they allow the programmer to do so if they wish.
Basically, since the code below has absolutely no static fields, the JIT is completely correct in simply not invoking static class constructors.

Jon Skeet wrote on this topic:
Type initialization changes in .NET 4.0

Writing C# GUI over a C++ dll or C++ exe

I have a C++ console Exe which does some progamming. Now i wanted to write a C# GUI which does some of the programming that the C++ exe does. I was thinking of few approaches,
Write the C# GUI with all programming in C++ done from scratch.(I do not want to do this for the amount of rework it entails)
Build a C++ dll which does the programming and have it imported in GUI app.(Now here i have a concern. How do i capture the output of the routines in c++ dll and display it in GUI? Should i return the output as string for every routine that the app calls.? Since i dont know managed c++ iam going to build an unmanaged C++ dll. )

Building a C++/CLI dll really isn't that hard. You basically use unmanaged C++ code except that you define a "public ref class" which hosts the functions you want the C# code to see.
What kind of data are you returning? Single numbers, matrices of numbers, complex objects?
UPDATE: Since it has been clarified that the "output" is iostreams, here is a project demonstrating redirection of cout to a .NET application calling the library. Redirecting clog or cerr would just require a handful of additional lines in DllMain patterned after the existing redirect.
The zip file includes VS2010 project files, but the source code should also work in 2005 or 2008.
The iostreams capture functionality is contained in the following code:
// compile this part without /clr
class capturebuf : public std::stringbuf
{
protected:
virtual int sync()
{
// ensure NUL termination
overflow(0);
// send to .NET trace listeners
loghelper(pbase());
// clear buffer
str(std::string());
return __super::sync();
}
};
BOOL WINAPI DllMain(_In_ HANDLE _HDllHandle, _In_ DWORD _Reason, _In_opt_ LPVOID _Reserved)
{
static std::streambuf* origbuf;
static capturebuf* altbuf;
switch (_Reason)
{
case DLL_PROCESS_ATTACH:
origbuf = std::cout.rdbuf();
std::cout.rdbuf(altbuf = new capturebuf());
break;
case DLL_PROCESS_DETACH:
std::cout.rdbuf(origbuf);
delete altbuf;
break;
}
return TRUE;
}
// compile this helper function with /clr
void loghelper(char* msg) { Trace::Write(gcnew System::String(msg)); }

So you just want to call c++ library from managed .net code?
Then you would need to either build a COM object or a p-invokable library in c++. Each approach has it's own pros and cons depending on your business need. You would have to marshall data in your consumer. There are tons and tons of material on both concepts.

Possibly relevant: Possible to call C++ code from C#?

You could just write a C# GUI wrapper (as you suggest in option 2) and spawn the C++ process; however, this will be a little slow (I don't know if that matters).
To run your C++ exe and capture the output you can use the ProcessRunner I put together. Here is the basic usage:
using CSharpTest.Net.Processes;
partial class Program
{
static int Main(string[] args)
{
ProcessRunner run = new ProcessRunner("svn.exe", "update");
run.OutputReceived += new ProcessOutputEventHandler(run_OutputReceived);
return run.Run();
}
static void run_OutputReceived(object sender, ProcessOutputEventArgs args)
{
Console.WriteLine("{0}: {1}", args.Error ? "Error" : "Output", args.Data);
}
}

See the first comment in this page to redirect stderr

Probably the best way to go here is use P/Invoke or Platform Invoke. Depending on the structure or your C++ dll interface you may want to wrap it in a pure C interface; it is easiest if your interface uses only blittable types. If you limit your dll interface to blittable types (Int32, Single, Boolean, Int32[], Single[], Double[] - the basics) you will not need to do any complex marshaling of data between the managed (C#) and unmanaged (C) memory spaces.
For example, in your c# code you define the available calls in your C/C++ dll using the DllImport attribute.
[DllImport, "ExactDllName.dll"]
static extern boolean OneOfMyCoolCRoutines([In] Double[] x, [In] Double[] y, [Out] Double result)
The little [In]'s and [Out]'s are not strictly required, but they can speed things along. Now having added your "ExactDllName.dll" as a reference to your C# project, you can call your C/C++ function from your C# code.
fixed(Double *x = &x[0], *y = &y[0] )
{
Boolean returnValue = OneOfMyCoolCRoutines(x, y, r);
}
Note that I'm essentially passing pointers back and fourth between my dll and C# code. That can cause memory bugs because the locations of those arrays may be changed by the CLR garbage collector, but the C/C++ dll will know nothing of it. So to guard against that, I've simply fixed those pointers in my C#, and now they won't move in memory while my dll is operating on those arrays. This is now unsafe code and I'll need to compile my C# code w/ that flag.
There are many fine details to language interop, but that should get you rolling. Sticking to a stateless C interface of blittable types is a great policy if possible. That will keep your language interop code the cleanest.
Good luck,
Paul

Exposing a C++ API to C#

So what I have is a C++ API contained within a *.dll and I want to use a C# application to call methods within the API.
So far I have created a C++ / CLR project that includes the native C++ API and managed to create a "bridge" class that looks a bit like the following:
// ManagedBridge.h
#include <CoreAPI.h>
using namespace __CORE_API;
namespace ManagedAPIWrapper
{
public ref class Bridge
{
public:
int bridge_test(void);
int bridge_test2(api_struct* temp);
}
}
.
// ManagedBridge.cpp
#include <ManagedBridge.h>
int Bridge::bridge_test(void)
{
return test();
}
int Bridge::bridge_test2(api_struct* temp)
{
return test2(temp);
}
I also have a C# application that has a reference to the C++/CLR "Bridge.dll" and then uses the methods contained within. I have a number of problems with this:
I can't figure out how to call bridge_test2 within the C# program, as it has no knowledge of what a api_struct actually is. I know that I need to marshal the object somewhere, but do I do it in the C# program or the C++/CLR bridge?
This seems like a very long-winded way of exposing all of the methods in the API, is there not an easier way that I'm missing out? (That doesn't use P/Invoke!)
EDIT: Ok, so I've got the basics working now thanks to responses below, however my struct (call it "api_struct2" for this example) has both a native enum and union in the C++ native code, like the following:
typedef struct
{
enum_type1 eEnumExample;
union
{
long lData;
int iData;
unsigned char ucArray[128];
char *cString;
void *pvoid;
} uData;
} api_struct2;
I think I have figured out how to get the enum working; I've re-declared it in managed code and am performing a "native_enum test = static_cast(eEnumExample)" to switch the managed version to native.
However the union has got me stumped, I'm not really sure how to attack it.. Ideas anyone?

Yes, you are passing an unmanaged structure by reference. That's a problem for a C# program, pointers are quite incompatible with garbage collection. Not counting the fact that it probably doesn't have the declaration for the structure either.
You can solve it by declaring a managed version of the structure:
public value struct managed_api_struct {
// Members...
};
Now you can declare the method as
int bridge_test2(managed_api_struct temp); // pass by value
or
int bridge_test2(managed_api_struct% temp); // pass by reference
Pick the latter if the structure has more than 4 fields (~16 bytes). The method needs to copy the structure members, one-by-one, into an unmanaged api_struct and call the unmanaged class method. This is unfortunately necessary because the memory layout of a managed structure is not predictable.
This is all pretty mechanical, you might get help from SWIG. Haven't used it myself, not sure if it is smart enough to deal with a passed structure.
A completely different approach is to make the wrapper class cleaner by giving it a constructor and/or properties that lets you build the content of an api_struct. Or you could declare a wrapper ref class for the structure, much like you would in managed code.

as it has no knowledge of what a api_struct actually is
You need to define a managed version in a .NET assembly, that uses attributes (like StructLayoutAttribute) to ensure it marshals correctly.
This seems like a very long-winded [...]
The other approach is to create a COM wrapper (e.g. using ATL) around your API. This might be more effort, but at least you avoid the double coding of struct and function definitions needed for P/Invoke.
Correction: You have created a C++/CLI project: so just add correct '#pragma' to tell the compiler this is .NET code, and then the output is an assembly the C# project can just reference.

Yuo are trying to do this way more complicated that it really is. What you want is two different structs. One managed and one unmanaged. You expose the managed version externally (to your C# app). It will be all ".Net-ish" with no concepts of unions or so.
In your bridge you receive the managed version of the struct, manually create the unmanaged struct and write code to move your data, field by field over to the unmanaged struct. Then call your unmanaged code and finally move the data back to the managed struct.
The beautiful thing about C++/CLI is that the managed code also can work with unmanaged code and data (and include the unmanaged .h files).

Callback unmanaged code from managed C#

Bit of a history lesson here. I'm working on a legacy C++/MFC application and am trying to start a incremental modernization by pushing components written in C# (WinForms and later WPF).
I'm stucking using .Net/1.1 and VS/2003 for many reasons which are impossible to resolve in the near future.
Currently, as a proof of concept, something like this works:
#pragma push_macro("new")
#undef new
WinFormA::Form1* myform;
myform = __gc new WinFormA::Form1();
myform->ShowDialog();
#pragma pop_macro("new")
The problem I'm having is this - I need the unmanaged C++/MFC code to pass a callback pointer into the managed C# WinForm code so that I can capture user interactions and have them processed by the application.
I've looked at some articles such as this MSDN article but it doesn't work in VS/2003 (the compiler doesn't like the delegate syntax).
Are there any other options? I don't think I can use DLLImport since I need to interact with the specific application instance not a flat API.
Thanks!

If the other answers don't work out, you could always write a C wrapper to flatten the classes. For example, if the C++ class is:
class TheClass {
public:
TheClass(int Param);
~TheClass();
bool SomeFunction(int Param1,int Param2);
};
I'll write a wrapper:
extern "C" void *TheClass_Create(int Param) {
return (void*) new TheClass(Param);
}
extern "C" void TheClass_Destroy(void *This) {
delete (TheClass*) This;
}
extern "C" bool TheClass_SomeFunction(void *This,int Param1,int Param2) {
return ((TheClass*) This)->SomeFunction(Param1,Param2);
}
Because the wrapper is straight C, you can P/Invoke in C# to your heart's content (the void *This should become an IntPtr to ensure compatibility if you move to 64-bit). Sometimes, if I'm really ambitious, I'll actually write a C# wrapper around the P/Invokes to 're-classify' the thing.

I already forgot .NET 1.*, but:
Define necessary interfaces and register your .NET components as COM objects. .NET utilities will usually provide reasonably good marshaling code.
If possible, access them as COM objects from C++ application without any Managed C++ at all. (Use interface pointers instead of functions for callbacks).
If COM is not an option, use .NET Reflector to see what's going on inside auto-generated interop assemblies - this might give an insight on how to do the same thing manually.

I have never tried it by myself, but did you check RuntimeMethodHandle struct which is definitely exists in .net1?
SomeDelegate Handler = new SomeDelegate(SomeMethod);
IntPtr HandlerPtr = Handler.Method.MethodHandle.GetFunctionPointer();
And some copy-paste from MSDN's description .net2 Marshal::GetDelegateForFunctionPointer Method:
In versions 1.0 and 1.1 of the .NET Framework, it was possible to pass a delegate representing a managed method to unmanaged code as a function pointer, allowing the unmanaged code to call the managed method through the function pointer. It was also possible for the unmanaged code to pass that function pointer back to the managed code, and the pointer was resolved properly to the underlying managed method.