Develop Reference

Proper way to return a List<String^> to a Vector<std::string> in Extern "C"?

I'm communicating between CLI and I need to return back a vector of strings from my C function. I have not done much cli between c++/c# so please tell me if there is a better way to accomplish this.
//Plugin.cpp
void PluginMain(){
ServiceClientHook();
}
//Plugin.h;
extern "C" {
vector<std::string> ServiceClientHook();
}
//ServiceClientHook.h
#include "Plugin.h"
//ServiceClientHook.cpp
string ServiceClientHook()
{
return "Foobar";
}
How do I handle ServiceClientHook()? I should add that I am using non cli on my Plugin and cli on my ServiceClientHook
Currently when I try and return a vector from 'Plugin.h': C linkage function cannot return C++ class 'std::vector>'

It sounds like you have two functions in the same C++/CLI DLL project:
PluginMain -- talks to an unmanaged EXE, maybe C++, maybe not
ServiceClientHook -- talks to a C# DLL
You don't need to do anything special at all to have these talk to each other. Get rid of the extern "C" on ServiceClientHook() and it will just work. Even though ServiceClientHook() is in a file using /clr and PluginMain is plain C++.
Now, PluginMain may need to have extern "C" and __declspec(dllexport) to help the EXE find it. But those requirements don't apply to the functions it uses.

.net wrapper for native dll - how to minimize risk of run-time error?

I am developing a C# application. Since I have some algorithms for least-squares fit in C/C++ that would be too cumbersome too translate, I have made the C++ code into a dll and then created a wrapper in C#.
In the C# code, I have defined a struct that is passed to the unmanaged C++ code as a pointer. The struct contains the initial guesstimates for the fitting functions, and it is also used to return the results of the fit.
It appears to me that you must define the struct in both the managed and the unmanaged code. However, someone using my source code in in the future might decide to change the fields of the struct in the C# application, without understanding that they also have to change the struct in the native code. This will result in a run-time error at best (and produce erroneous results at worse), but there will be no error message telling the developer / end user what is wrong.
From my understanding, it impossible to create a test in the unmanaged C++ DLL that checks if the struct contains the correct fields, but is it posible to have the DLL returning a struct of the correct format to the C# wrapper?
Otherwise, what ways are there to reduce the risk that some careless programmer in the future causes run-time errors that are hard to detect?
Sample code:
//C++
struct InputOutputStruct {
double a,b,c;
}
extern "C" __declspec(dllexport) void DoSomethingToStruct(InputOutputStruct* s)
{
// ... algorithm
}
//C#
using System.Runtime.InteropServices;
[StructLayout(LayoutKind.Sequential)]
public struct InputOutputStruct {
public double a,b,c;
}
[DllImport("CpluplusDll.dll")]
public static unsafe extern bool DoSomethingToStruct(InputOutputStruct* s);
class CSharpWrapper {
static void Main(string[] args)
{
InputOutputStruct s = new InputOutputStruct();
unsafe {
InputOutpustruct* sPtr = &s;
DoSomethingToStruct(sPtr);
s = *sPtr;
}
}
}

It appears to me that you must define the struct in both the managed
and the unmanaged code.
Not true. This is what C++/CLI was invented for- facilitate much easier interoperation with C++ and .NET.

but is it posible to have the DLL returning a struct of the correct format to the C# wrapper?
I don't think so, because you always need to define the structs on the C# side.
Here are a solution which may work ( never tested ):
Give each struct which is shared a unique identifier on both sides ( GUID, Macros )
Create a reflection for C++ which contains informations about the the types which are used on C# and C++ side. This could be done by using macros.
Compare the C++ structs and the C# structs on startup by using the GUIDs, reflection and macros. You can also use sizeof to compare sizes first.
That could be a lot of work
When working on C++ side you still can make a lot of things wrong when you not know about the macros

C++/CLI wrapper for C dll

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 );
}
};

how do i use the c++ dll in c#

i like import c++ dll in my c# application how can i Do this?, what is concept i need to use for this?

You need to look at different kinds of interop. The easiest is P/Invoke. Then is is COM Interop. And then finally you can use Managed C++

Have access to the C++ library source
The cleanest way is to use C++ Interop to create a wrapper as a mixed mode assembly. This allows you to use the pragmas managed and unmanaged to switch between native and managed code. This way you can nicely wrap your C++ class with a managed C++ wrapper and call it from (of course) managed C#.
Be aware this won't work under Mono.
Don't have access to the C++ library source, but know a bit of C++?
You can write a small managed wrapper in managed C++ to call the unmanaged C++ library due to C++'s unique ability to call COM natively.
This is done using custom runtime callable wrappers, or CRCWs. You can then call your managed wrapper directly from C# using native .Net types and all.
Benefits of this (as stated by MSDN):
The interop code is built into the application, so there is no dependency on a separate assembly. Also, the exposed managed interface is customized to be more .NET-like. For example, the RenderFile method takes a System.String instead of a char*. The managed version of the COM interface is called a Custom Runtime Callable Wrapper (CRCW).
Don't have access to the C++ library source and don't know C++?
Then you are stuck with C++ COM Interop which is a bit more messy. You'll need to generate a wrapper using Tlbimp.exe.
It's messy because:
Types that don't have a direct match in managed code are exposed as IntPtr pointer types (quite difficult to handle in C#)
The resulting assemlblies will be very large as interfaces for everything in the library are generated (not just the stuff you want).
You'll have to deloy an additional assembly with your application and manage versioning etc...

suppose this DLL that compile with MinGW.
in C-sharp you can follow this code:
using System.Runtime.InteropServices;
using System;
class call_dll {
[StructLayout(LayoutKind.Sequential, Pack=1)]
private struct STRUCT_DLL {
public Int32 count_int;
public IntPtr ints;
}
[DllImport("mingw_dll.dll")]
private static extern int func_dll(
int an_int,
[MarshalAs(UnmanagedType.LPArray)] byte[] string_filled_in_dll,
ref STRUCT_DLL s
);
public static void Main() {
byte[] string_filled_in_dll = new byte[21];
STRUCT_DLL struct_dll = new STRUCT_DLL();
struct_dll.count_int = 5;
int[] ia = new int[5];
ia[0] = 2; ia[1] = 3; ia[2] = 5; ia[3] = 8; ia[4] = 13;
GCHandle gch = GCHandle.Alloc(ia);
struct_dll.ints = Marshal.UnsafeAddrOfPinnedArrayElement(ia, 0);
int ret=func_dll(5,string_filled_in_dll, ref struct_dll);
Console.WriteLine("Return Value: " + ret);
Console.WriteLine("String filled in DLL: " + System.Text.Encoding.ASCII.GetString(string_filled_in_dll));
}
}

This is the concept you need to use :)

Expose Borland C++ methods to C#

I have following method in my Borland C++ code,
static bool UploadBitstream(void)
{
//Some code Implementation
}
And I'm trying to convert it to DLL and access it in C#.
What are the steps I need to follow to Convert the code DLL
and then use it in C# ??

First, you have to make sure that the methods are defined extern. Then you need to declare the method stdcall or pascal calling convention, and mark them dllexport. See code listing below (this is ancient memory for me, so pardon if I am a bit off on modern Borland C++ compilers).
// code.h
extern "C" {
#define FUNCTION __declspec(dllexport)
FUNCTION int __stdcall SomeFunction(int Value);
In the main
#include "code.h"
FUNCTION int __stdcall SomeFunction(int timesThree)
{
return timesThree * 3;
}

Once you've compiled your DLL all you should need to do in .NET to get access to it is use the DLLImport property.
public class stuff
{
[DLLImport("somedll.dll")]
public static extern void UploadBitstream();
}
If you need pointers or anything like that it gets more complicated but for void functions it's that simple.
It should be noted that once you invoke that function the dll will be loaded by your program and won't be released until your program is closed. You can dynamically load dlls but that is much more complicated. I can explain it here if you have a need.

Watch your terminology.
In your example UploadBitstream is function not a method.
If it is indeed a method in a class then it is very difficult to use classes from Borland compiled DLLs.
If your code is actually C not C++ you should be able to create a compatible DLL
for your simple C style functions with C++ Builder.
See the following SO question:
Use a dll from a c++ program. (borland c++ builder and in general)
where I list various compiler settings which will also apply to creating compatible DLLs.