Passing a C# string to C++/CLI does not show the string value in the CPP program

Question

I want to call a C# function from C++ , via CLI/C++.

C# code

private string _text = " ";

public void setText(string text)
{
    // _text = text;
    _text = "HI World";
}

Ideally setText shall have the commented line only. The _text = "HI World" is an example.

public string getText()
{
    return _text;
}

C++/CLI code

Header :

gcroot<Bridge> _managedObject;

virtual void setText(std::string text);
virtual std::string getText();

CPP file

std::string CStringBridge::getText()
{

// _managedObject = gcnew Bridge(); return (marshal_as(_managedObject->getText())); }

void CStringBridge::setText(std::string text)
{

// _managedObject = gcnew Bridge(); _managedObject->setText(gcnew System::String(text.c_str())); }

IStringBridgeWrapper* IStringBridgeWrapper::CreateInstance(void)
{
return ((IStringBridgeWrapper *)new CStringBridge());
}

Note : When I use the following code

virtual void setText(System::String^ text);
virtual System::String^ getText();

I get the following error 3395

*__declspec(dllexport) cannot be applied to a function with the __clrcall calling convention*

, and so I stuck to std::string

When I use the library from the C++/CLI code, and call from my C++ program, "Hi World" should be printed ; instead nothing gets printed

C++ console application

IStringBridgeWrapper *pBridge = IStringBridgeWrapper::CreateInstance();

pBridge->setText(std::string("I am here"));
pBridge->getText();

I think the string is not being properly passed .

Any ideas to solve it shall be appreciated.

EDIT

I have updated the code after the comments , yet nothing shows up.

gcroot creates a handle, but does not allocate memory for it. But as Bridge has no memory allocated , the application does not work.My code is in the same lines at the article here - http://www.codeproject.com/Articles/10020/Using-managed-code-in-an-unmanaged-application .

Answer 1

I want to call a C# function from C++ , via CLI/C++.

Wait... you want to call a C++ function from C#, right? That's what C++/CLI is good for. Wrapping C++ code to be accessible in managed environments. If you really want to call C# code from C++, you should look into COM registering your C# code. If you use C++/CLI for this, your whole C++ program will be dragged into the .NET world and you could have used C# from the start.

In C++/CLI, your whole public class interface of ref (.NET) classes should consist of only managed types. That would be System::String^ instead of std::string .

Answer 2

COM is your friend:

Create a an interface in C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;


namespace CsharpLibrary
{
   // Since the .NET Framework interface and coclass have to behave as 
   // COM objects, we have to give them guids.
   [Guid("56A868B1-0AD4-11CE-B03A-0020AF0BA770"),
    InterfaceType(ComInterfaceType.InterfaceIsDual)]
   public interface IStringHolder
   {
      String GetText();
      void SetText(String s);
   }
}

Implement the C# interface:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;

namespace CsharpLibrary
{
   [Guid("C6659361-1625-4746-931C-36014B146679")]
   public class MyStringHolder : IStringHolder
   {
      String _text;

      public String GetText()
      {
         return this._text;
      }

      public void SetText(String value)
      {
         _text = value;
      }

   }
}

Create and call your C# object from C++

#include <windows.h>
#include <stdio.h>

#pragma warning (disable: 4278)

// To use managed-code servers like the C# server, 
// we have to import the common language runtime:
#import <mscorlib.tlb> raw_interfaces_only


#pragma warning (disable: 4278)

// To use managed-code servers like the C# server, 
// we have to import the common language runtime:
#import <mscorlib.tlb> raw_interfaces_only

#import "..\CsharpLibrary\bin\Debug\CsharpLibrary.tlb" no_namespace named_guids





int main(int argc, char* argv[])
{
   HRESULT hr = S_OK;

   IStringHolder *pStringHolder = NULL;

   //
   // Initialize COM and create an instance of the InterfaceImplementation class:
   //
   CoInitialize(NULL);

   hr = CoCreateInstance(   __uuidof(MyStringHolder),
                           NULL,
                           CLSCTX_INPROC_SERVER,
                           __uuidof(IStringHolder),
                           reinterpret_cast<void**>(&pStringHolder));

   if(SUCCEEDED(hr))
   {
      _bstr_t sHelloWorld = SysAllocString( L"Hello, World" );

      hr = pStringHolder->SetText(sHelloWorld);

      SysFreeString(sHelloWorld);
   }


   //
   // Be a good citizen and clean up COM
   //
   CoUninitialize();

   return hr;
}

On the C# side you have to generate the type library and register the class by a post-build event:

generate the type library: "$(FrameworkSDKDir)bin\\NETFX 4.0 Tools\\tlbexp.exe" "$(TargetPath)" /out:"$(TargetDir)$(TargetName).tlb"

register the class: C:\\Windows\\Microsoft.NET\\Framework\\v4.0.30319\\RegAsm.exe "$(TargetPath)"

Enjoy!