CODEWITHSUNDEEP
c#c++dllclrstatic-libraries
Joseph
Joseph

Reputation: 1784

call a c++ function with char* from C#

I have a CLR C++ dll that wrraped a static C++ library. I have the following class:

#pragma once

#include <windows.h>
#include <sddl.h>

#include <LibEx.h>
using namespace System;
#using <mscorlib.dll>

namespace LIB_WrapperNamespace {

    public ref class LIB_WrapperClass
    {
    public:
        BOOL WINAPI T_LibEx_ConsoleConnect(IN DWORD num1, IN LPWSTR Name)
        {
            return LibEx_ConsoleConnect(num1,Name);
        }
        };
}

in C#, I add the reference to library

LIB_WrapperNamespace.LIB_WrapperClass myLib = new LIB_WrapperNamespace.LIB_WrapperClass();

Now how to call this function, how to send string to char*? from C#:

string myName = "NAME";
myLib.T_LibEx_ConsoleConnect(1,**myName**);

Upvotes: 0

Views: 1167

Answers (2)

Zverev Evgeniy
Zverev Evgeniy

Reputation: 3719

Why building C++\CLI project to wrap something when you "expose" marshaling problems onto the user of your wrapper? The idea of C++\CLI is to hide the marshaling troubles inside the wrapper. You should declare the function natively for .NET:

#pragma once

#include <Windows.h>
#include <stdlib.h>
#include <string.h>
#include <msclr\marshal_cppstd.h> 
#include <vector>

namespace ClassLibrary2 {
public ref class Class1
{
public:
    //Expose .NET types to .NET users.
    System::Boolean T_LibEx_ConsoleConnect(System::UInt64 num1, System::String^ Name);
};

}

And implementing this wrapper function you marshal as you feel proper, it could look like this:

#include "ClassLibrary2.h"

namespace ClassLibrary2 {
System::Boolean Class1::T_LibEx_ConsoleConnect(
    System::UInt64 num1, 
    System::String^ Name)
{
    //Initialize marshaling infrastructure. You can use its instance many times 
    //through out life span of your application.
    msclr::interop::marshal_context^ marshalContext = gcnew msclr::interop::marshal_context();

    //Turn System::String into LPWSTR. Keep in mind that you are now the owner of 
    //memory buffer allocated for unmanagedName. You need to release it somewhere.
    const wchar_t* clsConstChars = marshalContext->marshal_as<const wchar_t*>(Name);
    LPWSTR unmanagedName = const_cast<LPWSTR>(clsConstChars);

    //System::UInt64 num1 will be marshalled to DWORD natively by compiler.
    return LibEx_ConsoleConnect(num1, unmanagedName);
}}

Upvotes: 0

JaredPar
JaredPar

Reputation: 755477

The API should be exposing that parameter as a wchar_t* hence you need to provide a pointer value in C#. Try the following

IntPtr ptr = IntPtr.Zero;
try { 
  ptr = Marshal.StringToCoTaskMemUni("NAME");
  unsafe { 
    myLib.T_LibEx_Consoleconnect(1, (char*)(ptr.ToPointer()));
  }
} finally { 
  if (ptr != IntPtr.Zero) { 
    Marshal.FreeCoTaskMem(ptr);
  }
}

Unfortunately though since you have exposed the method with a raw pointer value there is no way to use this from C# without a unsafe code. An alternative approach would be to expose an overload which takes say a string^. This would be usable from C# and the C++/CLI code could take care of the marshalling from string^ to LPWSTR

BOOL WINAPI T_LibEx_ConsoleConnect(DWORD num1, String^ Name) { 
   IntPtr ip = Marshal::StringToHGlobalUni(Name);
   BOOL ret = T_LibEx_ConsoleConnect(num1, static_cast<LPWSTR>(ip.ToPointer()));
   Marshal::FreeHGlobal(ip);
   return ret;
}

// From C#
myLib.T_LibEx_ConsoleConnect(1, "NAME");

Upvotes: 2

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