C# DLL to .il, to C++ DLL: problem with strings

Question

I'm trying to export some functionality from C# so I can use it in my unmanaged C++ app. On my test project, I first I create a C# DLL with a simple function to write a string to a file. I then use ildasm to turn this into an intermediate language (.il file). The function in .il looks like so:

  // =============== CLASS MEMBERS DECLARATION ===================

  .class public auto ansi beforefieldinit MyTest.CSharpExportClass extends 

  [mscorlib]System.Object
  {
  .method public hidebysig static void        modopt([mscorlib]System.Runtime.CompilerServices.CallConvStdcall)   ExportNameOfFunction2(string test) cil managed
    {
  .vtentry 1 : 1
  .export [1] as ExportNameOfFunction2
      // Code size       25 (0x19)
      .maxstack  2
      .locals init ([0] class [mscorlib]System.IO.TextWriter tw)
      IL_0000:  ldstr      "date.txt"
      IL_0005:  newobj     instance void [mscorlib]System.IO.StreamWriter::.ctor(string)
      IL_000a:  stloc.0
    IL_000b:  ldloc.0
    IL_000c:  ldarg.0
    IL_000d:  callvirt   instance void [mscorlib]System.IO.TextWriter::WriteLine(string)
    IL_0012:  ldloc.0
    IL_0013:  callvirt   instance void [mscorlib]System.IO.TextWriter::Close()
    IL_0018:  ret
  } // end of method CSharpExportClass::ExportNameOfFunction2

.method public hidebysig specialname rtspecialname instance void  .ctor() cil managed
  {
    // Code size       7 (0x7)
    .maxstack  8
    IL_0000:  ldarg.0
    IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
    IL_0006:  ret
  } // end of method CSharpExportClass::.ctor

} // end of class MyTest.CSharpExportClass

I see the keyword "ansi" in there... After turning this into a C++ DLL using ilasm.exe, I try to load to use this function in my C++ app:

HMODULE hdll = LoadLibraryW(L"CpCsh.dll");
    if(!hdll)
    {       
        return(-1);
    }

    typedef void (__stdcall *_EXP_NAME_OF_FUNCT)(wchar_t*);
    _EXP_NAME_OF_FUNCT      ExportNameOfFunction;

    ExportNameOfFunction = (_EXP_NAME_OF_FUNCT)GetProcAddress(hdll, "ExportNameOfFunction2");

    if(ExportNameOfFunction == NULL)
    {
        return(-1);
    }

    ExportNameOfFunction(L"hello all");

But this will only write the first letter ("h") to file. If I declare the function as using char instead of wchar_t in C++, and use "hello all" instead of L"hello all" the whole string is written to file.

A few more notes: ildasm options: /nologo /quiet /out:MyTest.dll MyTest.il /unicode /DLL /resource=MyTest.res /optimize

ilasm options: /nologo /out:C:\temp\CpCsh.dll "MyTest.il" /DLL

Any help is appreciated!

Answer 1

This is exactly the symptom of a function expecting a char[] but receiving a wchar_t[]: the second byte of the first wchar_t is zero, interpreted as end-of-string

You can verify if your compiled dll exports it's function as a char* argument by using the dumpbin /exports yourdll.dll command.

You should try to find the compiler switch for ilasm that tells it to treat it's strings as wide characters.

Answer 2

I ended up going with Sandeep's suggestion and built a C++ mixed mode DLL.