Why this Explicit P/Invoke does not work ?

Question

The following .net to native C code does not work, any ideas

extern "C" {
   TRADITIONALDLL_API int TestStrRef( __inout char* c)    {
   int rc = strlen(c);
   std::cout << "the input to TestStrRef is: >>" << c << "<<" ;
   c = "This is from the C code ";
   return rc;
   }
 }

 [DllImport("MyDll.dll", SetLastError = true)]
 static extern int TestStrRef([MarshalAs(UnmanagedType.LPStr)] ref string s);
 String abc = "InOut string";
 TestStrRef(ref abc);

At this point Console.WriteLine(abc) should print "This is from the C code " but doesn't, Any ideas on what's wrong ?

FYI - i have another test function not using ref type string, it works just fine

Answer 1

Does this work for you? Basically just add CallingConvention = CallingConvention.Cdecl to the DllImport statement. You might also want to specify the CharSet (for example: CharSet:=CharSet.Unicode)

 [DllImport("MyDll.dll", CallingConvention = CallingConvention.Cdecl, SetLastError = true)]
 static extern int TestStrRef([MarshalAs(UnmanagedType.LPStr)] ref string s);

Answer 2

Your code wrong at C side also. __inout annotation just tell compiler you can change buffer to which "c" argument pointed. But pointer itself located in stack and does not return to caller if you modified "c" argument. Your declaration may look like:

extern "C" {
   TRADITIONALDLL_API int TestStrRef( __inout char** c)    {
   int rc = strlen(*c);
   std::cout << "the input to TestStrRef is: >>" << *c << "<<" ;
   *c = "This is from the C code ";
   return rc;
   }
 }

And C# side:

[DllImport("MyDll.dll", SetLastError = true)]
static extern int TestStrRef(ref IntPtr c);

{
    String abc = "InOut string";
    IntPtr ptrOrig = Marshal.StringToHGlobalAnsi(abc)        
    IntPtr ptr = ptrOrig; // Because IntPtr is structure, ptr contains copy of ptrOrig
    int len = TestStrRef(ref ptr);
    Marshal.FreeHGlobal(ptrOrig); // You need to free memory located to abc' native copy
    string newAbc = Marshal.PtrToStringAnsi(ptr); 
    // You cannot free memory pointed by ptr, because it pointed to literal string located in dll code.
}