DLL export array reference to C#

[ShadowLocke]

I am trying to pass a string array from c# into c++ and manipulate the data of that array. How can I do this? (I am c++ newb)

c++

__declspec(dllexport) int __stdcall Test(LPCTSTR* as_test)
{
    as_test[0] = L"This is new data";
                return 0;
}

c#

[DllImport("PBNI.dll", CharSet = CharSet.Unicode)]
            static extern int Test(ref String[] as_ret);
 
       public void Testing()
       {
                String[] ls = new String[] { "replace me", "more data", "even more" };
                Test(ref ls);
                Console.WriteLine(ls[0]);
       }

I tried this, but I get two japanese chars..

[weaknessforcats]

 as_test[0] = L"This is new data"; 

as_test is an LPCTSTR* but L"This is new data" is not a Unicode string.

You might try:

 as_test[0] = TEXT("This is new data"); 

[ShadowLocke]

Thanks for the post, just gave it a try but still getting the same results :(

[weaknessforcats]

Are you sure a String array is the same as an LPCTSTR?

I suspect interoperabilit y rules are causing this.

I did a Google advanced search using "interoperabili ty msdn String" and got a lot of hits. One of them said:

By default, .NET strings are marshalled by COM Interop to LPTSTR in C++ and, vice versa, and so you have to explicitly marshal any other type of unmanaged string (including BSTR) to and from a .NET string using the MarshalAs attribute.

There's no particular problem in inheriting from IUnknown or IDispatch but, if you inherit from the latter, you have to mark the C# interface declaration with the InterfaceType attribute.

I am leaving it to you to research this.

[ShadowLocke]

I've gotten a step closer by changing the c++ code to this..

__declspec(dllexport) int __stdcall Test(LPCWSTR * &as_test)
{ 
    _tcscpy((wchar_t*)as_test[0], L"new data");
 
    return 0;
}

But this replaces the string array with an array of the 1 element that was changed..I dont know why im losing the rest of the data

[weaknessforcats]

_tcscpy is a TCHAR macro that resolves to either strcpy(ASCII) or wcscpy(Unicode) . That means you need an LPCTSTR argument.

Then L"new data" does not produce the correct string. Use the TEXT or _T macro. This will call MultibyteToWide Char on the Unicode side.

__declspec(dllexport) int __stdcall Test(LPCTSTR as_test) 
{  
    _tcscpy(as_test, TEXT("new data"); 
 
    return 0; 
} 

I know you are passing in an array of C# strings but I suggest you pass in onyl one string until you get a result.

Next, _tcscpy does not check that you have enough memory to make a copy. Plus the address is the address of the C# string. That causes _tcscpy to overwrite the C# string and you have no idea what the format of a C# string object is. Therefore, I expect corruption here.

I don't see where you are marshaling the code on the C++ side and I don't see you using IDispatch on the C++ side. That is, COM does your marshaling aand it does it through IDispatch.

You can't simply copy from one language to the other.

[ShadowLocke]

@weaknessforcats
@ShadowLocke
Taking the array element out of the picture everything works as expected. I'm getting results, just not what im asking for. I need to return an edited array parameter..

Passing the array of strings, in the c++ code I am able to messagebox(0, as_test[any index], L"", 0) and see that each string is indeed there. So the pointer in is working.

I think i will end up turning the array into a delemited string but that just seems wrong..

@weaknessforcats
This is a test. I want to see results before I make it error proof.

@weaknessforcats
I know nothing of this IDispatch, please elaborate if you still think your on the right track. And since I see the data coming in properly, I think my problem here is arrays. Not strings.

...
You can't simply copy from one language to the other.

Hence...the orginal cry for help.

[weaknessforcats]

I assume you have researched stuff like this:

http://msdn.microsoft.com/en-us/library/ms173184.aspx
http://msdn.microsoft.com/en-us/library/ms172270.aspx
http://msdn.microsoft.com/en-us/library/8z2kbc1y.aspx

http://msdn.microsoft.com/en-us/library/be80xase.aspx

[ShadowLocke]

Problem solved with this:

c++

__declspec(dllexport) int __stdcall Test(LPCTSTR ** as_test)
{
    _tcscpy((wchar_t*)as_test[0], TEXT("a result."));
 
    return 3;
}

c#

[DllImportAttribute("PBNI.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.StdCall)]
            public static extern void Test([In, Out] String[] as_test);

Arrays require the [In, Out] attributes to come back.

[weaknessforcats]

Your solution fails if your code is compiled using ASCII.

Also, it assumes a pointer to a pointer is the same as a pointer. And it assumes there is enough allocated memory at that location to contain the source string. And it assumes a C# String has the same format as a C string considering you are overwriting the C# string.

You need a solution that does not rely on a cast and that works for both ASCII and Unicode.