Returning arrays from c++ dll with extern "C" to c#

Question

Hello!

I was wondering how on earth you can return an array (or string if possible) from a c++ DLL. This is going to be used with extern "C", so strings don't work it seems.

Here is the code:

1	extern "C" {
2	char* __declspec(dllexport) _fsgf_return_validity(wstring path) {
3	ifstream file;
4	char* validity=new char[4];
5	file.open(path.c_str(), ios::in|ios::binary);
6	file.read(validity, 4);
7	file.close();
8	return validity;
9	}
10	}

Many thanks!

(Edit: It seems that it doesn't like underscores. They appear within the function name and before declspec. Cheers!)

Darkzaelus

Answer 1

First, note that the function cannot take a wstring parameter; that should be const wchar_t* instead.

Second, you can return a char* or a const char* and C# can see it as either a System.String or as a System.Byte[], but with the way you have it coded now you're going to leak memory (who deletes the char* you created?). What you should do instead is have the function take a char* parameter to use as a buffer, so the function does no heap allocation -- the caller must take care of allocating and deallocating the buffer.

Answer 2

So if I use this c++ code:

1	void __declspec(dllexport) _fsgf_return_validity(wchar_t* path, char* &validity) {
2	ifstream file;
3	file.open(path, ios::in|ios::binary);
4	file.read(validity, 4);
5	file.close();
6	}

with this linker:

[DllImport("Core.dll")]

private static extern string _fsgf_return_validity(string path, string *buffer);

It should work?

Thanks for your patience,

Darkzaelus

Answer 3

No, both signatures are wrong.

C++:

extern "C" { void __declspec(dllexport) _fsgf_return_validity(const wchar_t* path, char* validity); }

C#:

[DllImport("Core.dll")]

private static extern void _fsgf_return_validity([MarshalAs(UnmanagedType.LPWStr)] string path, [MarshalAs(UnmanagedType.LPStr)] StringBuilder validity);

To call the function in C#, you'll need to create a StringBuilder with a capacity of at least 5 (to allow for null termination). To make this less error prone, you should wrap the above function with something like:

private static string FsgfReturnValidity(string path)

{

StringBuilder validity = new StringBuilder(5);

_fsgf_return_validity(path, validity);

return validity.ToString();

}

Answer 4

FANTASTIC!

Thank you very much for your time, it works perfectly!

this is brilliant!

Cheers!

Darkzaelus