Passing string from c++ to c#

Question

I'm trying to pass string from c++ to c#.

C++:
extern "C" __declspec(dllexport) void GetSurfaceName(wchar_t* o_name);

void GetSurfaceName(wchar_t* o_name)
{
  swprintf(o_name, 20, L"Surface name");
}

C#:
[DllImport("SurfaceReader.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern void GetSurfaceName(StringBuilder o_name);

StringBuilder name = new StringBuilder(20);
GetSurfaceName(name);

But only first symbol is passed: name[0] == 'S'. Other symbols are nulls. Could you tell me what is wrong here?

Thanks, Zhenya

Answer 1

You forgot to tell the pinvoke marshaller that the function is using a Unicode string. The default is CharSet.Ansi, you'll need to use CharSet.Unicode explicitly. Fix:

[DllImport("SurfaceReader.dll", 
           CallingConvention = CallingConvention.Cdecl, 
           CharSet = CharSet.Unicode)]
private static extern void GetSurfaceName(StringBuilder o_name);

You'll get a single "S" now because the utf-16 encoded value for "S" looks like a C string with one character.

Do in general avoid magic numbers like "20". Just add an argument that say how long the string buffer is. That way you'll never corrupt the GC heap by accident. Pass the StringBuilder.Capacity. And give the function a return value that can indicate success so you also won't ignore a buffer that's too small.

Answer 2

我不确定，但是我不是使用StringBuilder，而是将C＃char（wchar）数组传递给C ++，填充它，然后在C＃中对该数组进行操作。