How to get unicode string from C++ to C# using PInvoke

Question

I have the following signature of a C++ method. The last argument should return the device name as a 2 byte unicode string.

int GetDeviceIdentifier(DWORD deviceIndex, WCHAR** ppDeviceName);

I wrapped into C# with the following signature. It works but the string that I get is strange. Am I doing something wrong?

[DllImportAttribute("StclDevices.dll", EntryPoint = "GetDeviceIdentifier", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
public static extern int GetDeviceIdentifier(uint deviceIndex, StringBuilder ppDeviceName);

Answer 1

Passing a StringBuilder parameter would match a C++ parameter of type WCHAR*. In that case the memory would be allocated by the C# code by setting the capacity of the string builder object.

For your function it would appear that the memory is allocated by the C++ code. Hence the double pointer. So you would need this:

[DllImportAttribute("StclDevices.dll", 
    CallingConvention=CallingConvention.Cdecl)]
public static extern int GetDeviceIdentifier(
    uint deviceIndex, 
    out IntPtr ppDeviceName
);

You call it like this:

IntPtr ppDeviceName;
int retval = GetDeviceIdentifier(deviceIndex, out ppDeviceName);
string DeviceName = Marshal.PtrToStringUni(ppDeviceName);