Passing a C# string through Pinvoke

Question

C# side:

    [DllImport(@"FileGuidUtils.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
    private static extern long getReparseType([MarshalAsAttribute(UnmanagedType.LPWStr)] string linkPath);

C/C++ side:

__declspec(dllexport) ReparseType getReparseType(WCHAR *linkPath) {
    HANDLE hFile = CreateFile(linkPath, GENERIC_ALL, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OPEN_REPARSE_POINT, NULL);
    if (hFile == INVALID_HANDLE_VALUE)
    {
        CloseHandle(hFile);
        return NEITHER;
    }

    REPARSE_DATA_BUFFER *reparseDataBuffer = (REPARSE_DATA_BUFFER *)malloc(MAXIMUM_REPARSE_DATA_BUFFER_SIZE);
    DWORD dwRetLen = 0;
    BOOL bRet = DeviceIoControl(hFile, FSCTL_GET_REPARSE_POINT, NULL, 0, reparseDataBuffer, MAXIMUM_REPARSE_DATA_BUFFER_SIZE, &dwRetLen, NULL);
    if (bRet == FALSE)
    {
        free(reparseDataBuffer);
        CloseHandle(hFile);
        return NEITHER;
    }

    ULONG reparseType = reparseDataBuffer->ReparseTag;
    free(reparseDataBuffer);
    CloseHandle(hFile);

    if (reparseType == IO_REPARSE_TAG_SYMLINK) {
        return SYMLINK;
    }
    else if (reparseType == IO_REPARSE_TAG_MOUNT_POINT) {
        return JUNCTION;
    }

    return NEITHER;
}

How is linkPath passed? Is it passed as a malloc'd string and I need to free it up on the C/C++ side or will the CLR take care of it for me? If it is getting cleaned up, am I passing it correctly? Could I just pass it as an ordinary C# string without the [MarshalAsAttribute(UnmanagedType.LPWStr)]? Thanks!

pm100 · Accepted Answer · 2015-04-03 22:17:50Z

1

C# will make a copy of the C# string for you into unmanaged memory , pass a pointer to it and delete the string once the function call is finished

edited Apr 3, 2015 at 22:17

answered Apr 3, 2015 at 21:18

pm100

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1 Comment

Wes Over a year ago

OK so as I understand what you've said, there's nothing I need to change?

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