C＃DllImport函数返回指向结构的指针

Question

我正在尝试在C＃中使用C ++ DLL。
我只能使用一种功能。
这是C ++标头中的函数声明：

struct usb_relay_device_info EXPORT_API * usb_relay_device_enumerate(void);  

这是C ++中的结构：

enum usb_relay_device_type
    {
        USB_RELAY_DEVICE_ONE_CHANNEL = 1,
        USB_RELAY_DEVICE_TWO_CHANNEL = 2,
        USB_RELAY_DEVICE_FOUR_CHANNEL = 4,
        USB_RELAY_DEVICE_EIGHT_CHANNEL = 8  
    };

struct usb_relay_device_info
{
    unsigned char *serial_number;
    char *device_path;
    usb_relay_device_type type;
    usb_relay_device_info* next;
};

这就是我在C＃中导入的方式：

[DllImport("usb_relay_device.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr usb_relay_device_enumerate();

public enum usb_relay_device_type
    {
    USB_RELAY_DEVICE_ONE_CHANNEL = 1,

    USB_RELAY_DEVICE_TWO_CHANNEL = 2,

    USB_RELAY_DEVICE_FOUR_CHANNEL = 4,

    USB_RELAY_DEVICE_EIGHT_CHANNEL = 8,
}

[StructLayoutAttribute(LayoutKind.Sequential)]
public struct usb_relay_device_info
{
    /// unsigned char*
    [MarshalAsAttribute(UnmanagedType.LPStr)]
    public string serial_number;

    /// char*
    [MarshalAsAttribute(UnmanagedType.LPStr)]
    public string device_path;

    /// usb_relay_device_type
    public usb_relay_device_type type;

    /// usb_relay_device_info*
    public System.IntPtr next;
}

使用如下：

IntPtr intPtr = usb_relay_device_enumerate();

，但始终返回0。 需要帮助以了解我在做什么错以及如何纠正它。
非常感谢。