C#: marshalling a struct that contains arrays

Question

I am doing some C# interop work. I have the following struct:

#pragma pack(push,1)
typedef struct
{
    unsigned __int64 Handle;
    LinkType_t Type;
    LinkState_t State;
    unsigned __int64 Settings;
    signed __int8 Name[MAX_LINK_NAME];
    unsigned __int8 DeviceInfo[MAX_LINK_DEVINFO];
    unsigned __int8 Reserved[40];
} LinkInfo_t;

This is my attempt to convert it into a C# struct:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct LinkInfo_t
{
    [MarshalAs(UnmanagedType.U8)]
    public UInt64 Handle;
    MarshalAs(UnmanagedType.I4)]
    public LinkType_t Type;
    [MarshalAs(UnmanagedType.I4)]
    public LinkState_t State;
    [MarshalAs(UnmanagedType.U8)]
    public UInt64 Settings;
    [MarshalAs(UnmanagedType.LPStr, SizeConst = MAX_LINK_NAME)]
    public string Name;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX_LINK_DEVINFO, ArraySubType = UnmanagedType.U1)]
    public byte[] DeviceInfo;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 40, ArraySubType = UnmanagedType.U1)]
    public byte[] Reserved;
}

However, whenever I initialize the struct the Name, DeviceInfo, and Reserved fields are all set to null. How do I fix this?

Answer 1

For the arrays, try to use the fixed modifier :

    public fixed byte DeviceInfo[MAX_LINK_DEVINFO];
    public fixed byte Reserved[40];

Answer 2

whenever I initialize the struct the Name, DeviceInfo, and Reserved fields are all set to null

This is correct, and your definition looks OK to me (BTW, you don't need [MarshalAs] on the primitive fields, the default behaviour is to do what you specified there). Because your array fields are null, the marshaler won't do anything about them when marshaling your struct to unmanaged memory, but it's going to create the strings and arrays when unmarshaling.