What happens if a string in a struct is longer or shorter than the p/Invoked signature used?

Question

Case in point, I want to use DEV_BROADCAST_DEVICEINTERFACE_A from C#. However, I'm not sure how to declare the struct since the size of dbcc_name is dependent on dbcc_size (It's officially declared as char dbcc_name[1]).

According to this question it seems like I need to add

[MarshalAs(UnmanagedType.ByValTStr, SizeConst=255)]

over dbcc_name.

But why use SizeConst=255? We don't know the size. (And it seems from other answers I've seen, that there is no simple way to declare it such that it will know the correct size or a way to specify the size case by case.)

So what happens if I set a static length as in the linked answer. What would happen if the string is shorter or longer?

Testing has shown that if it's longer I get the correct string, and if shorter - I get a truncated string (e.g. if I set SizeConst to 2 and the real string is "abc", I get "ab".) But can I be sure that that's how it works, or is it dependent on something that just happens to be fine in this specific case?

Answer 1

You can't declare completely the structure, what you can do is something like this:

[StructLayout(LayoutKind.Sequential)]
private struct _DEV_BROADCAST_DEVICEINTERFACE_A
{
    public int dbcc_size;
    public uint dbcc_devicetype;
    public uint dbcc_reserved;
    public Guid dbcc_classguid;
    public char dbcc_name; // just for offset; don't use!
}

And use it like this:

// get ptr to structure from somewhere (lParam from WM_DEVICECHANGE ...)
IntPtr ptr = ...

// read structure
var iface = Marshal.PtrToStructure<_DEV_BROADCAST_DEVICEINTERFACE_A>(ptr);

// get name pointer
var namePtr = ptr + Marshal.OffsetOf<_DEV_BROADCAST_DEVICEINTERFACE_A>(nameof(_DEV_BROADCAST_DEVICEINTERFACE_A.dbcc_name)).ToInt32();

// get name
var name = Marshal.PtrToStringAnsi(namePtr);

Note if the name can contain zeros, you should instead use Marshal.PtrToStringAnsi(namePtr, len) with len = dbcc_size - offset of dbcc_name