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