structures, arrays marshalling from C#

I want to get a bite array that represent the following structure (using pseudocode):

struct MSG {
   int32 msg_type;
   int32 size;
   int32 num;
   struct USER {
       int32 id;
       char name[16];
   } [4]
}

The C# way to define this is something like

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public class USER
{
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst=16)]
    public string name = string.Empty;
    public int id = 0;
};

[StructLayout(LayoutKind.Sequential)]
public class MSG_USERS
{
   public int msg_type = (int)Enums.MessageTypes.USERS;
   public int size = 3*4 + 4*(16+4);
   public int num;
   public USER[] user = new USER[4] { new USER(), new USER(), new USER(), new USER() };
}

and the way to convert it as follows:

MSG_USERS msg = new MSG_USERS();
msg.num = 1;
msg.USER[0].id = 22;
msg.USER[0].name = "First User";
byte[] msgarr = new byte[msg.size];
Marshal.StructureToPtr(msg, Marshal.UnsafeAddrOfPinnedArrayElement(msgarr, 0), false);

This is not enough. I also need the following:
Marshal.StructureToPtr(msg.USER[0], Marshal.UnsafeAddrOfPinnedArrayElement(msgarr, 12), false);

Please help me make this code cleaner. That is:
 - get rid of the second explicit call to System.Runtime.InteropServices.Marshal.StructureToPtr.
 - have size calculated automatically
 - what if I have flexible amount of usersStart Free Trial