struct CallbackData
{
            ...
            [FieldOffset(16)]
            public TMyStruct1 MyStruct1;
            [FieldOffset(16)]
            public TMyStruct2 MyStruct2;
}

        [StructLayout(LayoutKind.Explicit, Pack = 1)]
        public struct CallbackData
        {
            [FieldOffset(0)]
            public uint TypeID;
            [FieldOffset(4)]
            public uint Version;
            [FieldOffset(8)]
            public uint Flags;
            [FieldOffset(12)]
            public uint UserData;
            [FieldOffset(16)]
            public FileStatusData FileStatusData;
            [FieldOffset(16)]
            public ErrorData ErrorData;
            [FieldOffset(20)]
            public uint AnyVar;
        }

        [StructLayout(LayoutKind.Sequential, Pack = 1)]
        public struct FileStatusData
        {
            public uint Flags;
            public uint ScanAnswer;
            public FileInfo FileInfo;
            public MalwareInfo MalwareInfo;
            public uint Warning;
            public uint Info;
        }

        [StructLayout(LayoutKind.Sequential, Pack = 1)]
        public struct ErrorData
        {
            public FileInfo FileInfo;
            public uint Category;
            public uint Level;
            public int Code;
            public KeyValue Options; // TODO PSAVAPIKeyValue
        }