Question

I have an unmanaged struct I’d like to marshal to c# that looks basically like this:

struct DateTimeStruct{
   double datetimestamp;
};   

struct MyStruct{
   char firstname[40];
   char lastname[40];
   DateTimeStruct bday;
   unsigned integer bool1;
   int val1;
};

What is the the correct c# declaration?

Answer 1

The struct isn’t a problem, it will marshal correctly as-is.

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
    struct MyStruct{
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 40]
        string firstname;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 40]
        string lastname;
        DateTimeStruct bday;
        uint bool1;
        int val1;
    }

Of course, it will be up to you to convert the double to a matching DateTime value. How it is encoded is unguessable from your question.