What is the file format that the C++ MFC object CArchive writes to?

Question

I am trying to read in with C# a file written with CArchive. From what I can tell the format is:

[length of next set of data][data]...etc

I'm still fuzzy on some of the data, though. How do I read in Date data? What about floats, ints, doubles, etc?

Also, [length of next set of data] could be a byte or word or dword. How do I know when it will be each? For instance, for a string "1.10" the data is:

04 31 2e 31 30

The 04 is the length, obviously and the rest are hex values for 1.10. Trivial. Later I have a string that is 41 characters long, but the [length] value is:

00 00 00 29

Why 4 bytes for the length? (0x29 = 41)

The main question is: Is there a spec for the format of CArchive output?