Porting from C++ with WINAPI

I'm trying to port a c++ class as a way to learn a bit more about c# but some things are bugging me.

If in c++ I have a struct like:

struct MYSTRUCT {
	int id;
	char some_text[FIXED_LENGTH_CONST];
}

public struct MYSTRUCT {
	public int id;
	[MarshalAs(UnmanagedType.ByValArray, SizeConst=FIXED_LENGTH_CONST)]
	public byte[] some_text
}

Once that is sorted out, how would I go about getting data from a string into some_text so that some_text is ansi c compliant (null terminated). The string needing to go into some_text isn't fixed in size, could be longer or shorter than FIXED_LENGTH_CONST in length. If it is longer then the string should just be copied up to FIXED_LENGTH_CONST.

Next, how do I "pack" the struct into a COPYDATASTRUCT so that it can be sent to another application via SendMessage() and WM_COPYDATA?

I know there are a lot of questions but any help would be great.

Thanks.