unmanaged code structure with a pointer

Question

my unmanaged code contains a structure with a stringpointer. This string pointer should point out to a string buffer typedef struct dat { LPSTR add; dword a; } what is the equivalent declaration in managed code of the above structure? i've used the following prototype: public class UserData { [ MarshalAs( UnmanagedType.ByValTStr, SizeConst=20000 )] public string buffer; public int callcount; } but it's not giving me the desired o/p. Is my declaration correct? please suggest me a way at the earliest.