marshalAsAttributes UnmanagedType.ByValArray ?

can anyone tell me how to define a fixed size unmanged array of fixed size strings? the code i began with was something like : MarshalAs(UnmanagedType.ByValArray, SizeConst = 3, ArraySubType = UnmanagedType.ByValTStr )] but now i need to define the fixed size of the string cell and i don't know how to do it, TIA