How to handle NA in C#, RDCOM

I am using RDCOM as a data-transfer between R and C#.net.
I got a question on datasets with missing data.
For instance, if list = c(1,2,3,NA,5), in R, typeof(list) is integer
but, in C#, I did not see a suitable data type for (1,2,3,NA,5).
Of course, one can use is.na to transfer (1,2,3,NA,5) into string[] in C#.
So, how to handle NA in this case?
Please help. Thank,

-james