vb.net - Conversion from type 'DBNull' to type 'Date' is not valid on tryparse -

i attempting datetime.tryparse date-formatted string, new column created part of dataview table. getting error when code runs conversion type 'dbnull' type 'date' not valid.

this line of code:

datetime.tryparse(dr("ix_articlestartdate"), dr("nstartdate")) 

and these values in watch when errors out.

+       dr("ix_articlestartdate")   "2015/3/11" {string}    object +       dr("nstartdate")    {}  object 

i under impression tryparse return null value if fails convert datatypes. there should doing different convert string datetime datatype?

dr instantiated datarow

dim dr datarow = dv0.table.rows(i)

vb implicitly try's cast dbnull value datetime, since method signature of datetime.tryparse is

public shared function tryparse(s string, byref result date) boolean 

which fails. can use variable instead:

dim startdate datetime if datetime.tryparse(dr("ix_articlestartdate").tostring(), startdate)     dr("startdate") = startdate end if