C# - If datetime return null show 'Unknown' message.

You cannot store a string value in a DateTime datatype. If it is mandatory to give such message like "unknown", you need to create a separate column for displaying such string by using the expression of dataTable.

 

Check below reference

http://stackoverflow.com/questions/19325064/how-do-i-use-if-else-or-case-in-datacolumn-expression

 

Hope it helps