Detecting a Null field on an NpgsqlDataReader object

Dear All,

How can I detect if a certain field of a record returned by a query is NULL ?

I'm sure there must be a direct way of doing this, but I googled up and down, back and forth and was unable to find out.

For ex, in Sql Server we just compare with DbNull singleton. 

What is the equivalent in Npgsql?

Thanks a million for your help

Best,
Oliveiros d'Azevedo Cristina