I need to do continuity check on data in table .
In SQL it could be done by simple join query like this:

select l.id + 1 as start
from ContSeq as l
WHERE NOT EXISTS(SELECT 1 from ContSeq as r WHERE l.id + 1 = r.id)

Can you help me to convert it to NHibernate criteria in C#?

I think you can use "Linq 2 Nhibernate" to solve your problem

Linq 2 NHibernate is for SQL Server database only.
Also it could be solved by HQL query but HQL is kind of script and this technic has problems like using dynamic SQL.

So i think the best solution is using crieteria object.
I made some progress this is my nhibernate criteria code



This code creates the this SQL Query:

select l.id as start
from ContSeq as l
WHERE NOT EXISTS(SELECT 1 from ContSeq as r WHERE l.id = r.id)

I need help to complate the criteria to produce this query

select l.id + 1 as start
from ContSeq as l
WHERE NOT EXISTS(SELECT 1 from ContSeq as r WHERE l.id + 1 = r.id)

??? I use "Linq to nHibernate" with PostgreSQL and it works fine. It's not limited to SQL-Server.