Question

I wonder how I can build a query expression which understands the given item being selected is the first or not. Say I’m selecting 10 items from DB:

var query = db.Table.Take(10).Select(t => IsFirst ? t.Value1 : t.Value2);

There is an indexed variant of Select but that is not supported in LINQ-to-SQL. If it was supported my problems would be solved. Is there any other trick?

I could have used ROW_NUMBER() on T-SQL for instance, which LINQ-to-SQL uses but does not give access to.

I know I can Concat two queries and use the first expression in the first and so forth but I don’t want to manipulate the rest of the query, just the select statement itself because the query is built at multiple places and this is where I want to behave differently on first row. I’ll consider other options if that is not possible.

Answer 1

You can use the indexed overload, but you need to use the LINQ to Objects version:

var query = 
    db.Table.Take(10).AsEnumreable()
    .Select((t, index) => index == 0 ? t.Value1 : t.Value2);